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Evaluation of Organic Spring Cover Crop Termination Practices to Enhance Rolling/Crimping

Evaluation of Organic Spring Cover Crop Termination Practices to Enhance Rolling/Crimping agronomy Article Evaluation of Organic Spring Cover Crop Termination Practices to Enhance Rolling/Crimping 1 , 1 2 2 Andrew J. Price *, Leah Duzy , J. Scott McElroy and Steve Li National Soil Dynamics Laboratory, Agricultural Research Service, United States Department of Agriculture, 411 South Donahue Drive, Auburn, AL 36832, USA Department of Crop, Soil, and Environmental Sciences, Auburn University, Auburn, AL 36831, USA * Correspondence: price@ars.usda.gov; Tel.: +1-334-887-8596 Received: 3 August 2019; Accepted: 3 September 2019; Published: 6 September 2019 Abstract: With organic farming hectarage and cover crop interest increasing throughout the United States, e ectively timed cover crop termination practices are needed that can be utilized in organic conservation tillage production systems. Four commercially available termination treatments approved by Organic Materials Review Institute (OMRI) were evaluated, immediately following mechanical termination with a cover crop roller/crimper and compared to a synthetic herbicide termination to access termination rates. Treatments included rolling/crimping followed by (1) 1 1 20% vinegar solution (28 L a.i. ha acetic acid), (2) 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove oil (eugenol, acetyl eugenol, caryophyllene) mixture, (3) 0.15 mm clear polyethylene sheeting applied with edges manually tucked into the soil for 28 days over the entire plot area (clear plastic), (4) broadcast flame emitting 1100 C applied at 1.2 k/h (flame), (5) glyphosate applied at 1.12 kg a.i. ha (this non-OMRI-approved, non-organic conservation tillage cover crop termination standard practice was included to help ascertain desiccation, regrowth, and economics), and (6) a non-treated control. Five cover crop species were evaluated: (1) hairy vetch (Vicia villosa Roth), (2) crimson clover (Trifolium incarnatum L.), (3) cereal rye (Secale cereale L.), (4) Austrian winter pea (Pisum sativum L.), and (5) rape (Brassica napus L.). Three termination timings occurred at four-week intervals beginning mid-March each year. In April or May, organic producers are most likely to be successful using a roller crimper as either a broadcast flamer for terminating all winter covers evaluated, or utilizing clear plastic for hairy vetch, winter peas, and cereal rye. Ine ectiveness and regrowth concerns following cover crop termination in March are substantial. Commercially available vinegar and cinnamon/clove oil solutions provided little predictable termination, and producers attempting to use these OMRI-approved products will likely resort to cover crop incorporation, or mowing, to terminate covers if no other practice is readily available. Keywords: cinnamon oil; clove oil; cover crop biomass; flame termination; organic agriculture; organic herbicides; roller/crimper; solarization; vinegar 1. Introduction In 2018, US vegetable farmers produced over 7.53 million hundredweight (cwt; 100 pounds) on over one million hectares, with a value of over US $12.9 billion [1]. On average, per-capita consumption of vegetables was estimated to be 170 kg person [2]. As the world’s largest market for organic food, organic vegetable production in the United States comprises 8.4% of total vegetable land area, and sales from organic production are more than US $5.5 million [3]. However, as of 2011, only 0.6% of US cropland was certified for organic production due to obstacles such as high management costs, Agronomy 2019, 9, 519; doi:10.3390/agronomy9090519 www.mdpi.com/journal/agronomy Agronomy 2019, 9, 519 2 of 14 risks associated with operational changes, limited knowledge of organic production methods, and marketing and infrastructure limitations [3]. For organic producers and producers considering transitioning to organic production, there are numerous management practices (e.g., reduced tillage, cover cropping, crop rotation, and intercropping) that are beneficial to soil quality [4,5]. When combined with reduced tillage, cover crops provide many benefits such as reduced soil erosion, utilizing legumes to provide nitrogen, the potential for reduced pesticide use, improved precipitation infiltration and subsequent soil moisture, enhanced soil organic matter, disruption of pest cycles, and weed suppression [6–11]. Integration of reduced tillage in vegetable production can be challenging due to lack of recommended practices, among others [12]. Specifically, loss of weed control provided by tillage and cultivation and integration challenges with polyethylene plastic production systems are the main concerns [5]. However, weed suppression in vegetable production can be attained utilizing residue from a variety of cover crops grown prior to the vegetable crop. Traditionally, in organic conventional tillage systems, winter cover crops are incorporated into the soil through primary and secondary tillage before crop seeding or transplanting [8,13–15]. Weed emergence is reduced while the cover crop is growing, as well as through the release of allelopathic compounds produced after soil incorporation [6,14,16–18]. In reduced tillage organic systems, cover crop residue remains on the soil surface after termination to suppress weed growth through both chemical and physical means [9,19–22]. Common cover crops include fall-planted species such as cereal rye, crimson clover (Trifolium spp.), pea (Pisum spp.), vetch (Vicia spp.), and radish (Raphanus spp.) [6,23]. Cover crop termination in organic conservation agriculture is primarily dependent on mechanical practices including rolling/crimping or flail mowing. Flail mowing increases mulch decomposition rate and creates planting issues when using a mechanical transplanter [20]. The Brazilian conservation tillage system, based on terminating cover crops by mechanically rolling/crimping winter covers (forming a dense residue mat on the soil surface into which crop seeds are planted), has been evaluated in many traditional row crop systems [24–27]. This system has also recently been adapted to specialty crop systems into which seedlings are transplanted [12,28]. The relatively flat cover crop biomass mat that is generated would allow for complementary termination methods to be subsequently applied to enhance termination rates (how fast the cover crop biomass desiccates and becomes brittle) [19,26,28,29]. Timely, e ective cover crop termination has been cited as a major adoption impedance to producers wanting to integrate conservation tillage practices in organic vegetable or row crops (Steve Li, personal communication). Integrating Organic Materials Review Institute (OMRI)-approved commercially available organic herbicides, solarization, or flaming following rolling/crimping could help enhance cover crop termination rates and prevent regrowth, similar to the e ects of utilizing glyphosate in non-organic systems following rolling/crimping [19]. Vinegar (acetic acid, 20%) and clove (Syzygium aromaticum L.) oil (eugenol, acetyl eugenol, caryophyllene, 60%–90%) are marketed organic herbicides and have been evaluated for weed control in sweet corn (Zea mays L.), potato (Solanum tuberosum L.), and onion (Allium cepa L.) [30]. Results reveal that unrealistically high application volumes of 636 L ha and 318 L ha were needed for vinegar and clove oil, respectively, for marginal initial herbicidal activity on relatively small weeds. Vinegar weed control decreased as weed size increased, application volume decreased, and control decreased over time due to regrowth. Vinegar has been shown to have higher herbicidal activity compared to clove oil, which was shown to be ine ective for weed control. However, these products following cover crop injury due to rolling/crimping may increase the cover crop termination rate, thus conserving soil moisture and facilitating earlier subsequent crop planting opportunities. Flame weeding has been evaluated for weed control before crop emergence or inter-row for onion, sweet corn, and cabbage (Brassica oleracea L.) [31–34]. Mechanical cover crop termination may be accelerated utilizing heat-induced cellular membrane disruption, such as flaming or solarization with clear polyethylene to trap solar radiation. Solarization has been utilized throughout warmer climates for weed and other pest control [35]. Synthetic mulches Agronomy 2019, 9, 519 3 of 14 are allowable in organic production systems as long as they are removed from the field at the end of the growing season [36]. Timely spring cover crop termination in organic production systems would allow for increased adoption and retention of conservation tillage practices. Cover crop termination in non-organic conservation agriculture has been primarily and broadly accomplished using glyphosate over decades. Producers transitioning from non-organic to organic systems will likely be familiar with glyphosate’s cover crop termination ecacy as well as economics. Thus, the objectives of this experiment were to determine the response of various cover crops species at di erent spring termination timings, to organic cover crop termination herbicides or practices, and determine their associated costs as compared to glyphosate, the non-OMRI-approved cover crop termination standard practice utilized in non-organic conservation systems [37]. 2. Materials and Methods A two-year cover crop termination experiment was established in the fall of 2013 in Alabama, following summer fallow in both years. The experiment was conducted at the Alabama Agricultural Experiment Station E.V. Smith Research Center Field Crops Unit, near Shorter, AL (32.42 N, 85.88 W) on Compass loamy sand. The experimental design was a randomized complete block design (r = 4) with a split block restriction on randomization. This design was chosen for practical reasons because it enabled ecient seeding of cover crops and application of termination treatments at three di erent application timings. We randomly assigned cover crop termination timings (3 timings) within main blocks. Within each termination timing block, we assigned cover crop species (5 species) to horizontal strips, and termination methods (6 methods) were randomly assigned vertically across strips. Therefore, we had three di erent sizes of experimental units [38]. The largest experimental unit (termination timing) equals one-third of the block size, the second largest (cover crop species) equals one-fifth of the block size, and the smallest (cover crop  termination treatment  termination timing combinations) equals 1/90 of the block size. This design also led to three di erent sources of experimental errors catering to each experimental unit. The smallest experimental unit (henceforth called plot) was 4 m wide and 8 m long. Cover crops were planted October 15 and October 5 in 2013 and 2014, respectively. Three termination timings occurred at four-week intervals beginning mid-March each year. Five cover crop species were evaluated: (1) Austrian winter peas drill seeded at 101 kg ha , (2) crimson 1 1 clover (cv. Dixie) drill seeded at 28 kg ha , (3) hairy vetch drill seeded at 22 kg ha , (4) rape (cv. Athena) 1 1 drill seeded at 11 kg ha , and (5) cereal rye (cv. Wrens Abruzzi) drill seeded at 101 kg ha in October each year. Immediately following a 3.66 m straight bar roller/crimper, six termination treatments 1 1 were applied: (1) 20% vinegar solution (28 L a.i. ha acetic acid), (2) 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove oil (eugenol, acetyl eugenol, caryophyllene) mixture, (3) 0.15 mm clear polyethylene sheeting applied with edges manually tucked into the soil for 28 d over the entire plot area (clear plastic), (4) broadcast flame emitting 1100 C applied at 1.2 km/h (flame), (5) glyphosate applied at 1.12 kg a.i. ha (this non-organic, conservation tillage cover crop termination standard practice treatment was included to help ascertain desiccation and regrowth and to make a monetary comparison), and (6) a non-treated control. A non-ionic surfactant at 0.25% (v/v) was included in all vinegar and cinnamon/clove oil treatments. Glyphosate, vinegar, and cinnamon/clove oil were applied with a compressed CO backpack sprayer delivering 1 ® 140 L ha at 147 kPa. Flame was applied with a tractor mounted Red Dragon Flamer (PL-8750 Poultry House Flame Sanitizer (Figure 1), Flame Engineering, Inc., P.O. Box 577, West Highway 4, LaCrosse, KS, 1 1 USA, 67548) at 1.2 km ha and 345 kPa operating pressure utilizing 382 L/ha (192 kg ha ) propane. The flame equipment utilized in this experiment used substantially more propane compared to others reported in the literature [31–34]. Agronomy 2019, 9, 519 4 of 14 Agronomy 2019, 9, x FOR PEER REVIEW 4 of 15 Figure 1. Rolled/crimped hairy vetch winter cover crop being desiccated by a Red Dragon Flamer Figure 1. Rolled/crimped hairy vetch winter cover crop being desiccated by a Red Dragon Flamer PL-8750 Poultry House Flame Sanitizer, Flame Engineering, Inc., moving at 1.2 k/h and emitting 1100 PL-8750 Poultry House Flame Sanitizer, Flame Engineering, Inc., moving at 1.2 k/h and emitting °C. 1100 C. Since weather and solar radiation attributes a ects cover crop biomass and solarization potential, Since weather and solar radiation attributes affects cover crop biomass and solarization average daily temperature, precipitation, and solar radiation are reported for each year in Figure 2. potential, average daily temperature, precipitation, and solar radiation are reported for each year in At each termination timing prior to rolling/crimping, cover crop biomass samples were collected by Figure 2. At each termination timing prior to rolling/crimping, cover crop biomass samples were clipping all aboveground plant parts close to the soil surface from one randomly selected 0.25 m collected by clipping all aboveground plant parts close to the soil surface from one randomly selected section 2 in each plot. Plant material was dried at 60 C for 72 h and weighed. Cover termination ratings 0.25 m section in each plot. Plant material was dried at 60 °C for 72 h and weighed. Cover termination were visually estimated over the whole plot, assessing greenness, and recorded on a scale from 0% (no ratings were visually estimated over the whole plot, assessing greenness, and recorded on a scale a ect compared to non-treated control) to 100% (complete termination) [39]. Three termination ratings from 0% (no affect compared to non-treated control) to 100% (complete termination) [39]. Three were recorded 7, 14, and 21 days after termination (DAT) treatments per plot in both years. termination ratings were recorded 7, 14, and 21 days after termination (DAT) treatments per plot in Total variable costs (TVC) were estimated using a partial budgeting approach for each cover crop both years. and termination combination. Only variable costs that di ered between treatments were included, Total variable costs (TVC) were estimated using a partial budgeting approach for each cover and were assumed to be constant across the two years. These costs included cover crop seed and crop and termination combination. Only variable costs that differed between treatments were termination costs (labor, machinery repair and maintenance, fuel, and product). Cover crop seed costs included, and were assumed to be constant across the two years. These costs included cover crop were based on actual seed costs paid during the experiment: US $1.74 kg for Austrian winter pea, seed and termination costs (labor, machinery repair and maintenance, fuel, and product). Cover crop 1 1 1 −1 US $1.98 kg for crimson clover, US $4.30 kg for hairy vetch, US $3.53 kg for rapeseed, and US seed costs were based on actual seed costs paid during the experiment: US $1.74 kg for Austrian −1 −1 −1 $0.69 kg for cereal rye. Table 1 displays the termination costs by termination method [40,41]. Cover winter pea, US $1.98 kg for crimson clover, US $4.30 kg for hairy vetch, US $3.53 kg for rapeseed, 1 1 −1 crop planting (~US $19 ha ) and rolling/crimping costs (~US $13.10 ha ) were not included since and US $0.69 kg for cereal rye. Table 1 displays the termination costs by termination method [40,41]. −1 −1 these operations were applied to all plots. Machinery costs were based on a two-wheel drive, 37 kW Cover crop planting (~US $19 ha ) and rolling/crimping costs (~US $13.10 ha ) were not included 1 1 closed cab tractor with a fuel price of US $0.57 L . Labor costs included both operator (US $12.55 h ) since these operations were applied to all plots. Machinery costs were based on a two-wheel drive, and hand labor (US $9.87 h ) as applicable. 1 37 kW closed cab tractor with a fuel price of US $0.57 L . Labor costs included both operator Mixed models procedures as implemented in SAS PROC GLIMMIX (SAS Institute Inc., Cary, 1 1 − − (US $12.55 h ) and hand labor (US $9.87 h ) as applicable. NC, USA) were used to analyze winter cover biomass and visual termination as a percentage. Winter Mixed models procedures as implemented in SAS PROC GLIMMIX (SAS Institute Inc., Cary, cover crop, termination timing, and termination treatment were considered fixed e ects. Year and the NC, USA) were used to analyze winter cover biomass and visual termination as a percentage. Winter interaction of replication with termination timing and termination treatment were considered random cover crop, termination timing, and termination treatment were considered fixed effects. Year and e ects. Error terms appropriate to the split design with repeated measures were used to test main the interaction of replication with termination timing and termination treatment were considered random effects. Error terms appropriate to the split design with repeated measures were used to test main effects and interactions at P ≤ α = 0.10. We chose this α prior to analysis knowing inherent Agronomy 2019, 9, 519 5 of 14 Agronomy 2019, 9, x FOR PEER REVIEW 5 of 15 e ects and interactions at P  = 0.10. We chose this prior to analysis knowing inherent variability variability in visually estimated data is higher than other quantitative data. Non-transformed data in visually for visu estimated al terminadata tion is evhigher aluation than s weother re presen quantitative ted becau data. se tran Non-transform sformation did edno data t affe for ct visual data termination interpreta evaluations tion. Treatmen wer t di e ff pr er esented ences wer because e calcula transformation ted by Tukey–Kr did amer not groupin a ectgdata least interpr square etation. means test. Treatment di erences were calculated by Tukey–Kramer grouping least square means test. Figure 2. The 2014 and 2015 average daily temperature ( C), solar radiation (Wh/m ), and Figure 2. The 2014 and 2015 average daily temperature (°C), solar radiation (Wh/m ), and precipitation (mm). precipitation (mm). Table 1. Total variable costs by termination method ($US ha ). −1 Table 1. Total variable costs by termination method ($US ha ). Product Product Application Total Variable Costs Product Product Application Total Variable Costs Termination Method Termination Method $US ha −1 $US ha Non-treated 0 0 0 Non-treated 0 0 0 Glyphosate 5.76 8.93 14.69 Glyphosate 5.76 8.93 14.69 Vinegar 1074.54 22.06 1096.60 Vinegar 1074.54 22.06 1096.60 45% clove/45% cinnamon oil 148.24 22.06 170.31 45% clove/45% cinnamon oil 148.24 22.06 170.31 Clear plastic 790.88 306.73 1097.61 Clear plastic 790.88 306.73 1097.61 Flame 266.87 76.18 343.05 Flame 266.87 76.18 343.05 Agronomy 2019, 9, 519 6 of 14 3. Results and Discussion 3.1. Cover Crop Biomass ANOVA revealed significant e ects for cover crop, termination treatment, and termination timing. ANOVA also showed that the year by species interactions were significant (p < 0.10). Therefore, each species is discussed separately by year. In 2014, a maximum May biomass of 8838 kg ha was attained 1 1 in cereal rye plots, followed by Austrian winter peas (7177 kg ha ), hairy vetch (6213 kg ha ), and 1 1 crimson clover (5124 kg ha ) (Table 2). Rape provided the least biomass (1657 kg ha ). Terminating covers in April resulted in approximately 50% less biomass in all comparisons, while terminating covers in March resulted in less than 1000 kg ha regardless of cover crop. These results are similar to previously reported biomass and are indicative of proper management and environmental conditions (Figure 2) [19,29]. Conservation agriculture specialists consider cover crop biomass amounts exceeding 4000 kg ha as ‘high biomass’ systems [25]. However, for more e ective weed suppression, weed scientists recommend >6000 kg ha [20–22,29]. In 2015, a relatively warm winter and higher early spring temperatures (Figure 2) resulted in quicker spring cover crop growth compared to 2014 and covers attained maximum biomass at the April (2nd) termination date (Table 2). Similar to 2014, cereal rye attained the highest biomass at 5828 kg ha followed by crimson clover and hairy vetch (3995 and 3787 kg ha , respectively). Austrian winter pea 1 1 provided 1623 kg ha while rape again resulted in the least biomass (558 kg ha ) when terminated in April. Again, terminating covers in March again reduced cereal rye and rape biomass by half and decreased the legumes’ biomass by a third. Early termination due to planting schedules for early cash crops is a challenge to producing high biomass, especially as you move to more northern latitudes [20]. Table 2. Cover Crop Biomass at Three Termination Timings, E.V. Smith 2014 and 2015. Cover Crop Dry Biomass (kg/ha) Cover Crop 1st Termination Timing 2nd Termination Timing 3rd Termination Timing hg** fe b Austrian Winter Peas 959 2587 7177 h fe cd Crimson Clover 336 2369 5124 hg cb Hairy Vetch 3094 928 6213 i h fg Rape 116 370 1657 hg d a Cereal Rye 719 4769 8838 LSD ( = 0.10) 634.08 — — Cover Crop dc d Austrian Winter Peas 1762 1623 dc b Crimson Clover 2408 3995 dc b Hairy Vetch 1991 3787 e e Rape 84 558 c a Cereal Rye 2453 5828 LSD ( = 0.10) 448.7 — Cover crop biomass was obtained on March 14, April 16, and May 14, 2014; March 11, and April 12, 2015. Cover crop biomass was not sampled in May 2015 as covers had reached maximum maturity in April. ** Mean separations with the same letter are not significantly di erent. 3.2. Cover Crop Termination As previously stated, cover termination ratings were estimated by visually accessing greenness over the whole plot and recorded on a scale from 0% (no a ect compared to non-treated control) to 100% (complete termination). ANOVA revealed significant e ects for cover crop, termination treatment, and termination timing. ANOVA again showed year by cover crop species interactions were significant (p < 0.10). Therefore, each species is discussed separately by year. To facilitate discussion, the 7 (DAT) % termination estimation will be referred to as ‘quick’ or ‘quickly’, whereas the 21 DAT % termination estimation will be referred to as ‘waiting longer ’. As previously stated, rolling/crimping Agronomy 2019, 9, 519 7 of 14 was accomplished on all plots, therefore rolling/crimping alone will be discussed first, followed by the remaining treatments applied immediately following this practice. Hairy Vetch. In March 2014, rolling/crimping alone provided no quick termination or after waiting longer (Table 3). This result shows additional/alternative practices are required in March to attain termination. The highest (90%) quick termination was attained following flaming; however, regrowth occurred, and termination decreased to 23% after waiting longer. Similarly, significant regrowth was observed in e ective quick (90%) glyphosate non-organic comparison treatment after waiting longer, further highlighting hairy vetch resiliency. In March, 95% termination was attained after covering with clear plastic sheeting after waiting longer, and regrowth did not occur following plastic removal (data not shown). In March, other termination practices were not e ective quickly or after waiting longer. In April, rolling/crimping alone again provided no termination quickly or after waiting longer (Table 3). Flaming in April resulted in 93% termination quickly, and again following regrowth, only 68% termination was observed after waiting longer. In May, rolling/crimping alone provided 90% termination after waiting longer (Table 3). Cinnamon/clove oil provided a quick 28% increase in termination in April, compared to the ine ective rolled/crimped alone treatment; while vinegar failed to injure hairy vetch. However, regrowth again occurred and negated the control attained with oil. In 2015, likely due to advanced maturity in March, rolling/crimping alone provided 32% termination quickly but did not increase to adequate termination after waiting longer. The addition of flaming provided 93% termination quickly and near complete termination after waiting longer (Table 4). Clear plastic provided 85% termination after waiting longer, with no regrowth after removal (data not shown). In April, hairy vetch termination was 87% following rolling/crimping alone quickly and increased to 99% after waiting longer (Table 4). Similar to 2014, cinnamon/clove oil and vinegar showed little to no substantial activity on hairy vetch in 2015 (Table 4). Thus, when selecting a cover crop proceeding a March- or April-planted cash crop, hairy vetch might not be the best choice if quick termination is needed due to both resiliency and regrowth concerns. If slower termination can be utilized due to subsequent cash crop planting timing, clear plastic will likely be e ective in terminating hairy vetch. Overall, hairy vetch termination rates increased, and regrowth was reduced by delaying termination. These results are similar to those reported by Mirsky et al. [42]. Crimson Clover. In 2014, crimson clover response to treatments was similar to hairy vetch in most comparisons (Table 3). In March and April, rolling/crimping alone provided no termination quickly or after waiting longer (Table 3). Flaming clover was less e ective in March compared to hairy vetch likely due to the lack of biomass to injure. Clear plastic was also moderately e ective in March after waiting longer, and was highly e ective in April and May quickly. In April, clover was terminated 42% with cinnamon/clove oil quickly, and this was the second highest susceptibility observed compared to the non-treated, albeit still inadequate, for any species evaluated in this experiment terminated with cinnamon/clove oil. Rolling/crimping alone was 99% e ective after waiting 14 d when terminated in May. Vinegar resulted in no termination quickly, or after waiting longer, for any application timing when compared to the rolled/crimped non-treated. In 2015, March clover termination was similar to April 2014 (Table 4). In April, rolling/crimping alone resulted in 87% control quickly and 99% termination after waiting longer (Table 4). Clear plastic was again highly e ective in April and May quickly. Similar to 2014, cinnamon/clove oil and vinegar were non-e ective at enhancing clover termination quickly or after waiting longer, at any application timing. Thus, similar to hairy vetch, selecting a cover crop proceeding a March-planted cash crop, crimson clover might not be the best choice if quick termination is needed, due to resiliency. If slower termination can be utilized due to subsequent cash crop planting timing, or use in April or later, clear plastic will likely be e ective in terminating crimson clover. These results agree with those of Evans and Bellinder [30] that reported termination rates increased with clover maturation. Agronomy 2019, 9, 519 8 of 14 Cereal Rye. In 2014, rolling/crimping alone provided no termination and no treatment e ectively terminated rye in March (Table 3). In April, rolling/crimping alone resulted in 60% termination quickly. Flaming resulted in 97% termination; however, termination decreased to 70% following regrowth after waiting longer (Table 3). Clear plastic covering rye resulted in 97% termination after waiting 14 d. Cinnamon/clove oil increased termination 13 percentage points over rolling/crimping alone after waiting longer. In May, rolling/crimping alone provided 94% termination quickly and increased to 99% after waiting an additional week (Table 3). In 2015, rolling crimping in April alone provided 75% termination quickly and increased to 98% by waiting longer (Table 3). Flaming increased the rate of termination 23% and 11% quickly and after 14 d, respectively. In May rolling/crimping alone provided 99% control quickly, with no additional treatment increasing rye termination (Table 4). Notably, the cinnamon and clover oil mixture increased termination rates for cereal rye in one year after waiting longer. Generally, these results are similar to those reported by researchers that show termination rates following rolling/crimping increase as cereal cover crop growth stage progresses [24,26]. For March-planted cash crops, producers likely would benefit by not choosing cereal rye, as it is relatively hard to terminate at early growth stages. Sequential rolling/crimping or flaming operations would likely be needed. Austrian Winter Pea. In 2014, March rolling/crimping alone resulted in no termination quickly or after waiting longer (Table 3). Flaming provided 82% pea termination quickly, however aggressive regrowth resulted in only 10% termination after waiting longer. Clear plastic covering peas after waiting longer resulted in 58% termination, with no regrowth after removal (data not shown). Cinnamon/clove oil and vinegar provided no pea termination quickly or after waiting longer at any application timing. In April, rolling/crimping provided 25% termination alone with flaming and clear plastic treatment resulting in 98% termination after waiting 14 d (Table 3). In May, rolling/crimping alone provided 20% termination quickly, however, termination increased to 96% after waiting longer (Table 2). In 2015, rolling/crimping alone provided 48% termination quickly and increasing to 79% after waiting longer, again, likely due to later maturity (Table 4). Cinnamon/clove oil increased pea termination by 23 and 11 percentage points over rolling/crimping alone after 14 d and longer, respectively. Similar to the previous cover crops, March-planted conservation tillage cash crop establishment may be challenging due to insucient winter pea termination. Rape. In March and April 2014, rolling/crimping provided no termination quickly or after waiting longer (Table 3). Flaming provided 95% termination quickly, however, rape recovered completely after waiting longer (Table 3). Clear plastic provided 58% termination after waiting longer, with no regrowth after removal (data not shown). Cinnamon/clove oil and vinegar provided no termination at any application timing quickly or after waiting longer. In April, flaming provided only 33% termination quickly and did not increase after waiting longer (Table 3). Clear plastic provided 48% termination quickly, increasing to 78% after waiting longer. In 2015, rolling/crimping in March provided no rape termination quickly or after waiting longer (Table 3). Clear plastic was very e ective, providing 95% termination quickly increasing to 99% after waiting longer, likely due to increasing solar radiation and relatively high air temperatures. Flaming provided 99% termination quickly, however, termination decreased to 70% after waiting longer due to regrowth. Similar to 2014, cinnamon/clove oil and vinegar provided no rape termination at any application timing quickly or after waiting longer. Agronomy 2019, 9, 519 9 of 14 Table 3. Cover crop response to termination method by termination date in 2014. 1st Termination Date (March) 2nd Termination Date (April) 3rd Termination Date (May) Cover Crop Termination Method 7 DAT 14 DAT 21 DAT 7 DAT 14 DAT 21 DAT 7 DAT 14 DAT 21 DAT % Termination * 1 cb c c c c b b b a Non-treated 3 0 0 0 0 0 37 78 90 2 b a ba bc b a b a a Glyphosate 13 90 60 43 72 83 58 98 99 c c c c a 3 cb b b ba Vinegar 2 2 0 0 0 0 53 80 81 4 cb c c bc c b ba ba a Hairy Vetch 0 0 8 93 Clove/cinnamon oil 8 28 5 62 85 5 c c a ba a a a a a Clear plastic 1 3 95 66 97 66 99 99 99 6 a b bc a b a a a a Flame 90 63 23 93 73 68 98 99 99 LSD ( = 0.10) 6.7 9.4 29.5 29.5 6.1 34.9 22.8 11.8 11.4 b c c c c c b a a Non-treated 2 0 0 0 0 0 67 99 99 a a a a a a a b ba Glyphosate 57 98 98 38 93 97 78 99 99 b c bc c c c b a a Vinegar 3 2 2 2 0 0 57 99 99 b c bc b b b b a a Crimson Clover (Dixie) Clove/cinnamon oil 5 5 10 42 33 30 57 66 66 c a a a a a a a Clear plastic 8 3 72 82 93 97 99 99 99 a b ba a a a a a a Flame 67 96 96 96 99 99 99 68 52 LSD ( = 0.10) 23.0 14.3 31.0 18.6 11.0 10.6 17.2 34.0 34.0 c c b ba b ba b a a Non-treated 0 0 0 60 63 67 94 99 99 a a a a a a a a a Glyphosate 48 98 69 97 99 99 99 99 99 c c b b b b b a a Vinegar 0 0 0 45 53 53 94 99 99 Cereal Rye (Wrens c c b ba a ba ba a a Clove/cinnamon oil 8 0 0 75 85 80 96 99 99 Abruzzi) c c ba ba a a ba a a Clear plastic 2 2 48 82 97 96 98 99 99 a a a a a b b b ba Flame 28 15 12 97 96 70 99 99 99 LSD ( = 0.10) 8.0 3.5 31.5 24.3 12.6 22.8 2.7 **** **** c b b b b b c a ba Non-treated 0 0 0 27 28 25 20 87 96 ba a a ba a a b a a Glyphosate 67 98 99 60 95 98 72 99 99 c a bac b b b b b b Vinegar 31 17 10 28 28 28 32 88 93 bc b b b b b b a a Austrian Winter Peas Clove/cinnamon oil 97 99 27 20 25 27 28 25 57 c b a ba a a a a a Clear plastic 9 11 93 62 98 99 99 99 99 a a b a a a a a a Flame 82 68 10 98 98 98 98 99 99 LSD ( = 0.10) 31.2 23.5 18.9 35.8 15.4 15.0 13.3 7.8 3.4 c b b c c c c b c Non-treated 0 0 0 0 0 0 2 0 0 a a a a a a b b ba Glyphosate 48 96 69 80 97 98 47 90 94 c b b c c c c b c Vinegar 2 0 0 0 0 0 2 0 0 c b b bc c c c b c Rape (Athena) Clove/cinnamon oil 2 0 0 7 0 0 2 0 0 c a a a a b ba ba b Clear plastic 0 1 58 48 63 78 95 99 99 a a b bc b b a a b Flame 95 95 99 70 3 33 37 33 70 LSD ( = 0.10) 27.3 1.6 36.5 25.7 16.5 17.8 24.0 22.5 16.9 1 2 No herbicide or other termination method was used to kill the cover crop other than mechanically rolling/crimping. Mechanically rolling/crimping followed by glyphosate (not Organic Materials Review Institute (OMRI)-approved) applied to covers at 1.12 a.i. kg/ha. Mechanically rolling/crimping followed by 20% vinegar solution applied to covers with an application 4 1 volume of 140.25 L/ha. Mechanically rolling/crimping followed by 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove 5 6 oil (eugenol, acetyl eugenol, caryophyllene) mixture. Mechanically rolling/crimping followed by clear plastic laid over the entire plot 3.7m  3m area for 28 days. Mechanically rolling/crimping followed by broadcast flaming emitting 1100 C applied at 1.2 k/h. **** Mean separations with the same letter are not significantly di erent within a cover crop and termination timing. Treatment di erences were calculated by Tukey–Kramer grouping least square means. Agronomy 2019, 9, 519 10 of 14 Table 4. Cover crop response to termination method by termination date in 2015. 1st Termination Date (March) 2nd Termination Date (April) Cover Crop Termination Method 7 DAT 14 DAT 21 DAT 7 DAT 14 DAT 21 DAT % Termination * c a a a 1 b b Non-treated 32 37 38 87 99 99 2 b a a a a a Glyphosate 63 93 98 98 99 99 3 c b b a a a 32 88 99 99 Vinegar 40 35 4 c b b a a a Hairy Vetch Clove/cinnamon oil 17 15 15 90 99 99 5 b a a a a a Clear plastic 65 87 85 90 99 99 a a a a a a Flame 93 92 98 99 99 99 LSD ( = 0.10) 15.3 18.0 25.4 7.7 **** **** c c a a a Non-treated 5 3 3 99 99 99 a a a a a ba Glyphosate 77 82 96 99 99 99 b c c a a a Vinegar 0 0 99 99 99 b c c a a a Crimson Clover (Dixie) Clove/cinnamon oil 8 8 99 99 99 b b b a a a Clear plastic 30 58 5 8 98 99 99 a a ba a a a Flame 90 94 88 99 99 99 LSD ( = 0.10) 19.7 20.6 21.8 **** **** **** bc b a a a a Non-treated 75 87 98 99 99 99 a a a a a a Glyphosate 99 99 99 99 99 99 c a a a a ba Vinegar 73 90 98 99 99 99 bac ba a a a a Cereal Rye (Wrens Abruzzi) Clove/cinnamon oil 83 92 96 99 99 99 c ba a a a a Clear plastic 67 93 99 99 99 ba a a a a a Flame 98 98 99 99 99 99 LSD ( = 0.10) 13.7 6.2 5.6 **** **** **** a a a a a a Non-treated 48 50 79 98 99 99 a a a a a a Glyphosate 62 67 69 98 99 99 a a a a a a Vinegar 40 50 72 99 99 99 a a a a a a Austrian Winter Peas Clove/cinnamon oil 50 80 90 99 99 99 a a a a a a Clear plastic 57 73 79 99 99 99 a a a a a a Flame 96 97 98 99 99 99 LSD ( = 0.10) **** **** **** **** **** **** a a a a a a Non-treated 99 99 99 99 99 99 a a a a a a Glyphosate 99 99 99 99 99 99 a a a a a a Vinegar 99 99 99 99 99 99 a a a a a Rape (Athena) Clove/cinnamon oil 99 99 99 97 99 99 a a a c a a Clear plastic 99 99 99 95 99 99 a a a a a a Flame 99 99 99 99 99 99 LSD ( = 0.10) **** **** **** **** **** **** No herbicide or other termination method was used to kill the cover crop other than mechanically rolling/crimping. Mechanically rolling/crimping followed by glyphosate (not OMRI-approved) applied to covers at 1.12 a.i. kg/ha. 3 1 4 Mechanically rolling/crimping followed by 20% vinegar solution (28 L a.i. ha acetic acid). Mechanically rolling/crimping followed by 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove oil (eugenol, acetyl eugenol, caryophyllene) mixture. Mechanically rolling/crimping followed by clear plastic laid over the entire plot 3.7m  3m area for 28 days. Mechanically rolling/crimping followed by broadcast flaming emitting 1100 C applied at 1.2 k/h. * Mean separations with the same letter are not significantly di erent within a cover crop and termination timing. Treatment di erences were calculated by Tukey–Kramer grouping least square means. **** Indicates no significance. 3.3. Cover Crop Termination Economics Cover crop seed costs drive cover crop establishment costs. Austrian winter peas had the highest per unit seed costs, and at a seeding rate of 101 kg ha , it was the highest cost cover crop. Hairy vetch and cereal rye were also more expensive due to seed costs and seeding rate, respectively. However, since one of the main objectives of cover crops is to produce adequate biomass, the cover crop seed cost per 100 kg of dry biomass was calculated to compare the production potential in terms of dollars spent (Table 5). At the early termination date (March), rape had the highest cost per 100 kg of biomass, with biomass averaged across both years; however, at the later termination dates (April and May), there was little numerical di erence between Austrian winter peas and rape. Biomass from Austrian winter peas and rape was the most expensive to produce across all three termination dates. Crimson clover and cereal rye consistently had lower seed costs per 100 kg produced biomass. Glyphosate was the least expensive treatment option compared to non-treated; however, it obviously does not meet organic production standards (Table 6). Of the organic methods, Agronomy 2019, 9, 519 11 of 14 cinnamon/clove oil was the least expensive and one of the least e ective termination methods. Vinegar, along with clear plastic, had the highest costs as driven by product costs and application rates; however, in general, vinegar was not an e ective termination method, while expensive, clear plastic was an e ective termination method after waiting longer. Clear plastic also requires additional labor and machinery time as compared to the other treatments. Furthermore, there may be disposal costs associated with clear plastic, which were not accounted for in this analysis. The cost of flame termination was almost four times the cost of glyphosate; however, it proved to be an e ective termination option for most covers evaluated in April or May. However, there may be additional costs if regrowth occurs after termination. Furthermore, the cost to purchase the equipment and ownership costs were not included since they di er greatly between farms. From an economic perspective, organic farmers should look at their operations, goal and objectives, and potential constraints, such as labor, before deciding on the most appropriate termination methods. Table 5. Cover crop seed costs per 100 kg of biomass averaged over 2014–2015. Cover Crop Seed Costs per 100 kg of Biomass ($US 100 kg ) Cover Crop 1st Termination Timing 2nd Termination Timing 3rd Termination Timing Austrian Winter Peas 5.23 3.38 0.99 Crimson Clover 1.64 0.71 0.44 Hairy Vetch 2.67 1.13 0.63 Rape 16.00 3.45 0.97 Cereal Rye 1.79 0.54 0.32 Table 6. Total variable costs (including seed and termination method costs) by cover crop and termination method ($US ha ). Termination Method Cover Crop Glyphosate Vinegar 45% Clove Oil/45% Cinnamon Oil Clear Plastic Flame $US ha Austrian Winter Peas 212 1272 346 1273 519 Crimson Clover 92 1152 226 1153 399 Hairy Vetch 133 1193 267 1194 439 Rape (Athena) 76 1136 210 1137 383 Cereal Rye 106 1167 240 1168 413 4. Conclusions Five winter cover crops were evaluated, however, the Brassica (rape) never attained biomass at any termination timing that would benefit either soil quality or weed suppression. Because of the very low biomass produced, it was the most expensive to produce on a weight basis. In addition, Austrian winter pea was also expensive to produce on a weight basis. Crimson clover and cereal rye were both relatively less expensive on a weight bases and produced adequate biomass to both a ect soil quality and provide weed suppressive qualities. Organic producers seeking to terminate winter covers would most likely be successful using a broadcast flamer on most any winter cover in April or May, or utilizing clear plastic in hairy vetch, winter peas, or cereal rye, as ambient temperature increases along with solar radiation, both following a roller/crimper. Obvious limitations to using plastic include cooler climates, and/or higher latitudes. Termination rates increase as cover crops mature, thus, producers with early crop planting timing that necessitate early cover crop termination timings will likely need to use multiple recurrent practices to terminate most cover crops. Commercially available vinegar and cinnamon/clove oil solutions provided little predictable termination. Producers attempting to use these OMRI-approved products will likely resort to utilize cover crop incorporation, or mowing, to terminate covers if no other e ective practice is readily available, and these practices fail to adequately terminate the cover. Agronomy 2019, 9, 519 12 of 14 Author Contributions: A.J.P., J.S.M., and S.L. collaborated on experimental conceptualization, A.J.P. managed experimental investigation, A.J.P. and J.S.M. contributed to resources, and L.D. provided economic analysis, and A.J.P. and L.D. contributed to statistical analysis and writing-original draft preparation. Funding: This research received no external funding. Acknowledgments: Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by USDA and does not imply endorsement of a product to the exclusion of others that may be suitable. Conflicts of Interest: The authors declare no conflicts of interest. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Agronomy Multidisciplinary Digital Publishing Institute

Evaluation of Organic Spring Cover Crop Termination Practices to Enhance Rolling/Crimping

Price, Andrew J.; Duzy, Leah; McElroy, J. Scott; Li, Steve
Agronomy , Volume 9 (9) – Sep 6, 2019
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agronomy Article Evaluation of Organic Spring Cover Crop Termination Practices to Enhance Rolling/Crimping 1 , 1 2 2 Andrew J. Price *, Leah Duzy , J. Scott McElroy and Steve Li National Soil Dynamics Laboratory, Agricultural Research Service, United States Department of Agriculture, 411 South Donahue Drive, Auburn, AL 36832, USA Department of Crop, Soil, and Environmental Sciences, Auburn University, Auburn, AL 36831, USA * Correspondence: price@ars.usda.gov; Tel.: +1-334-887-8596 Received: 3 August 2019; Accepted: 3 September 2019; Published: 6 September 2019 Abstract: With organic farming hectarage and cover crop interest increasing throughout the United States, e ectively timed cover crop termination practices are needed that can be utilized in organic conservation tillage production systems. Four commercially available termination treatments approved by Organic Materials Review Institute (OMRI) were evaluated, immediately following mechanical termination with a cover crop roller/crimper and compared to a synthetic herbicide termination to access termination rates. Treatments included rolling/crimping followed by (1) 1 1 20% vinegar solution (28 L a.i. ha acetic acid), (2) 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove oil (eugenol, acetyl eugenol, caryophyllene) mixture, (3) 0.15 mm clear polyethylene sheeting applied with edges manually tucked into the soil for 28 days over the entire plot area (clear plastic), (4) broadcast flame emitting 1100 C applied at 1.2 k/h (flame), (5) glyphosate applied at 1.12 kg a.i. ha (this non-OMRI-approved, non-organic conservation tillage cover crop termination standard practice was included to help ascertain desiccation, regrowth, and economics), and (6) a non-treated control. Five cover crop species were evaluated: (1) hairy vetch (Vicia villosa Roth), (2) crimson clover (Trifolium incarnatum L.), (3) cereal rye (Secale cereale L.), (4) Austrian winter pea (Pisum sativum L.), and (5) rape (Brassica napus L.). Three termination timings occurred at four-week intervals beginning mid-March each year. In April or May, organic producers are most likely to be successful using a roller crimper as either a broadcast flamer for terminating all winter covers evaluated, or utilizing clear plastic for hairy vetch, winter peas, and cereal rye. Ine ectiveness and regrowth concerns following cover crop termination in March are substantial. Commercially available vinegar and cinnamon/clove oil solutions provided little predictable termination, and producers attempting to use these OMRI-approved products will likely resort to cover crop incorporation, or mowing, to terminate covers if no other practice is readily available. Keywords: cinnamon oil; clove oil; cover crop biomass; flame termination; organic agriculture; organic herbicides; roller/crimper; solarization; vinegar 1. Introduction In 2018, US vegetable farmers produced over 7.53 million hundredweight (cwt; 100 pounds) on over one million hectares, with a value of over US $12.9 billion [1]. On average, per-capita consumption of vegetables was estimated to be 170 kg person [2]. As the world’s largest market for organic food, organic vegetable production in the United States comprises 8.4% of total vegetable land area, and sales from organic production are more than US $5.5 million [3]. However, as of 2011, only 0.6% of US cropland was certified for organic production due to obstacles such as high management costs, Agronomy 2019, 9, 519; doi:10.3390/agronomy9090519 www.mdpi.com/journal/agronomy Agronomy 2019, 9, 519 2 of 14 risks associated with operational changes, limited knowledge of organic production methods, and marketing and infrastructure limitations [3]. For organic producers and producers considering transitioning to organic production, there are numerous management practices (e.g., reduced tillage, cover cropping, crop rotation, and intercropping) that are beneficial to soil quality [4,5]. When combined with reduced tillage, cover crops provide many benefits such as reduced soil erosion, utilizing legumes to provide nitrogen, the potential for reduced pesticide use, improved precipitation infiltration and subsequent soil moisture, enhanced soil organic matter, disruption of pest cycles, and weed suppression [6–11]. Integration of reduced tillage in vegetable production can be challenging due to lack of recommended practices, among others [12]. Specifically, loss of weed control provided by tillage and cultivation and integration challenges with polyethylene plastic production systems are the main concerns [5]. However, weed suppression in vegetable production can be attained utilizing residue from a variety of cover crops grown prior to the vegetable crop. Traditionally, in organic conventional tillage systems, winter cover crops are incorporated into the soil through primary and secondary tillage before crop seeding or transplanting [8,13–15]. Weed emergence is reduced while the cover crop is growing, as well as through the release of allelopathic compounds produced after soil incorporation [6,14,16–18]. In reduced tillage organic systems, cover crop residue remains on the soil surface after termination to suppress weed growth through both chemical and physical means [9,19–22]. Common cover crops include fall-planted species such as cereal rye, crimson clover (Trifolium spp.), pea (Pisum spp.), vetch (Vicia spp.), and radish (Raphanus spp.) [6,23]. Cover crop termination in organic conservation agriculture is primarily dependent on mechanical practices including rolling/crimping or flail mowing. Flail mowing increases mulch decomposition rate and creates planting issues when using a mechanical transplanter [20]. The Brazilian conservation tillage system, based on terminating cover crops by mechanically rolling/crimping winter covers (forming a dense residue mat on the soil surface into which crop seeds are planted), has been evaluated in many traditional row crop systems [24–27]. This system has also recently been adapted to specialty crop systems into which seedlings are transplanted [12,28]. The relatively flat cover crop biomass mat that is generated would allow for complementary termination methods to be subsequently applied to enhance termination rates (how fast the cover crop biomass desiccates and becomes brittle) [19,26,28,29]. Timely, e ective cover crop termination has been cited as a major adoption impedance to producers wanting to integrate conservation tillage practices in organic vegetable or row crops (Steve Li, personal communication). Integrating Organic Materials Review Institute (OMRI)-approved commercially available organic herbicides, solarization, or flaming following rolling/crimping could help enhance cover crop termination rates and prevent regrowth, similar to the e ects of utilizing glyphosate in non-organic systems following rolling/crimping [19]. Vinegar (acetic acid, 20%) and clove (Syzygium aromaticum L.) oil (eugenol, acetyl eugenol, caryophyllene, 60%–90%) are marketed organic herbicides and have been evaluated for weed control in sweet corn (Zea mays L.), potato (Solanum tuberosum L.), and onion (Allium cepa L.) [30]. Results reveal that unrealistically high application volumes of 636 L ha and 318 L ha were needed for vinegar and clove oil, respectively, for marginal initial herbicidal activity on relatively small weeds. Vinegar weed control decreased as weed size increased, application volume decreased, and control decreased over time due to regrowth. Vinegar has been shown to have higher herbicidal activity compared to clove oil, which was shown to be ine ective for weed control. However, these products following cover crop injury due to rolling/crimping may increase the cover crop termination rate, thus conserving soil moisture and facilitating earlier subsequent crop planting opportunities. Flame weeding has been evaluated for weed control before crop emergence or inter-row for onion, sweet corn, and cabbage (Brassica oleracea L.) [31–34]. Mechanical cover crop termination may be accelerated utilizing heat-induced cellular membrane disruption, such as flaming or solarization with clear polyethylene to trap solar radiation. Solarization has been utilized throughout warmer climates for weed and other pest control [35]. Synthetic mulches Agronomy 2019, 9, 519 3 of 14 are allowable in organic production systems as long as they are removed from the field at the end of the growing season [36]. Timely spring cover crop termination in organic production systems would allow for increased adoption and retention of conservation tillage practices. Cover crop termination in non-organic conservation agriculture has been primarily and broadly accomplished using glyphosate over decades. Producers transitioning from non-organic to organic systems will likely be familiar with glyphosate’s cover crop termination ecacy as well as economics. Thus, the objectives of this experiment were to determine the response of various cover crops species at di erent spring termination timings, to organic cover crop termination herbicides or practices, and determine their associated costs as compared to glyphosate, the non-OMRI-approved cover crop termination standard practice utilized in non-organic conservation systems [37]. 2. Materials and Methods A two-year cover crop termination experiment was established in the fall of 2013 in Alabama, following summer fallow in both years. The experiment was conducted at the Alabama Agricultural Experiment Station E.V. Smith Research Center Field Crops Unit, near Shorter, AL (32.42 N, 85.88 W) on Compass loamy sand. The experimental design was a randomized complete block design (r = 4) with a split block restriction on randomization. This design was chosen for practical reasons because it enabled ecient seeding of cover crops and application of termination treatments at three di erent application timings. We randomly assigned cover crop termination timings (3 timings) within main blocks. Within each termination timing block, we assigned cover crop species (5 species) to horizontal strips, and termination methods (6 methods) were randomly assigned vertically across strips. Therefore, we had three di erent sizes of experimental units [38]. The largest experimental unit (termination timing) equals one-third of the block size, the second largest (cover crop species) equals one-fifth of the block size, and the smallest (cover crop  termination treatment  termination timing combinations) equals 1/90 of the block size. This design also led to three di erent sources of experimental errors catering to each experimental unit. The smallest experimental unit (henceforth called plot) was 4 m wide and 8 m long. Cover crops were planted October 15 and October 5 in 2013 and 2014, respectively. Three termination timings occurred at four-week intervals beginning mid-March each year. Five cover crop species were evaluated: (1) Austrian winter peas drill seeded at 101 kg ha , (2) crimson 1 1 clover (cv. Dixie) drill seeded at 28 kg ha , (3) hairy vetch drill seeded at 22 kg ha , (4) rape (cv. Athena) 1 1 drill seeded at 11 kg ha , and (5) cereal rye (cv. Wrens Abruzzi) drill seeded at 101 kg ha in October each year. Immediately following a 3.66 m straight bar roller/crimper, six termination treatments 1 1 were applied: (1) 20% vinegar solution (28 L a.i. ha acetic acid), (2) 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove oil (eugenol, acetyl eugenol, caryophyllene) mixture, (3) 0.15 mm clear polyethylene sheeting applied with edges manually tucked into the soil for 28 d over the entire plot area (clear plastic), (4) broadcast flame emitting 1100 C applied at 1.2 km/h (flame), (5) glyphosate applied at 1.12 kg a.i. ha (this non-organic, conservation tillage cover crop termination standard practice treatment was included to help ascertain desiccation and regrowth and to make a monetary comparison), and (6) a non-treated control. A non-ionic surfactant at 0.25% (v/v) was included in all vinegar and cinnamon/clove oil treatments. Glyphosate, vinegar, and cinnamon/clove oil were applied with a compressed CO backpack sprayer delivering 1 ® 140 L ha at 147 kPa. Flame was applied with a tractor mounted Red Dragon Flamer (PL-8750 Poultry House Flame Sanitizer (Figure 1), Flame Engineering, Inc., P.O. Box 577, West Highway 4, LaCrosse, KS, 1 1 USA, 67548) at 1.2 km ha and 345 kPa operating pressure utilizing 382 L/ha (192 kg ha ) propane. The flame equipment utilized in this experiment used substantially more propane compared to others reported in the literature [31–34]. Agronomy 2019, 9, 519 4 of 14 Agronomy 2019, 9, x FOR PEER REVIEW 4 of 15 Figure 1. Rolled/crimped hairy vetch winter cover crop being desiccated by a Red Dragon Flamer Figure 1. Rolled/crimped hairy vetch winter cover crop being desiccated by a Red Dragon Flamer PL-8750 Poultry House Flame Sanitizer, Flame Engineering, Inc., moving at 1.2 k/h and emitting 1100 PL-8750 Poultry House Flame Sanitizer, Flame Engineering, Inc., moving at 1.2 k/h and emitting °C. 1100 C. Since weather and solar radiation attributes a ects cover crop biomass and solarization potential, Since weather and solar radiation attributes affects cover crop biomass and solarization average daily temperature, precipitation, and solar radiation are reported for each year in Figure 2. potential, average daily temperature, precipitation, and solar radiation are reported for each year in At each termination timing prior to rolling/crimping, cover crop biomass samples were collected by Figure 2. At each termination timing prior to rolling/crimping, cover crop biomass samples were clipping all aboveground plant parts close to the soil surface from one randomly selected 0.25 m collected by clipping all aboveground plant parts close to the soil surface from one randomly selected section 2 in each plot. Plant material was dried at 60 C for 72 h and weighed. Cover termination ratings 0.25 m section in each plot. Plant material was dried at 60 °C for 72 h and weighed. Cover termination were visually estimated over the whole plot, assessing greenness, and recorded on a scale from 0% (no ratings were visually estimated over the whole plot, assessing greenness, and recorded on a scale a ect compared to non-treated control) to 100% (complete termination) [39]. Three termination ratings from 0% (no affect compared to non-treated control) to 100% (complete termination) [39]. Three were recorded 7, 14, and 21 days after termination (DAT) treatments per plot in both years. termination ratings were recorded 7, 14, and 21 days after termination (DAT) treatments per plot in Total variable costs (TVC) were estimated using a partial budgeting approach for each cover crop both years. and termination combination. Only variable costs that di ered between treatments were included, Total variable costs (TVC) were estimated using a partial budgeting approach for each cover and were assumed to be constant across the two years. These costs included cover crop seed and crop and termination combination. Only variable costs that differed between treatments were termination costs (labor, machinery repair and maintenance, fuel, and product). Cover crop seed costs included, and were assumed to be constant across the two years. These costs included cover crop were based on actual seed costs paid during the experiment: US $1.74 kg for Austrian winter pea, seed and termination costs (labor, machinery repair and maintenance, fuel, and product). Cover crop 1 1 1 −1 US $1.98 kg for crimson clover, US $4.30 kg for hairy vetch, US $3.53 kg for rapeseed, and US seed costs were based on actual seed costs paid during the experiment: US $1.74 kg for Austrian −1 −1 −1 $0.69 kg for cereal rye. Table 1 displays the termination costs by termination method [40,41]. Cover winter pea, US $1.98 kg for crimson clover, US $4.30 kg for hairy vetch, US $3.53 kg for rapeseed, 1 1 −1 crop planting (~US $19 ha ) and rolling/crimping costs (~US $13.10 ha ) were not included since and US $0.69 kg for cereal rye. Table 1 displays the termination costs by termination method [40,41]. −1 −1 these operations were applied to all plots. Machinery costs were based on a two-wheel drive, 37 kW Cover crop planting (~US $19 ha ) and rolling/crimping costs (~US $13.10 ha ) were not included 1 1 closed cab tractor with a fuel price of US $0.57 L . Labor costs included both operator (US $12.55 h ) since these operations were applied to all plots. Machinery costs were based on a two-wheel drive, and hand labor (US $9.87 h ) as applicable. 1 37 kW closed cab tractor with a fuel price of US $0.57 L . Labor costs included both operator Mixed models procedures as implemented in SAS PROC GLIMMIX (SAS Institute Inc., Cary, 1 1 − − (US $12.55 h ) and hand labor (US $9.87 h ) as applicable. NC, USA) were used to analyze winter cover biomass and visual termination as a percentage. Winter Mixed models procedures as implemented in SAS PROC GLIMMIX (SAS Institute Inc., Cary, cover crop, termination timing, and termination treatment were considered fixed e ects. Year and the NC, USA) were used to analyze winter cover biomass and visual termination as a percentage. Winter interaction of replication with termination timing and termination treatment were considered random cover crop, termination timing, and termination treatment were considered fixed effects. Year and e ects. Error terms appropriate to the split design with repeated measures were used to test main the interaction of replication with termination timing and termination treatment were considered random effects. Error terms appropriate to the split design with repeated measures were used to test main effects and interactions at P ≤ α = 0.10. We chose this α prior to analysis knowing inherent Agronomy 2019, 9, 519 5 of 14 Agronomy 2019, 9, x FOR PEER REVIEW 5 of 15 e ects and interactions at P  = 0.10. We chose this prior to analysis knowing inherent variability variability in visually estimated data is higher than other quantitative data. Non-transformed data in visually for visu estimated al terminadata tion is evhigher aluation than s weother re presen quantitative ted becau data. se tran Non-transform sformation did edno data t affe for ct visual data termination interpreta evaluations tion. Treatmen wer t di e ff pr er esented ences wer because e calcula transformation ted by Tukey–Kr did amer not groupin a ectgdata least interpr square etation. means test. Treatment di erences were calculated by Tukey–Kramer grouping least square means test. Figure 2. The 2014 and 2015 average daily temperature ( C), solar radiation (Wh/m ), and Figure 2. The 2014 and 2015 average daily temperature (°C), solar radiation (Wh/m ), and precipitation (mm). precipitation (mm). Table 1. Total variable costs by termination method ($US ha ). −1 Table 1. Total variable costs by termination method ($US ha ). Product Product Application Total Variable Costs Product Product Application Total Variable Costs Termination Method Termination Method $US ha −1 $US ha Non-treated 0 0 0 Non-treated 0 0 0 Glyphosate 5.76 8.93 14.69 Glyphosate 5.76 8.93 14.69 Vinegar 1074.54 22.06 1096.60 Vinegar 1074.54 22.06 1096.60 45% clove/45% cinnamon oil 148.24 22.06 170.31 45% clove/45% cinnamon oil 148.24 22.06 170.31 Clear plastic 790.88 306.73 1097.61 Clear plastic 790.88 306.73 1097.61 Flame 266.87 76.18 343.05 Flame 266.87 76.18 343.05 Agronomy 2019, 9, 519 6 of 14 3. Results and Discussion 3.1. Cover Crop Biomass ANOVA revealed significant e ects for cover crop, termination treatment, and termination timing. ANOVA also showed that the year by species interactions were significant (p < 0.10). Therefore, each species is discussed separately by year. In 2014, a maximum May biomass of 8838 kg ha was attained 1 1 in cereal rye plots, followed by Austrian winter peas (7177 kg ha ), hairy vetch (6213 kg ha ), and 1 1 crimson clover (5124 kg ha ) (Table 2). Rape provided the least biomass (1657 kg ha ). Terminating covers in April resulted in approximately 50% less biomass in all comparisons, while terminating covers in March resulted in less than 1000 kg ha regardless of cover crop. These results are similar to previously reported biomass and are indicative of proper management and environmental conditions (Figure 2) [19,29]. Conservation agriculture specialists consider cover crop biomass amounts exceeding 4000 kg ha as ‘high biomass’ systems [25]. However, for more e ective weed suppression, weed scientists recommend >6000 kg ha [20–22,29]. In 2015, a relatively warm winter and higher early spring temperatures (Figure 2) resulted in quicker spring cover crop growth compared to 2014 and covers attained maximum biomass at the April (2nd) termination date (Table 2). Similar to 2014, cereal rye attained the highest biomass at 5828 kg ha followed by crimson clover and hairy vetch (3995 and 3787 kg ha , respectively). Austrian winter pea 1 1 provided 1623 kg ha while rape again resulted in the least biomass (558 kg ha ) when terminated in April. Again, terminating covers in March again reduced cereal rye and rape biomass by half and decreased the legumes’ biomass by a third. Early termination due to planting schedules for early cash crops is a challenge to producing high biomass, especially as you move to more northern latitudes [20]. Table 2. Cover Crop Biomass at Three Termination Timings, E.V. Smith 2014 and 2015. Cover Crop Dry Biomass (kg/ha) Cover Crop 1st Termination Timing 2nd Termination Timing 3rd Termination Timing hg** fe b Austrian Winter Peas 959 2587 7177 h fe cd Crimson Clover 336 2369 5124 hg cb Hairy Vetch 3094 928 6213 i h fg Rape 116 370 1657 hg d a Cereal Rye 719 4769 8838 LSD ( = 0.10) 634.08 — — Cover Crop dc d Austrian Winter Peas 1762 1623 dc b Crimson Clover 2408 3995 dc b Hairy Vetch 1991 3787 e e Rape 84 558 c a Cereal Rye 2453 5828 LSD ( = 0.10) 448.7 — Cover crop biomass was obtained on March 14, April 16, and May 14, 2014; March 11, and April 12, 2015. Cover crop biomass was not sampled in May 2015 as covers had reached maximum maturity in April. ** Mean separations with the same letter are not significantly di erent. 3.2. Cover Crop Termination As previously stated, cover termination ratings were estimated by visually accessing greenness over the whole plot and recorded on a scale from 0% (no a ect compared to non-treated control) to 100% (complete termination). ANOVA revealed significant e ects for cover crop, termination treatment, and termination timing. ANOVA again showed year by cover crop species interactions were significant (p < 0.10). Therefore, each species is discussed separately by year. To facilitate discussion, the 7 (DAT) % termination estimation will be referred to as ‘quick’ or ‘quickly’, whereas the 21 DAT % termination estimation will be referred to as ‘waiting longer ’. As previously stated, rolling/crimping Agronomy 2019, 9, 519 7 of 14 was accomplished on all plots, therefore rolling/crimping alone will be discussed first, followed by the remaining treatments applied immediately following this practice. Hairy Vetch. In March 2014, rolling/crimping alone provided no quick termination or after waiting longer (Table 3). This result shows additional/alternative practices are required in March to attain termination. The highest (90%) quick termination was attained following flaming; however, regrowth occurred, and termination decreased to 23% after waiting longer. Similarly, significant regrowth was observed in e ective quick (90%) glyphosate non-organic comparison treatment after waiting longer, further highlighting hairy vetch resiliency. In March, 95% termination was attained after covering with clear plastic sheeting after waiting longer, and regrowth did not occur following plastic removal (data not shown). In March, other termination practices were not e ective quickly or after waiting longer. In April, rolling/crimping alone again provided no termination quickly or after waiting longer (Table 3). Flaming in April resulted in 93% termination quickly, and again following regrowth, only 68% termination was observed after waiting longer. In May, rolling/crimping alone provided 90% termination after waiting longer (Table 3). Cinnamon/clove oil provided a quick 28% increase in termination in April, compared to the ine ective rolled/crimped alone treatment; while vinegar failed to injure hairy vetch. However, regrowth again occurred and negated the control attained with oil. In 2015, likely due to advanced maturity in March, rolling/crimping alone provided 32% termination quickly but did not increase to adequate termination after waiting longer. The addition of flaming provided 93% termination quickly and near complete termination after waiting longer (Table 4). Clear plastic provided 85% termination after waiting longer, with no regrowth after removal (data not shown). In April, hairy vetch termination was 87% following rolling/crimping alone quickly and increased to 99% after waiting longer (Table 4). Similar to 2014, cinnamon/clove oil and vinegar showed little to no substantial activity on hairy vetch in 2015 (Table 4). Thus, when selecting a cover crop proceeding a March- or April-planted cash crop, hairy vetch might not be the best choice if quick termination is needed due to both resiliency and regrowth concerns. If slower termination can be utilized due to subsequent cash crop planting timing, clear plastic will likely be e ective in terminating hairy vetch. Overall, hairy vetch termination rates increased, and regrowth was reduced by delaying termination. These results are similar to those reported by Mirsky et al. [42]. Crimson Clover. In 2014, crimson clover response to treatments was similar to hairy vetch in most comparisons (Table 3). In March and April, rolling/crimping alone provided no termination quickly or after waiting longer (Table 3). Flaming clover was less e ective in March compared to hairy vetch likely due to the lack of biomass to injure. Clear plastic was also moderately e ective in March after waiting longer, and was highly e ective in April and May quickly. In April, clover was terminated 42% with cinnamon/clove oil quickly, and this was the second highest susceptibility observed compared to the non-treated, albeit still inadequate, for any species evaluated in this experiment terminated with cinnamon/clove oil. Rolling/crimping alone was 99% e ective after waiting 14 d when terminated in May. Vinegar resulted in no termination quickly, or after waiting longer, for any application timing when compared to the rolled/crimped non-treated. In 2015, March clover termination was similar to April 2014 (Table 4). In April, rolling/crimping alone resulted in 87% control quickly and 99% termination after waiting longer (Table 4). Clear plastic was again highly e ective in April and May quickly. Similar to 2014, cinnamon/clove oil and vinegar were non-e ective at enhancing clover termination quickly or after waiting longer, at any application timing. Thus, similar to hairy vetch, selecting a cover crop proceeding a March-planted cash crop, crimson clover might not be the best choice if quick termination is needed, due to resiliency. If slower termination can be utilized due to subsequent cash crop planting timing, or use in April or later, clear plastic will likely be e ective in terminating crimson clover. These results agree with those of Evans and Bellinder [30] that reported termination rates increased with clover maturation. Agronomy 2019, 9, 519 8 of 14 Cereal Rye. In 2014, rolling/crimping alone provided no termination and no treatment e ectively terminated rye in March (Table 3). In April, rolling/crimping alone resulted in 60% termination quickly. Flaming resulted in 97% termination; however, termination decreased to 70% following regrowth after waiting longer (Table 3). Clear plastic covering rye resulted in 97% termination after waiting 14 d. Cinnamon/clove oil increased termination 13 percentage points over rolling/crimping alone after waiting longer. In May, rolling/crimping alone provided 94% termination quickly and increased to 99% after waiting an additional week (Table 3). In 2015, rolling crimping in April alone provided 75% termination quickly and increased to 98% by waiting longer (Table 3). Flaming increased the rate of termination 23% and 11% quickly and after 14 d, respectively. In May rolling/crimping alone provided 99% control quickly, with no additional treatment increasing rye termination (Table 4). Notably, the cinnamon and clover oil mixture increased termination rates for cereal rye in one year after waiting longer. Generally, these results are similar to those reported by researchers that show termination rates following rolling/crimping increase as cereal cover crop growth stage progresses [24,26]. For March-planted cash crops, producers likely would benefit by not choosing cereal rye, as it is relatively hard to terminate at early growth stages. Sequential rolling/crimping or flaming operations would likely be needed. Austrian Winter Pea. In 2014, March rolling/crimping alone resulted in no termination quickly or after waiting longer (Table 3). Flaming provided 82% pea termination quickly, however aggressive regrowth resulted in only 10% termination after waiting longer. Clear plastic covering peas after waiting longer resulted in 58% termination, with no regrowth after removal (data not shown). Cinnamon/clove oil and vinegar provided no pea termination quickly or after waiting longer at any application timing. In April, rolling/crimping provided 25% termination alone with flaming and clear plastic treatment resulting in 98% termination after waiting 14 d (Table 3). In May, rolling/crimping alone provided 20% termination quickly, however, termination increased to 96% after waiting longer (Table 2). In 2015, rolling/crimping alone provided 48% termination quickly and increasing to 79% after waiting longer, again, likely due to later maturity (Table 4). Cinnamon/clove oil increased pea termination by 23 and 11 percentage points over rolling/crimping alone after 14 d and longer, respectively. Similar to the previous cover crops, March-planted conservation tillage cash crop establishment may be challenging due to insucient winter pea termination. Rape. In March and April 2014, rolling/crimping provided no termination quickly or after waiting longer (Table 3). Flaming provided 95% termination quickly, however, rape recovered completely after waiting longer (Table 3). Clear plastic provided 58% termination after waiting longer, with no regrowth after removal (data not shown). Cinnamon/clove oil and vinegar provided no termination at any application timing quickly or after waiting longer. In April, flaming provided only 33% termination quickly and did not increase after waiting longer (Table 3). Clear plastic provided 48% termination quickly, increasing to 78% after waiting longer. In 2015, rolling/crimping in March provided no rape termination quickly or after waiting longer (Table 3). Clear plastic was very e ective, providing 95% termination quickly increasing to 99% after waiting longer, likely due to increasing solar radiation and relatively high air temperatures. Flaming provided 99% termination quickly, however, termination decreased to 70% after waiting longer due to regrowth. Similar to 2014, cinnamon/clove oil and vinegar provided no rape termination at any application timing quickly or after waiting longer. Agronomy 2019, 9, 519 9 of 14 Table 3. Cover crop response to termination method by termination date in 2014. 1st Termination Date (March) 2nd Termination Date (April) 3rd Termination Date (May) Cover Crop Termination Method 7 DAT 14 DAT 21 DAT 7 DAT 14 DAT 21 DAT 7 DAT 14 DAT 21 DAT % Termination * 1 cb c c c c b b b a Non-treated 3 0 0 0 0 0 37 78 90 2 b a ba bc b a b a a Glyphosate 13 90 60 43 72 83 58 98 99 c c c c a 3 cb b b ba Vinegar 2 2 0 0 0 0 53 80 81 4 cb c c bc c b ba ba a Hairy Vetch 0 0 8 93 Clove/cinnamon oil 8 28 5 62 85 5 c c a ba a a a a a Clear plastic 1 3 95 66 97 66 99 99 99 6 a b bc a b a a a a Flame 90 63 23 93 73 68 98 99 99 LSD ( = 0.10) 6.7 9.4 29.5 29.5 6.1 34.9 22.8 11.8 11.4 b c c c c c b a a Non-treated 2 0 0 0 0 0 67 99 99 a a a a a a a b ba Glyphosate 57 98 98 38 93 97 78 99 99 b c bc c c c b a a Vinegar 3 2 2 2 0 0 57 99 99 b c bc b b b b a a Crimson Clover (Dixie) Clove/cinnamon oil 5 5 10 42 33 30 57 66 66 c a a a a a a a Clear plastic 8 3 72 82 93 97 99 99 99 a b ba a a a a a a Flame 67 96 96 96 99 99 99 68 52 LSD ( = 0.10) 23.0 14.3 31.0 18.6 11.0 10.6 17.2 34.0 34.0 c c b ba b ba b a a Non-treated 0 0 0 60 63 67 94 99 99 a a a a a a a a a Glyphosate 48 98 69 97 99 99 99 99 99 c c b b b b b a a Vinegar 0 0 0 45 53 53 94 99 99 Cereal Rye (Wrens c c b ba a ba ba a a Clove/cinnamon oil 8 0 0 75 85 80 96 99 99 Abruzzi) c c ba ba a a ba a a Clear plastic 2 2 48 82 97 96 98 99 99 a a a a a b b b ba Flame 28 15 12 97 96 70 99 99 99 LSD ( = 0.10) 8.0 3.5 31.5 24.3 12.6 22.8 2.7 **** **** c b b b b b c a ba Non-treated 0 0 0 27 28 25 20 87 96 ba a a ba a a b a a Glyphosate 67 98 99 60 95 98 72 99 99 c a bac b b b b b b Vinegar 31 17 10 28 28 28 32 88 93 bc b b b b b b a a Austrian Winter Peas Clove/cinnamon oil 97 99 27 20 25 27 28 25 57 c b a ba a a a a a Clear plastic 9 11 93 62 98 99 99 99 99 a a b a a a a a a Flame 82 68 10 98 98 98 98 99 99 LSD ( = 0.10) 31.2 23.5 18.9 35.8 15.4 15.0 13.3 7.8 3.4 c b b c c c c b c Non-treated 0 0 0 0 0 0 2 0 0 a a a a a a b b ba Glyphosate 48 96 69 80 97 98 47 90 94 c b b c c c c b c Vinegar 2 0 0 0 0 0 2 0 0 c b b bc c c c b c Rape (Athena) Clove/cinnamon oil 2 0 0 7 0 0 2 0 0 c a a a a b ba ba b Clear plastic 0 1 58 48 63 78 95 99 99 a a b bc b b a a b Flame 95 95 99 70 3 33 37 33 70 LSD ( = 0.10) 27.3 1.6 36.5 25.7 16.5 17.8 24.0 22.5 16.9 1 2 No herbicide or other termination method was used to kill the cover crop other than mechanically rolling/crimping. Mechanically rolling/crimping followed by glyphosate (not Organic Materials Review Institute (OMRI)-approved) applied to covers at 1.12 a.i. kg/ha. Mechanically rolling/crimping followed by 20% vinegar solution applied to covers with an application 4 1 volume of 140.25 L/ha. Mechanically rolling/crimping followed by 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove 5 6 oil (eugenol, acetyl eugenol, caryophyllene) mixture. Mechanically rolling/crimping followed by clear plastic laid over the entire plot 3.7m  3m area for 28 days. Mechanically rolling/crimping followed by broadcast flaming emitting 1100 C applied at 1.2 k/h. **** Mean separations with the same letter are not significantly di erent within a cover crop and termination timing. Treatment di erences were calculated by Tukey–Kramer grouping least square means. Agronomy 2019, 9, 519 10 of 14 Table 4. Cover crop response to termination method by termination date in 2015. 1st Termination Date (March) 2nd Termination Date (April) Cover Crop Termination Method 7 DAT 14 DAT 21 DAT 7 DAT 14 DAT 21 DAT % Termination * c a a a 1 b b Non-treated 32 37 38 87 99 99 2 b a a a a a Glyphosate 63 93 98 98 99 99 3 c b b a a a 32 88 99 99 Vinegar 40 35 4 c b b a a a Hairy Vetch Clove/cinnamon oil 17 15 15 90 99 99 5 b a a a a a Clear plastic 65 87 85 90 99 99 a a a a a a Flame 93 92 98 99 99 99 LSD ( = 0.10) 15.3 18.0 25.4 7.7 **** **** c c a a a Non-treated 5 3 3 99 99 99 a a a a a ba Glyphosate 77 82 96 99 99 99 b c c a a a Vinegar 0 0 99 99 99 b c c a a a Crimson Clover (Dixie) Clove/cinnamon oil 8 8 99 99 99 b b b a a a Clear plastic 30 58 5 8 98 99 99 a a ba a a a Flame 90 94 88 99 99 99 LSD ( = 0.10) 19.7 20.6 21.8 **** **** **** bc b a a a a Non-treated 75 87 98 99 99 99 a a a a a a Glyphosate 99 99 99 99 99 99 c a a a a ba Vinegar 73 90 98 99 99 99 bac ba a a a a Cereal Rye (Wrens Abruzzi) Clove/cinnamon oil 83 92 96 99 99 99 c ba a a a a Clear plastic 67 93 99 99 99 ba a a a a a Flame 98 98 99 99 99 99 LSD ( = 0.10) 13.7 6.2 5.6 **** **** **** a a a a a a Non-treated 48 50 79 98 99 99 a a a a a a Glyphosate 62 67 69 98 99 99 a a a a a a Vinegar 40 50 72 99 99 99 a a a a a a Austrian Winter Peas Clove/cinnamon oil 50 80 90 99 99 99 a a a a a a Clear plastic 57 73 79 99 99 99 a a a a a a Flame 96 97 98 99 99 99 LSD ( = 0.10) **** **** **** **** **** **** a a a a a a Non-treated 99 99 99 99 99 99 a a a a a a Glyphosate 99 99 99 99 99 99 a a a a a a Vinegar 99 99 99 99 99 99 a a a a a Rape (Athena) Clove/cinnamon oil 99 99 99 97 99 99 a a a c a a Clear plastic 99 99 99 95 99 99 a a a a a a Flame 99 99 99 99 99 99 LSD ( = 0.10) **** **** **** **** **** **** No herbicide or other termination method was used to kill the cover crop other than mechanically rolling/crimping. Mechanically rolling/crimping followed by glyphosate (not OMRI-approved) applied to covers at 1.12 a.i. kg/ha. 3 1 4 Mechanically rolling/crimping followed by 20% vinegar solution (28 L a.i. ha acetic acid). Mechanically rolling/crimping followed by 2.5 L a.i. ha 45% cinnamon (Cinnamomum verum L.) oil (cinnamaldehyde, eugenol, eugenol acetate)/45% clove oil (eugenol, acetyl eugenol, caryophyllene) mixture. Mechanically rolling/crimping followed by clear plastic laid over the entire plot 3.7m  3m area for 28 days. Mechanically rolling/crimping followed by broadcast flaming emitting 1100 C applied at 1.2 k/h. * Mean separations with the same letter are not significantly di erent within a cover crop and termination timing. Treatment di erences were calculated by Tukey–Kramer grouping least square means. **** Indicates no significance. 3.3. Cover Crop Termination Economics Cover crop seed costs drive cover crop establishment costs. Austrian winter peas had the highest per unit seed costs, and at a seeding rate of 101 kg ha , it was the highest cost cover crop. Hairy vetch and cereal rye were also more expensive due to seed costs and seeding rate, respectively. However, since one of the main objectives of cover crops is to produce adequate biomass, the cover crop seed cost per 100 kg of dry biomass was calculated to compare the production potential in terms of dollars spent (Table 5). At the early termination date (March), rape had the highest cost per 100 kg of biomass, with biomass averaged across both years; however, at the later termination dates (April and May), there was little numerical di erence between Austrian winter peas and rape. Biomass from Austrian winter peas and rape was the most expensive to produce across all three termination dates. Crimson clover and cereal rye consistently had lower seed costs per 100 kg produced biomass. Glyphosate was the least expensive treatment option compared to non-treated; however, it obviously does not meet organic production standards (Table 6). Of the organic methods, Agronomy 2019, 9, 519 11 of 14 cinnamon/clove oil was the least expensive and one of the least e ective termination methods. Vinegar, along with clear plastic, had the highest costs as driven by product costs and application rates; however, in general, vinegar was not an e ective termination method, while expensive, clear plastic was an e ective termination method after waiting longer. Clear plastic also requires additional labor and machinery time as compared to the other treatments. Furthermore, there may be disposal costs associated with clear plastic, which were not accounted for in this analysis. The cost of flame termination was almost four times the cost of glyphosate; however, it proved to be an e ective termination option for most covers evaluated in April or May. However, there may be additional costs if regrowth occurs after termination. Furthermore, the cost to purchase the equipment and ownership costs were not included since they di er greatly between farms. From an economic perspective, organic farmers should look at their operations, goal and objectives, and potential constraints, such as labor, before deciding on the most appropriate termination methods. Table 5. Cover crop seed costs per 100 kg of biomass averaged over 2014–2015. Cover Crop Seed Costs per 100 kg of Biomass ($US 100 kg ) Cover Crop 1st Termination Timing 2nd Termination Timing 3rd Termination Timing Austrian Winter Peas 5.23 3.38 0.99 Crimson Clover 1.64 0.71 0.44 Hairy Vetch 2.67 1.13 0.63 Rape 16.00 3.45 0.97 Cereal Rye 1.79 0.54 0.32 Table 6. Total variable costs (including seed and termination method costs) by cover crop and termination method ($US ha ). Termination Method Cover Crop Glyphosate Vinegar 45% Clove Oil/45% Cinnamon Oil Clear Plastic Flame $US ha Austrian Winter Peas 212 1272 346 1273 519 Crimson Clover 92 1152 226 1153 399 Hairy Vetch 133 1193 267 1194 439 Rape (Athena) 76 1136 210 1137 383 Cereal Rye 106 1167 240 1168 413 4. Conclusions Five winter cover crops were evaluated, however, the Brassica (rape) never attained biomass at any termination timing that would benefit either soil quality or weed suppression. Because of the very low biomass produced, it was the most expensive to produce on a weight basis. In addition, Austrian winter pea was also expensive to produce on a weight basis. Crimson clover and cereal rye were both relatively less expensive on a weight bases and produced adequate biomass to both a ect soil quality and provide weed suppressive qualities. Organic producers seeking to terminate winter covers would most likely be successful using a broadcast flamer on most any winter cover in April or May, or utilizing clear plastic in hairy vetch, winter peas, or cereal rye, as ambient temperature increases along with solar radiation, both following a roller/crimper. Obvious limitations to using plastic include cooler climates, and/or higher latitudes. Termination rates increase as cover crops mature, thus, producers with early crop planting timing that necessitate early cover crop termination timings will likely need to use multiple recurrent practices to terminate most cover crops. Commercially available vinegar and cinnamon/clove oil solutions provided little predictable termination. Producers attempting to use these OMRI-approved products will likely resort to utilize cover crop incorporation, or mowing, to terminate covers if no other e ective practice is readily available, and these practices fail to adequately terminate the cover. Agronomy 2019, 9, 519 12 of 14 Author Contributions: A.J.P., J.S.M., and S.L. collaborated on experimental conceptualization, A.J.P. managed experimental investigation, A.J.P. and J.S.M. contributed to resources, and L.D. provided economic analysis, and A.J.P. and L.D. contributed to statistical analysis and writing-original draft preparation. Funding: This research received no external funding. Acknowledgments: Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by USDA and does not imply endorsement of a product to the exclusion of others that may be suitable. Conflicts of Interest: The authors declare no conflicts of interest. 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Agronomy – Multidisciplinary Digital Publishing Institute

Published: Sep 6, 2019

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