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Burn and mechanical residue removal methods on production‐life of Kentucky bluegrass

Burn and mechanical residue removal methods on production‐life of Kentucky bluegrass AbbreviationsBBbale then burnFfallFLBfull load burnMECbale then mow then harrowNPVnet present valueNSBnonstanding biomassPSpre‐swathingPTpost‐residue treatmentSPspringSTBstanding biomassSYSrotation systemINTRODUCTIONIt has long been known that accumulation of postharvest residues reduces Kentucky bluegrass (Poa pratensis L.) seed yields due to the interruption of necessary light (Ensign et al., 1983) and temperature (Aamlid, 1992; Carlson et al., 1995) stimuli required for optimal seed production. Burning not only removes much of this excess residue but also kills many of the surviving older, nonreproductive tillers, allowing the development of new tillers (Chilcote et al., 1978). Mechanical methods such as mowing, raking, baling, harrowing, grazing, gapping, and/or vacuuming have been studied and were widely used in bluegrass seed production systems in Washington and Oregon (Chastain et al., 1997; Evans, 1980; Holman, Hunt, Johnson‐Maynard, et al., 2007; Lamb & Murray, 1999; Youngberg, 1980). Generally, these mechanical methods provide seed yields comparable with burning during the first 1–2 yr of seed production, yet with certain methods and cultivars, comparable yields were achieved through the third (Lamb & Murray, 1999) and fifth (Chastain et al., 1997) seed harvest. Nonthermal residue management, therefore, generally results in premature stand decline as compared with field burning. In addition, postharvest residue is low in forage nutritive value, economic value, and demand (Holman, Hunt, & Thill, 2007; Holman, Hunt, Johnson‐Maynard, et al., 2007; Holman et al., 2011), all of which are disincentives to manage postharvest residue other than through field burning.Regardless of residue removal method, maintaining high seed yields for many years prolongs the expense of stand reestablishment and strongly influences the profitability of growing dryland Kentucky bluegrass seed. Newly established stands produce little to no seed yield the first year in dryland production areas like northern Idaho. In addition, reestablishment involves tillage and the planting of annual crops prior to Kentucky bluegrass reestablishment, increasing the rates of soil erosion and loss. Even fields that are burned annually eventually succumb to weakening of the stand, decreased yields, weed and disease pressure, and ultimately crop failure. Since seed yield can be influenced by residue management (Ensign et al., 1983), as well as stand age (Canode & Law, 1975), the long‐term comparisons of residue management practices in this study are critical to understanding the factors affecting seed yield in the latter years of a Kentucky bluegrass stand.Past published reports on Kentucky bluegrass stand decline have focused on environmental factors such as light and temperature. Relatively few studies have focused on biomass and residue dynamics overtime as a stand aged. In addition to direct effects of residue management and stand age on seed production, indirect effects may occur due to changes in nutrient cycling. For example, continued application of high rates of nitrogen (N) fertilizer resulted in faster yield decline, whereas lower N rates maintained yields as the stand aged in a study conducted in Prosser, WA (Evans, 1980). This is consistent with the idea that fertilizer rates and timing need to be optimized for a particular residue management practice (Lamb & Murray, 1999). Residue management may influence N availability in multiple ways. Although burning may result in the loss of a fraction of N through volatilization, it may also cause a relatively rapid flush of N. Nitrogen may also be lost in nonthermal practices when residue N is baled and removed from the field (Banowetz et al., 2009). Finally, depending on the chemical composition of the specific cultivar grown (Holman, Hunt, & Thill, 2007), accumulation of residue in nonthermal practices may result in either the loss of available N through microbial immobilization or the return of N through mineralization.Many areas where bluegrass seed is produced in northern Idaho are characterized as having moderately to strongly acid soils, in part due to continual applications of relatively high rates of N fertilizer. Acidification of near surface soil in no‐till agricultural systems in this region has been noted (Barth, et al., 2018; Brown et al., 2008; Mahler & McDole, 1987). Similar to no‐till systems, bluegrass residue may be left on the surface to decompose in nonburn systems, and tillage is restricted to stand take out. Acidification of near‐surface soil in these already acid soils may lead to fertility problems including aluminum (Al) toxicity and alteration of the availability of other nutrients.The purpose of this research was to investigate, over the entire expected lifespan of a Kentucky bluegrass stand in northern Idaho, the integration of thermal and nonthermal residue management practices on seed yield and profitability. This work represents one of the few studies to focus on the entire expected life of a Kentucky bluegrass stand in dryland production regions. In this study, we propose new explanations for stand and seed yield decline and new methods for predicting seed yields. A thorough understanding of seed yield potential over the entire life of the stand across various residue management practices is critically important to seed producers, policy makers, and the economic sustainability of the region.Core IdeasSeed yields were the greatest in full load burn, followed by bale then burn treatments.Seed yield was negatively influenced by nonstanding residue and positively by N content of fall standing biomass.Like seed yield, profitability was also greatest in bale then burn and full load burn treatments.Profitability of mechanical management practices is dependent on highly variable forage and feed prices.MATERIAL AND METHODSSite, experimental design, and managementThe experimental site was located in Kootenai County, Idaho (47°28′0′′ N, 116°57′0′′ W) on a grower‐cooperator field. The field was planted to the cultivar ‘Alene’, characterized as a nonaggressive Kentucky bluegrass cultivar. The soil type was a Taney silt loam (fine‐silty, mixed, superactive, frigid Vitrandic Argixerolls) (Soil Survey Staff, 2020). The site receives approximately 600 mm of precipitation annually, the majority of which occurs during the winter and early spring. Our experimental design was a randomized complete block with four blocks and a total of 16 plots. Plots measured 18–21 m wide and 91 m long in order to accommodate full‐sized commercial farm equipment and plot burning. Four residue removal treatments including full load burn (FLB), bale then burn (BB), bale then mow then harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3) were evaluated over a 7‐yr period from establishment in 2001–2007. The treatments were selected to study the effect of burn, reduced burn, and mechanical residue management on residue dynamics, stand productivity, and seed yield. After harvest, the FLB treatments were burned without removal of any plant material, BB treatments were baled prior to burning, and the MEC plots were baled, mowed, and harrowed. Either a rotary or flail mower and a heavy flex harrow was used. Since mowing occurred only after significant regrowth, harrowing sometimes preceded mowing. Fertilization with ammonium nitrate occurred annually in late fall with application of 168 kg N ha−1 applied across all treatments. Plots were swathed in early July and harvested between mid‐July and early August using a pickup header on a field‐scale combine (Table 1). Whole‐plot seed weight was measured, and a subsample was processed through screens to determine clean seed weight. Residue removal treatments were applied mid‐August to late September. Herbicide applications of pendimethalin, metolachlor, dicamba and clopyralid were made to the plots as necessary to primarily control Ventenata [Ventenata dubia (Leers) Cross] and Cirsium spp. (‘thistle’).1TABLESampling dates of Kentucky bluegrass standing biomass and nonstanding residue at specific time periods throughout the studyTime period200220032004200520062007Spring (SP)–7 May6 May11 May9 May7 MayPre‐swath (PS)–23 June16 June1 July28 June15 JunePost‐treatment (PT)9 Sept.19 Sept.30 Aug.22 Sept.25 Sept .–Fall (F)22 Oct.22 Oct.4 Nov.24 Oct.22 Oct.–Data collected, sampling, and laboratory analysisKentucky bluegrass standing biomass (STB) and nonstanding biomass (NSB) were collected four times throughout the cropping year: spring (SP), pre‐swathing (PS), post‐residue treatment (PT), and fall (F). The sampling dates within each period varied slightly from year to year due to field operations and/or inclement weather (Table 1). Bluegrass samples were collected from each of three randomly placed, 0.25‐m2 quadrats per plot. A small rake was used to remove the NSB from the quadrat before clipping and removal of STB. Standing biomass and NSB samples were dried in an oven at 60 °C for 48 h. Any visible soil aggregates or rocks were removed, and then STB and NSB were weighed and ground. Residue removal through baling was calculated as the difference between residue measured pre‐swath and post‐baling. Ground subsamples were ashed at 500 °C for 4 h to correct for mineral content. Three soil samples from each plot were collected at the 0‐to‐10‐cm depth and combined for pH analysis after swathing in the first and last years of the study. A 1:2 soil/water slurry was used for pH measurements adapted from Thomas (1996). Total carbon (C) and N in STB and NSB was measured by dry combustion in a VarioMax C/N/S analyzer (Elementar) (Tabatabai & Bremner, 1991). The harvest index was calculated as the clean seed yield (kg ha−1) divided by the amount of STB (kg ha−1) (Thompson & Clark, 1989).Statistical data analysisInfluence of residue management techniques on yield, harvest index, STB, NSB, and N content were analyzed by repeated measures ANOVA in SAS (SAS Institute, 2008). A one‐way ANOVA was used to test for soil pH changes over time. Significant effects were determined at P ≤ .05, and Fishers LSD was conducted for mean separation.The linear relationship between seed yield and stand age was studied by regressing measured seed yield against standing age using PROC REG procedure of SAS. The relationship between seed yield to STB, NSB, %C, and %N measured during each sampling period (PT, F, SP, and PS) was determined using correlation analysis in PROC CORR procedure of SAS.Economic returns for each management practice were compared over the life of the stand by estimating the net present value (NPV) for each year using an enterprise budget approach and a discount rate of 5.75% (Table 2). Input and return prices were based on current costs of inputs and expected returns in this region. Both variable and total costs were determined for each production practice. Costs varied by production practice due to differences in inputs and management. Variable costs included fertilizer, pesticide, and annual machinery and harvest expense. Fixed costs included machinery depreciation and interest, land rent, overhead, and management.2TABLEInput prices for establishing Kentucky bluegrass production budgets from a survey of input providers in the regionExpenseItemUnitPrice unit−1FuelDiesel, offroad, bulkL$0.74GasL$0.66FertilizerNitrogen (liquid)kg$0.97Phosphorusakg$1.32Sulfur (liquid)kg$0.88Potassium (dry)kg$0.95Gypsumkg$0.35AdjuvantsAmmonium sulfate (20‐0‐0‐24)kg$0.97Ammonium sulfate (liquid)L$0.02Adjuvant (antifoam)L$0.0008Crop oil concentrateL$4.22Nonionic surfactantL$0.0001Pesticides2,4‐Dichlorophenoxyacetic acid (47.3% a.i.b)L$0.0004Quizalofop‐p‐ethyl (10.3% a.i.)L$0.0027Primisfulforn‐methyl urea (75% a.i.)L$0.12Bromoxynil octanoate (28% a.i.)L$0.0013Diuron (80% a.i.)L$0.0048Flucarbazone‐sodium (66% a.i.)L$0.024Tribenuron methyl (75% a.i.)L$0.043Glyphosate‐potassium salt (48.8% a.i.)L$0.0004SeedBluegrass seed for establishment (common)kg$3.31Custom hireChemical applicator (aerial)ha$22.12Chemical applicator (ground)ha$21.00Fertilizer applicatorha$4.94LaborcHourly machine laborh$20.00Other laborh$12.00Other costsOverheadd%2.50%Management feee%5.00%InterestOperating loan%5.75%Machinery loan/investment%5.75%SalesLow bluegrass seed uncleanedkg$1.65High bluegrass seed uncleanedkg$2.78Low bluegrass straw price (100% dry matter)kg$0.06High bluegrass straw price (100% dry matter)kg$0.09aAverage of dry and liquid formulation.ba.i. = pesticide % active ingredient.cCovers all applicable state and federal taxes.dCovers legal, accounting, and utility fees as percentage of operating expenses.eCalculated as a percentage of gross revenue.RESULTS AND DISCUSSIONNonstanding biomass dynamicsOf the time periods measured (SP‐PS‐PT‐F), the greatest amounts of NSB were present during PT and F periods (Figure 1a). Each year during the PT period, the burn treatments, FLB and BB, resulted in lower amounts of NSB compared with the MEC treatment, except in 2005 when all treatments were similar (Table 3). The higher PT residue levels following field burning were due to a spotty burn, which was caused by precipitation prior to burning and subsequent damp postharvest residue and stand regrowth. The SYS treatment had higher NSB levels after BB in fall 2003 (2004 crop year) than did FLB or BB, which may have been the result of not burning in fall 2002 (MEC). Again, in 2006, there was more NSB in SYS than in FLB or BB when it was not burned in fall 2005 (MEC). In the 2007 crop year, NSB levels in SYS were not different than those measured in the other treatments. These results indicate that occasional burning did not consistently reduce NSB in the SYS treatment to the extent found in FLB or BB.1FIGURE(a) Average nonstanding biomass (NSB) and (b) standing biomass (STB) by time period from 2003–2007. Kentucky bluegrass samples collected in 2002 were not separated into nonstanding residue and standing biomass. Time periods sampled were post‐treatment (PT), fall (F), spring (SP), and pre‐swath (PS). Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3). Significant residue differences between treatments within a time period are noted by letters; treatments that share the same letter or have no letters are not significantly different at P < .053TABLENonstanding Kentucky bluegrass residue amount at post‐treatment (PT) and fall (F) time periodsStand age (harvest year)Residue removal treatmentaFLBBBMECSYSResidue amount (Mg ha−2) at PT4 (2004)1.64ab2.16a3.61b2.96b5 (2005)1.13a1.22a1.80a1.54a6 (2006)1.70a1.03a3.87b3.57b7 (2007)0.72a1.17a2.43b1.57abResidue amount (Mg ha−2) at F4 (2004)1.60ab1.92a4.34c3.51b5 (2005)0.24a0.54a1.94b0.60a6 (2006)1.56a1.29a3.12b4.28c7 (2007)0.31a0.70b1.73c0.65baFLB, full load burn; BB, bale then burn; MEC, bale–mow–harrow; SYS, practice (BB [2003]–FLB [2004]–MEC [2005]–BB [2006]).bDifferent letters indicate differences within a harvest year (P ≤ .05).Within the remaining sampling periods (SP, PS, F), NSB amounts were similar between the burn treatments (FLB and BB) and were different between the burn and MEC treatment (Figure 1a). Amounts of NSB in the SYS treatment were generally between those measured in the MEC, BB, or FLB treatments, reflecting the fact that the SYS treatment consisted of each treatment on a rotating basis. Throughout the growth cycle beginning with the PT period, the greatest decrease in NSB was between F and SP, when moist conditions and insulating snow cover promoted rapid residue decomposition. Amounts of NSB in the MEC treatment decreased by an average of 2.1 Mg ha−1, between F and SP, significantly more than the decreases observed in FLB (0.8 Mg ha−1) or BB (1.0 Mg ha−1) (Figure 1a). Greater amounts of fall residue in the MEC treatment likely promoted over‐winter decomposition by providing a suitable local environment for decomposer communities. Likewise, first‐order decomposition rate equations as used by Berndt (2008) for thatch and Kopp and Guillard (2004) for grass clippings predict greater decomposition losses when greater amounts of residue are initially present. Although in situ decomposition of NSB as in the MEC treatment has long‐term benefits such as the return of nutrients to the soil, burning resulted in lower NSB during the critical F time period (Table 3), most certainly promoting seed yield.Standing biomass dynamicsStanding stubble biomass remaining after harvest (Thompson & Clark, 1989) and late summer tiller regrowth (Chastain et al., 1997) were both considered STB in this study. Standing stubble interferes with light and temperature induction stimuli and results in lower seed production. During the PT period, average STB was greater every year in the MEC treatment than in the FLB or BB treatments (Table 3). Average amounts of STB were 1.40 Mg ha−1 in MEC, 0.14 Mg ha−1 in SYS, 0.13 Mg ha−1 in BB, and 0.11 Mg ha−1 in FLB (Figure 1b). Lower amounts of STB in the burn treatments are a result of efficient burning (removal) of both growing and nongrowing (stubble) STB.Regrowth between the PT and F periods tended to be significantly greater in the SYS (1.12 Mg ha−1), FLB (0.76 Mg ha−1), and BB (0.77 Mg ha−1) treatments as compared with the MEC (0.31 Mg ha−1) treatment (Figure 1b). The growth rate and amount of STB in SYS varied by year and averaged between MEC and burn treatments, again reflecting the year‐to‐year variability when the treatment was burned or not. The regrowth in STB from PT to F periods highlights the transition from summer dormancy, induced by dry, hot conditions, into cool, sometimes moist, fall conditions. If moisture is not limiting, higher mid‐ to late‐fall soil temperature promoted by the blackened surface in the burn treatments may be a factor in determining the amount of regrowth. Greater regrowth of burned vs. unburned red fescue was attributed to higher November soil surface temperatures when burned (Chilcote et al., 1978). In addition to soil temperature, burning of excess tillers around the base of the plant may also have a stimulatory effect on new tiller production (Chilcote et al., 1978).Annual seed yieldMany factors influence annual seed yield of bluegrass including environment, cultivar, age, fertility, and moisture availability (Holman & Thill, 2005). Seed yield was affected by residue management treatment each year (Table 4) after establishment (2001). The MEC treatment yielded less than FLB and BB every year except the first year (Table 4). Previous studies also reported lower yields when mechanical methods of residue removal were compared with burning postharvest residue (Adams et al., 1976; Chastain et al., 1995; Ensign et al., 1983). Bale‐burn yielded less than FLB in 2006 and 2007, which was likely due to spotty burns caused by precipitation wetting postharvest residue and initiating some bluegrass regrowth prior to burning. Bluegrass yield might be particularly sensitive to spotty burns in BB or SYS as stands age, evident in Years 6 and 7 of this stand. Seed yields in the SYS treatment fluctuated from year to year depending on the residue management treatment implemented. When the SYS treatment was BB or FLB yields were high and similar to the FLB treatment, yet in years that the SYS treatment was MEC, yields were lower than those measured in FLB or BB. Across the life of the stand, yields tended to decrease as less burning was used FLB > BB > SYS > MEC (Table 4). The harvest index (seed yield/STB, Figure 2), was very similar to seed yield, indicating that residue management practice affected plant biomass production as well as seed yield. Seed yield in SYS was significantly less than that measured in the burn treatments in three of the study years. Although reduced yield affects profitability, alternatives to FLB may be viable and allow producers to sustain stand productivity while reducing negative impacts to air quality.4TABLEWithin‐year and average across‐year treatment effect on seed yieldStand age (harvest rear)Treatmenta1 (2001)2 (2002)3 (2003)4 (2004)5 (2005)6 (2006)7 (2007)Meankg ha−1FLB5041,087a846a710a734a595a545a774aBB5041,101a878a726a642a476b439b742bMEC504986b669b506b387b191d227c525dSYS504890c765ab685a369b302c489ab600cLSDNS6515095159917623aFLB, full load burn; BB, bale then burn; MEC, bale–mow–harrow; SYS, practice (MEC [2002]–BB [2003]–FLB [2004]–MEC [2005]–BB [2006]–FLB[2007]).2FIGUREHarvest index (HI) comparisons by year. Different letters indicate differences within a year (P ≤ .05). Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3). Treatments applied each year in the SYS treatment are shown. Harvest index = seed yield (kg ha−1)/standing biomass (STB) (kg ha−1), where STB is sampled pre‐swath (PS)Seed yield decline with stand ageAlthough seed yields typically decline as Kentucky bluegrass stands age (Canode & Law, 1975; Lamb & Murray, 1999), some residue removal strategies maintain annual yields longer than others, thereby increasing economic longevity of the stand (Evans, 1980; Holman & Thill, 2005; Holman, Hunt, Johnson‐Maynard, et al., 2007; Lamb & Murray, 1999). In this study, rate of yield decline was highly correlated with stand age (R2 = .92), and it was greater for MEC than for burn treatments (Figure 3). During the 7‐yr period (2001–2007), seed yield decreased an average of 98 kg ha−1 yr−1 in FLB, 106 kg ha−1 yr−1 in SYS, 132 kg ha−1 yr−1 in BB, and 153 kg ha−1 yr−1 MEC (Figure 3). The rate of decline in SYS may have been underestimated due to unexpectedly high yields in this treatment in 2007 (Figure 3), attributed to FLB management in fall 2006. The seed yield decline under FLB was comparable with that reported in a previous study that found seed production decreased 81 kg ha−1 yr−1 from the first through the fifth seed harvests when burned annually (Canode & Law, 1975). Decreased seed yield with burning was attributed to a decrease in panicles per plant (Canode & Law, 1975), and the formation of a sod‐bound stand (Canode & Law, 1979) over time. These are the first published rates of stand decline over the life of a stand in a long‐term, side‐by‐side comparisons of burn, reduced‐burn, and mechanical residue management practices.3FIGUREDeclining seed productivity of treatments between 2002 and 2007. Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3)Another factor that might have influenced the rate of yield decline across treatments is related to management effects on soil pH. Although soil pH was not significantly different across treatments, it did show an average decrease from 5.10 in the beginning of our study to 4.96 in Year 5 (data not presented). The availability of essential nutrients and potentially toxic elements such as Al, depend on soil pH. A decline in surface soil pH has occurred in other agricultural practices in the region that use reduced tillage and ammonium‐based fertilization (Brown et al., 2008; Umiker et al., 2009). Palazzo and Duell (1974) achieved maximum aboveground growth of Kentucky bluegrass when soil pH, at 0‐to‐15‐cm depth, ranged between 6.0 and 6.5, with lower pH resulting in reduced aboveground biomass production. At pH levels below 5.0 (0‐to‐20‐cm depth), poor seed and straw yields in Kentucky bluegrass were attributed to Al toxicity (Aamlid, 1991). Acidic pH in our study may have increased the availability of Al and contributed to decreases in yield and aboveground biomass as the stand aged. The lack of tillage with Kentucky bluegrass might also be causing stratification of low pH in the top 10 cm, similar to that noted in no‐till fields within the region (Brown et al., 2008).Factors affecting seed yieldNitrogen content in STB (STB %N) at PT and F were positively correlated to seed yield the following year, whereas STB and NSB were negatively correlated to seed yield the following year (Table 2). Measurements taken during SP indicated NSB was negatively correlated to seed yield that year, whereas STB and the amount of N in STB (STB%N) were positively correlated to seed yield (Table 2). Measurements taken PS had little to no impact on seed yield. These results show the importance of removing residue (both STB and NSB) after seed harvest, high N uptake of regrowth (STB %N) PT and F, good stand overwintering, and spring regrowth (STB in SP) on seed production. Yield correlation with STB %N in F indicates the importance of fall N uptake for seed yield the following year. The factor that tended to negatively affect seed yield the most was NSB in the fall (Table 5; Figure 4a). The amount of NSB in the F was not significantly correlated to yield in the FLB treatment but was in all the other treatments (Figure 4a), suggesting FLB reduced NSB levels across all years and to levels low enough to support high seed yields. The factor that most positively affected seed yield was N content of STB (STB %N) measured in the fall (Table 5; Figure 4b). This research, like previous studies, showed the importance of the fall period for determining seed production the following year (Holman, Hunt, Johnson‐Maynard, et al., 2007). Nonstanding biomass remaining in the field during late summer and fall have long been associated with decreased yields (Lamb & Murray, 1999). The data presented here, however, highlight the importance of linking residue and N management practices to reduce NSB and STB and increase N uptake between seed harvest and fall. Furthermore, through these findings and further testing, it might be possible to measure these variables the year prior to seed harvest and predict the seed yield potential of the next crop. The best N management practice might vary by residue management practice, and future research should test methods that would increase N uptake in the fall, particularly within reduced‐burn and no‐burn residue management practices. Yield may be predicted by STB %N, although more research is needed to determine the exact causes for yield relationships and if similar correlations are found for multiple cultivars and locations. If yield can be reasonably predicted in the fall long before seed harvest, then in years of low predicted seed yield, bluegrass biomass can be used as forage or the field can be taken out of production. Growers may also choose to alter fertilizer amounts and/or timing depending upon expected crop usage and seed yield.5TABLEAcross‐treatment correlation between seed yield and select residue parameters, standing biomass (STB), nonstanding residue (NSB), N concentration in standing biomass (STB %N), and amount of N in standing biomass (STBN) measured at sampling periods during the cropping years 2003–2007Correlation coefficient (r)Time periodaSTBNSBSTB %NSTBNPT−.42−.65.28−.39F−.35−.78.80.18SP.38−.38−.26.38PS.19−.18−.39−.03Note. Bold values indicate significant correlation (P < .05).aPT, post‐treatment; F, fall; SP, spring; PS, pre‐swath.4FIGUREThe relationship between (a) nonstanding residue (NSR) and (b) standing biomass %N (STB %N) to yield during the fall sampling period. Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3)Economic assessmentWolfley et al. (2006) reported the profitability for the first 4 yr of this study (2001–2004), before seed yield declined in mechanical treatments. They reported BB as the most profitable system, with a NPV of US$867 ha−1, including the amortized cost of establishment, followed closely by FLB with a positive per‐acre NPV of $862. The NPV for MEC and SYS were $343 and $346 ha−1, respectively. Both MEC and SYS were hindered by lower seed yields and higher costs.This study presents economic returns for all residue management systems and years for the full stand life (2001–2007). Stand establishment was amortized over the 7‐yr stand life for each system. Net present value for each system was calculated annually, based on a discount rate of 5.75%, in order to determine the optimal stand life given current price assumptions and compare profitability across treatments. Input prices (Table 2) and machinery cost estimates were used to develop annual budgets. Together with crop price estimates, annual budgets for all practices and years were used to estimate profitability (Table 6). Straw price is dependent on market demand (regional drought and shortage of feed resources), and seed price is dependent on whether the seed is a nonprotected cultivar or proprietary cultivar grown under contract. Revenue is highly dependent on the market price of straw and seed, so a range of NPV was provided (Table 6). Total revenue varied by residue management system, seed yield, and stand life. Total and variable costs varied across production practices due to differences in management, inputs, and production. Variable costs were $486 ha−1 in FLB, varied between $482 and $706 ha−1 for BB, varied between $545 and $696 ha−1 in MEC, and varied between $486 and $594 ha−1 in SYS (Table 6). Harvesting costs were dependent on bluegrass yield (i.e., the greater the yield, the higher the harvest cost). Therefore, there was variation in harvest cost per year. Stand establishment was $723 ha−1, value of seed sold was $1.65 kg−1 (low seed price) and $2.78 kg−1 (high seed price), and value of straw sold was $0.06 kg−1 (low straw price) and $0.09 kg−1 (high straw price) (Table 6).6TABLESummary of economic returns: total revenue (TR), total costs (TC), returns over TC, variable costs (VC), returns over VC, and net present value (NPV) for Kentucky bluegrass production practicesTreatmentStand ageYearTRTCReturns over TCVCReturns over VCNPVLSLSLSHSHSLSHSHSUS$FLBEstablishment0723−723482−4820000120011,2561,144112486770−164−164100100220022,7071,5791,1284862,2212512511,0541,054320032,1071,3997084861,6214204201,6201,620420041,7701,2984714861,2834614611,9761,976520051,8311,3175144861,345521a5212,3442,344620061,4821,2122704869964684682,5272,527720071,3591,2171424868733543542,6182,618BBEstablishment0723−723482−4820000120011,2561,266−10542714−273−273−9−9220023,4111,8791,5327062,7054767591,2871,569320032,5161,6288896231,8947751,1891,9972,411420042,4051,5818246881,7171,0761,7142,6213,259520052,0401,4795616511,3891,2082,0043,0223,818620061,5081,3551536518581,1222,0273,1254,030720071,0811,271−1916514308641,8243,0033,963MECEstablishment0723−723000000120011,2561,308−52545711−311−311−46−46220022,9201,8091,1116592,2611403258941,079320031,8941,5003946011,2931404111,2081,479420041,8701,4943756961,1742006891,4921,982520051,2621,311−4961964335941,4582,049620066281,120−49258345−4052351,1251,766720076401,123−48356377−799−1368161,479SYSEstablishment0723−723000000120011,2561,368−112594662−364−369−100−104220022,6811,5961,0855832,0981113028171,008320031,9061,3395674861,4202063971,2711,46242004,2311,5037285941,6374538391,8212,207520051,1991,207−85836162937781,8162,30162006751992−242486265104951,6532,138 720071,3321,357−25594738−1903281,6362,154Note. Production practices were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and a rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3). Establishment costs were amortized over the 7‐yr stand life with a 5.75% discount rate. The NPV of each rotation was calculated annually in order to compare systems across time and determine the year in which net returns were maximized for each system. Net present value with low seed and low straw value (LSLS), low seed and low straw value (LSLS), high seed and low straw value (HSLS), and high seed and high straw value (HSHS).aHighest NPV in bold indicates the year in which the stand should be terminated in order to maximize profits.Net present value represents all previous cash flows less all costs, adjusted for the discount rate. Using these cost and price assumptions, the BB system was most profitable, maximizing returns with a NPV between $1,208 and $4,030 ha−1 in Year 6, or Year 4 when both seed and straw price were low (Table 6). The second most profitable treatment was FLB with a NPV between $521 and $2,618 ha−1 in Year 5 when seed price was low or Year 7 when seed price was high. The third most profitable system was SYS with a NPV between $453 and $2,301 ha−1 in Year 4, or in Year 5 when both seed and straw price were high. Lastly, MEC was the least profitable treatment, with a NPV between $200 and $2,049 ha−1 in Year 4, or Year 5 when both seed and straw price were high (Table 6).Revenue from bluegrass straw, particularly for the higher seed yielding BB system, explains higher profitability for systems with residue baling. The relative profitability of FLB vs. the other systems depends on the value and market for bluegrass straw. At times, the price for bluegrass straw may be too low to justify harvesting. In that case, only FLB would be profitable.CONCLUSIONResidue removal after seed harvest, particularly that portion of the residue that is not standing, was critical for obtaining high seed yield. Nonstanding residue removal is difficult to accomplish using mechanical methods, and FLB performed the best at removing this fraction of the residue. Reduced‐burn methods (BB and SYS), however, were able to keep this residue level lower than mechanical methods (MEC), and therefore also yielded more than MEC.Another factor that contributed to high seed yield was the percent N of regrowth in the fall. Future research on how to increase fall N uptake across burn, reduced burn, and mechanical production systems is needed. Additional research is also needed to determine the exact causes for these yield relationships and if similar relationships are found for multiple cultivars and locations. This study suggests it might be possible to predict yield potential by measuring the amount of nonstanding residue and N in the fall. Being able to estimate yield potential a year early would allow a producer to make an informed decision whether it would be most profitable to keep a stand in production or take it out and plant another crop. Making informed decisions on seed yield potential would result in significant economic improvements for the Kentucky bluegrass growers.Profitability, as determined by annual revenue less expenses and NPV over time, was followed the order of BB > FLB > SYS > MEC. These results are dependent upon the price assumptions used in this study. Without a bluegrass straw market, only the FLB system is profitable. If both seed and straw can be marketed above the cost of production, then BB was the most profitable treatment. Reduced‐burn systems have greater input costs and lower yields over time. This research suggests reduced burning can maintain productive stands in the first 4–5 yr of the stand life. The complete elimination of field burning in northern Idaho would result in lower seed yield and reduced stand longevity. Producers would need a stable bluegrass straw market in order to profitably raise bluegrass seed without the option of field burning. In order to maintain profitability when producing this seed crop, producers need to know market prices for both seed and straw. We concluded that profitability was highest with BB and FLB. Overall, understanding the factors that contribute to stand decline may help growers prolong stand life and increase profitability, while mitigating air quality issues.ACKNOWLEDGMENTSThe authors would like to thank Karl Umiker and Janice Reed for assistance with data collection.AUTHOR CONTRIBUTIONSJohn Holman: Conceptualization; Data curation; Funding acquisition; Investigation; Methodology; Project administration; Writing – original draft; Writing – review & editing. Jack D. Robertson: Data curation; Investigation; Methodology. Jodi Johnson‐Maynard: Conceptualization; Funding acquisition; Investigation; Methodology; Project administration; Writing – review & editing. Kathleen Painter: Formal analysis; Investigation; Methodology; Project administration; Writing – review & editing. Yared Assefa: Formal analysis; Validation; Visualization; Writing – review & editing.CONFLICT OF INTERESTThe authors declare no conflict of interest.REFERENCESAamlid, T. S. (1991). Seed production of smooth meadow grass (Poa pratensis L.) as influenced by soil type, pH and compaction. II. Seed yields and other plant characteristics. Norwegian Journal of Agricultural Sciences, 5, 321–338.Aamlid, T. S. (1992). Effects of temperature and photoperiod on growth and development of tillers and rhizomes in Poa pratensis L. ecotypes. Annals of Botany, 69, 289–296. https://doi.org/10.1093/oxfordjournals.aob.a088344Adams, D., Law, A. G., Canode, C. L., Jensen, M., McCool, D. K., Papendick, R. I., Bruehl, W., Oetting, R. D., Anderson, C., Wirth, M. E., & Burt, C. (1976). Alternatives to open field burning of grass seed field residues (Progress Report). Washington State University and USDA‐ARS.Banowetz, G. M., Griffith, S. M., & El‐Nashaar, H. M. (2009). Mineral content of grasses grown for seed in low rainfall areas of the Pacific Northwest and analysis of ash from gasification of bluegrass (Poa pratensis L.) straw. Energy and Fuels, 23, 502–506. https://doi.org/10.1021/ef800490wBarth, V. P., Reardon, C. L., Coffey, T., Klein, A. M., Mcfarland, C., Huggins, D. R., & Sullivan, T. S. (2018). Stratification of soil chemical and microbial properties under no‐till after liming. Applied Soil Ecology, 130, 169–177. https://doi.org/10.1016/j.apsoil.2018.06.001Berndt, W. L. (2008). Double exponential model describes decay of hybrid bermudagrass thatch. Crop Science, 48, 2437–2446. https://doi.org/10.2135/cropsci2008.01.0056Brown, T. T., Koenig, R. T., Huggins, D. R., Harsh, J. B., & Rossi, R. E. (2008). Lime effects on soil acidity, crop yield, and aluminum chemistry in direct‐seeded cropping systems. Soil Science Society of America journal, 72, 634–640. https://doi.org/10.2136/sssaj2007.0061Canode, C. L., & Law, A. G. (1975). Seed production of Kentucky bluegrass associated with age of stand. Agronomy Journal, 67, 790–794. https://doi.org/10.2134/agronj1975.00021962006700060016xCanode, C. L., & Law, A. G. (1979). Thatch and tiller size as influenced by residue management in Kentucky bluegrass seed production. Agronomy Journal, 71, 289–291. https://doi.org/10.2134/agronj1979.00021962007100020017xCarlson, J. M., Ehlke, N. J., & Wyse, D. L. (1995). Environmental control of floral induction and development in Kentucky bluegrass. Crop Science, 35, 1127–1132. https://doi.org/10.2135/cropsci1995.0011183X003500040035xChastain, T. G., Kiemnec, G. L., Cook, G. H., Garbacik, C. J., & Quebbeman, B. M. (1995). Potential alternatives to field burning in the Grande Ronde valley (Seed Production Research Report). Oregon State University and USDA‐ARS.Chastain, T. G., Kiemnec, G. L., Cook, G. H., Garbacik, C. J., Quebbeman, B. M., & Crowe, F. J. (1997). Residue management strategies for Kentucky bluegrass seed production. Crop Science, 37, 1836–1840. https://doi.org/10.2135/cropsci1997.0011183X003700060029xChilcote, D. O., Youngberg, H. W., Stanwood, P. C., & Kim, S. (1978). Post‐harvest residue burning effects on perennial grass development and seed yield. In Hebblethwaite, P. D. (Ed.), Seed production (pp. 91–103). Butterworths.Ensign, R. D., Hickey, V. G., & Bernardo, M. D. (1983). Seed yield of Kentucky bluegrass as affected by post‐harvest residue removal. Agronomy Journal, 75, 549–551. https://doi.org/10.2134/agronj1983.00021962007500030030xEvans, D. W. (1980). Stand thinning in seed production of Cougar Kentucky bluegrass. Agronomy Journal, 72, 525–527. https://doi.org/10.2134/agronj1980.00021962007200030027xHolman, J. D., Hunt, C., Johnson‐Maynard, J., Vantassell, L., & Thill, D. (2007). Livestock use as a non‐thermal residue management practice in Kentucky bluegrass seed production systems. Agronomy Journal, 99, 203–210. https://doi.org/10.2134/agronj2006.0048Holman, J. D., Hunt, C., & Thill, D. (2007). Structural composition, growth stage, and cultivar effects on Kentucky bluegrass forage yield and nutrient composition. Agronomy Journal, 99(1), 195–202. https://doi.org/10.2134/agronj2006.0047Holman, J. D., Hunt, C., & Thill, D. (2011). Effect of harvest processes on the nutritive value of Kentucky bluegrass residue from seed harvest. Forage and Grazinglands, 9(1), https://doi.org/10.1094/FG‐2011‐1228‐01‐RSHolman, J. D., & Thill, D. C. (2005). Kentucky bluegrass seed production. University of Idaho Extension.Kopp, K. L., & Guillard, K. (2004). Decomposition rates and nitrogen release of turf grass clippings. In New directions for a diverse planet. Proceedings of the 4th International Crop Science Congress. Regional Institute. http://www.cropscience.org.au/icsc2004/poster/2/5/2/860_koppk.htmLamb, P. F., & Murray, G. A. (1999). Kentucky bluegrass seed and vegetative responses to residue management and fall nitrogen. Crop Science, 39, 1416––1423. https://doi.org/10.2135/cropsci1999.3951416xMahler, R. L., & McDole, R. E. (1987). The relationship of soil pH and crop yields in northern Idaho (Cooperative Extension Publication, Current Information Series No. 811). University of Idaho.Palazzo, A. J., & Duell, R. W. (1974). Responses of grasses and legumes to soil pH. Agronomy Journal, 66, 678–682. https://doi.org/10.2134/agronj1974.00021962006600050022xSAS Institute. (2008). SAS version 9.2. SAS Institute.Soil Survey Staff. (2020). Web soil survey. USDA‐NRCS. http://websoilsurvey.nrcs.usda.gov/Tabatabai, M. A., & Bremner, J. M. (1991). Automated instruments for determination of total carbon, nitrogen, and sulfur in soils by combustion techniques. In Smith, K. A. (Ed.), Soil analysis (pp. 261–286). Marcel Dekker.Thomas, G. W. (1996). Soil pH and soil acidity. In Sparks, D. L. et al. (Eds.), Methods of soil analysis. Part 3. Chemical methods (pp. 475–490). SSSA and ASA. https://doi.org/10.2136/sssabookser5.3.c16Thompson, D. J., & Clark, K. W. (1989). Influence of nitrogen fertilization and mechanical stubble removal on seed production of Kentucky bluegrass in Manitoba. Canadian Journal of Plant Science, 69, 939–943. https://doi.org/10.4141/cjps89‐114Umiker, K. J., Johnson‐Maynard, J. L., Hatten, T. D., Eigenbrode, S. D., & Bosque‐Pérez, N. A. (2009). Soil carbon, nitrogen, pH, and earthworm density as influenced by cropping practices in the inland Pacific Northwest. Soil Tillage Research, 105, 184–191.Wolfley, J., Van Tassell, L., Smathers, R., Holman, J., Thill, D., & Reed, J. (2006). Economic analysis of experimental thermal and non‐thermal residue managements for Kentucky bluegrass seed (Bulletin 847). University of Idaho Extension.Youngberg, H. W. (1980). Techniques of seed production in Oregon. In Hebblethwaite, P. D. (Ed.), Seed production (pp. 203–213). Butterworths. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Agrosystems, Geosciences & Environment" Wiley

Burn and mechanical residue removal methods on production‐life of Kentucky bluegrass

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Wiley
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© 2022 Crop Science Society of America and American Society of Agronomy.
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2639-6696
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10.1002/agg2.20282
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Abstract

AbbreviationsBBbale then burnFfallFLBfull load burnMECbale then mow then harrowNPVnet present valueNSBnonstanding biomassPSpre‐swathingPTpost‐residue treatmentSPspringSTBstanding biomassSYSrotation systemINTRODUCTIONIt has long been known that accumulation of postharvest residues reduces Kentucky bluegrass (Poa pratensis L.) seed yields due to the interruption of necessary light (Ensign et al., 1983) and temperature (Aamlid, 1992; Carlson et al., 1995) stimuli required for optimal seed production. Burning not only removes much of this excess residue but also kills many of the surviving older, nonreproductive tillers, allowing the development of new tillers (Chilcote et al., 1978). Mechanical methods such as mowing, raking, baling, harrowing, grazing, gapping, and/or vacuuming have been studied and were widely used in bluegrass seed production systems in Washington and Oregon (Chastain et al., 1997; Evans, 1980; Holman, Hunt, Johnson‐Maynard, et al., 2007; Lamb & Murray, 1999; Youngberg, 1980). Generally, these mechanical methods provide seed yields comparable with burning during the first 1–2 yr of seed production, yet with certain methods and cultivars, comparable yields were achieved through the third (Lamb & Murray, 1999) and fifth (Chastain et al., 1997) seed harvest. Nonthermal residue management, therefore, generally results in premature stand decline as compared with field burning. In addition, postharvest residue is low in forage nutritive value, economic value, and demand (Holman, Hunt, & Thill, 2007; Holman, Hunt, Johnson‐Maynard, et al., 2007; Holman et al., 2011), all of which are disincentives to manage postharvest residue other than through field burning.Regardless of residue removal method, maintaining high seed yields for many years prolongs the expense of stand reestablishment and strongly influences the profitability of growing dryland Kentucky bluegrass seed. Newly established stands produce little to no seed yield the first year in dryland production areas like northern Idaho. In addition, reestablishment involves tillage and the planting of annual crops prior to Kentucky bluegrass reestablishment, increasing the rates of soil erosion and loss. Even fields that are burned annually eventually succumb to weakening of the stand, decreased yields, weed and disease pressure, and ultimately crop failure. Since seed yield can be influenced by residue management (Ensign et al., 1983), as well as stand age (Canode & Law, 1975), the long‐term comparisons of residue management practices in this study are critical to understanding the factors affecting seed yield in the latter years of a Kentucky bluegrass stand.Past published reports on Kentucky bluegrass stand decline have focused on environmental factors such as light and temperature. Relatively few studies have focused on biomass and residue dynamics overtime as a stand aged. In addition to direct effects of residue management and stand age on seed production, indirect effects may occur due to changes in nutrient cycling. For example, continued application of high rates of nitrogen (N) fertilizer resulted in faster yield decline, whereas lower N rates maintained yields as the stand aged in a study conducted in Prosser, WA (Evans, 1980). This is consistent with the idea that fertilizer rates and timing need to be optimized for a particular residue management practice (Lamb & Murray, 1999). Residue management may influence N availability in multiple ways. Although burning may result in the loss of a fraction of N through volatilization, it may also cause a relatively rapid flush of N. Nitrogen may also be lost in nonthermal practices when residue N is baled and removed from the field (Banowetz et al., 2009). Finally, depending on the chemical composition of the specific cultivar grown (Holman, Hunt, & Thill, 2007), accumulation of residue in nonthermal practices may result in either the loss of available N through microbial immobilization or the return of N through mineralization.Many areas where bluegrass seed is produced in northern Idaho are characterized as having moderately to strongly acid soils, in part due to continual applications of relatively high rates of N fertilizer. Acidification of near surface soil in no‐till agricultural systems in this region has been noted (Barth, et al., 2018; Brown et al., 2008; Mahler & McDole, 1987). Similar to no‐till systems, bluegrass residue may be left on the surface to decompose in nonburn systems, and tillage is restricted to stand take out. Acidification of near‐surface soil in these already acid soils may lead to fertility problems including aluminum (Al) toxicity and alteration of the availability of other nutrients.The purpose of this research was to investigate, over the entire expected lifespan of a Kentucky bluegrass stand in northern Idaho, the integration of thermal and nonthermal residue management practices on seed yield and profitability. This work represents one of the few studies to focus on the entire expected life of a Kentucky bluegrass stand in dryland production regions. In this study, we propose new explanations for stand and seed yield decline and new methods for predicting seed yields. A thorough understanding of seed yield potential over the entire life of the stand across various residue management practices is critically important to seed producers, policy makers, and the economic sustainability of the region.Core IdeasSeed yields were the greatest in full load burn, followed by bale then burn treatments.Seed yield was negatively influenced by nonstanding residue and positively by N content of fall standing biomass.Like seed yield, profitability was also greatest in bale then burn and full load burn treatments.Profitability of mechanical management practices is dependent on highly variable forage and feed prices.MATERIAL AND METHODSSite, experimental design, and managementThe experimental site was located in Kootenai County, Idaho (47°28′0′′ N, 116°57′0′′ W) on a grower‐cooperator field. The field was planted to the cultivar ‘Alene’, characterized as a nonaggressive Kentucky bluegrass cultivar. The soil type was a Taney silt loam (fine‐silty, mixed, superactive, frigid Vitrandic Argixerolls) (Soil Survey Staff, 2020). The site receives approximately 600 mm of precipitation annually, the majority of which occurs during the winter and early spring. Our experimental design was a randomized complete block with four blocks and a total of 16 plots. Plots measured 18–21 m wide and 91 m long in order to accommodate full‐sized commercial farm equipment and plot burning. Four residue removal treatments including full load burn (FLB), bale then burn (BB), bale then mow then harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3) were evaluated over a 7‐yr period from establishment in 2001–2007. The treatments were selected to study the effect of burn, reduced burn, and mechanical residue management on residue dynamics, stand productivity, and seed yield. After harvest, the FLB treatments were burned without removal of any plant material, BB treatments were baled prior to burning, and the MEC plots were baled, mowed, and harrowed. Either a rotary or flail mower and a heavy flex harrow was used. Since mowing occurred only after significant regrowth, harrowing sometimes preceded mowing. Fertilization with ammonium nitrate occurred annually in late fall with application of 168 kg N ha−1 applied across all treatments. Plots were swathed in early July and harvested between mid‐July and early August using a pickup header on a field‐scale combine (Table 1). Whole‐plot seed weight was measured, and a subsample was processed through screens to determine clean seed weight. Residue removal treatments were applied mid‐August to late September. Herbicide applications of pendimethalin, metolachlor, dicamba and clopyralid were made to the plots as necessary to primarily control Ventenata [Ventenata dubia (Leers) Cross] and Cirsium spp. (‘thistle’).1TABLESampling dates of Kentucky bluegrass standing biomass and nonstanding residue at specific time periods throughout the studyTime period200220032004200520062007Spring (SP)–7 May6 May11 May9 May7 MayPre‐swath (PS)–23 June16 June1 July28 June15 JunePost‐treatment (PT)9 Sept.19 Sept.30 Aug.22 Sept.25 Sept .–Fall (F)22 Oct.22 Oct.4 Nov.24 Oct.22 Oct.–Data collected, sampling, and laboratory analysisKentucky bluegrass standing biomass (STB) and nonstanding biomass (NSB) were collected four times throughout the cropping year: spring (SP), pre‐swathing (PS), post‐residue treatment (PT), and fall (F). The sampling dates within each period varied slightly from year to year due to field operations and/or inclement weather (Table 1). Bluegrass samples were collected from each of three randomly placed, 0.25‐m2 quadrats per plot. A small rake was used to remove the NSB from the quadrat before clipping and removal of STB. Standing biomass and NSB samples were dried in an oven at 60 °C for 48 h. Any visible soil aggregates or rocks were removed, and then STB and NSB were weighed and ground. Residue removal through baling was calculated as the difference between residue measured pre‐swath and post‐baling. Ground subsamples were ashed at 500 °C for 4 h to correct for mineral content. Three soil samples from each plot were collected at the 0‐to‐10‐cm depth and combined for pH analysis after swathing in the first and last years of the study. A 1:2 soil/water slurry was used for pH measurements adapted from Thomas (1996). Total carbon (C) and N in STB and NSB was measured by dry combustion in a VarioMax C/N/S analyzer (Elementar) (Tabatabai & Bremner, 1991). The harvest index was calculated as the clean seed yield (kg ha−1) divided by the amount of STB (kg ha−1) (Thompson & Clark, 1989).Statistical data analysisInfluence of residue management techniques on yield, harvest index, STB, NSB, and N content were analyzed by repeated measures ANOVA in SAS (SAS Institute, 2008). A one‐way ANOVA was used to test for soil pH changes over time. Significant effects were determined at P ≤ .05, and Fishers LSD was conducted for mean separation.The linear relationship between seed yield and stand age was studied by regressing measured seed yield against standing age using PROC REG procedure of SAS. The relationship between seed yield to STB, NSB, %C, and %N measured during each sampling period (PT, F, SP, and PS) was determined using correlation analysis in PROC CORR procedure of SAS.Economic returns for each management practice were compared over the life of the stand by estimating the net present value (NPV) for each year using an enterprise budget approach and a discount rate of 5.75% (Table 2). Input and return prices were based on current costs of inputs and expected returns in this region. Both variable and total costs were determined for each production practice. Costs varied by production practice due to differences in inputs and management. Variable costs included fertilizer, pesticide, and annual machinery and harvest expense. Fixed costs included machinery depreciation and interest, land rent, overhead, and management.2TABLEInput prices for establishing Kentucky bluegrass production budgets from a survey of input providers in the regionExpenseItemUnitPrice unit−1FuelDiesel, offroad, bulkL$0.74GasL$0.66FertilizerNitrogen (liquid)kg$0.97Phosphorusakg$1.32Sulfur (liquid)kg$0.88Potassium (dry)kg$0.95Gypsumkg$0.35AdjuvantsAmmonium sulfate (20‐0‐0‐24)kg$0.97Ammonium sulfate (liquid)L$0.02Adjuvant (antifoam)L$0.0008Crop oil concentrateL$4.22Nonionic surfactantL$0.0001Pesticides2,4‐Dichlorophenoxyacetic acid (47.3% a.i.b)L$0.0004Quizalofop‐p‐ethyl (10.3% a.i.)L$0.0027Primisfulforn‐methyl urea (75% a.i.)L$0.12Bromoxynil octanoate (28% a.i.)L$0.0013Diuron (80% a.i.)L$0.0048Flucarbazone‐sodium (66% a.i.)L$0.024Tribenuron methyl (75% a.i.)L$0.043Glyphosate‐potassium salt (48.8% a.i.)L$0.0004SeedBluegrass seed for establishment (common)kg$3.31Custom hireChemical applicator (aerial)ha$22.12Chemical applicator (ground)ha$21.00Fertilizer applicatorha$4.94LaborcHourly machine laborh$20.00Other laborh$12.00Other costsOverheadd%2.50%Management feee%5.00%InterestOperating loan%5.75%Machinery loan/investment%5.75%SalesLow bluegrass seed uncleanedkg$1.65High bluegrass seed uncleanedkg$2.78Low bluegrass straw price (100% dry matter)kg$0.06High bluegrass straw price (100% dry matter)kg$0.09aAverage of dry and liquid formulation.ba.i. = pesticide % active ingredient.cCovers all applicable state and federal taxes.dCovers legal, accounting, and utility fees as percentage of operating expenses.eCalculated as a percentage of gross revenue.RESULTS AND DISCUSSIONNonstanding biomass dynamicsOf the time periods measured (SP‐PS‐PT‐F), the greatest amounts of NSB were present during PT and F periods (Figure 1a). Each year during the PT period, the burn treatments, FLB and BB, resulted in lower amounts of NSB compared with the MEC treatment, except in 2005 when all treatments were similar (Table 3). The higher PT residue levels following field burning were due to a spotty burn, which was caused by precipitation prior to burning and subsequent damp postharvest residue and stand regrowth. The SYS treatment had higher NSB levels after BB in fall 2003 (2004 crop year) than did FLB or BB, which may have been the result of not burning in fall 2002 (MEC). Again, in 2006, there was more NSB in SYS than in FLB or BB when it was not burned in fall 2005 (MEC). In the 2007 crop year, NSB levels in SYS were not different than those measured in the other treatments. These results indicate that occasional burning did not consistently reduce NSB in the SYS treatment to the extent found in FLB or BB.1FIGURE(a) Average nonstanding biomass (NSB) and (b) standing biomass (STB) by time period from 2003–2007. Kentucky bluegrass samples collected in 2002 were not separated into nonstanding residue and standing biomass. Time periods sampled were post‐treatment (PT), fall (F), spring (SP), and pre‐swath (PS). Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3). Significant residue differences between treatments within a time period are noted by letters; treatments that share the same letter or have no letters are not significantly different at P < .053TABLENonstanding Kentucky bluegrass residue amount at post‐treatment (PT) and fall (F) time periodsStand age (harvest year)Residue removal treatmentaFLBBBMECSYSResidue amount (Mg ha−2) at PT4 (2004)1.64ab2.16a3.61b2.96b5 (2005)1.13a1.22a1.80a1.54a6 (2006)1.70a1.03a3.87b3.57b7 (2007)0.72a1.17a2.43b1.57abResidue amount (Mg ha−2) at F4 (2004)1.60ab1.92a4.34c3.51b5 (2005)0.24a0.54a1.94b0.60a6 (2006)1.56a1.29a3.12b4.28c7 (2007)0.31a0.70b1.73c0.65baFLB, full load burn; BB, bale then burn; MEC, bale–mow–harrow; SYS, practice (BB [2003]–FLB [2004]–MEC [2005]–BB [2006]).bDifferent letters indicate differences within a harvest year (P ≤ .05).Within the remaining sampling periods (SP, PS, F), NSB amounts were similar between the burn treatments (FLB and BB) and were different between the burn and MEC treatment (Figure 1a). Amounts of NSB in the SYS treatment were generally between those measured in the MEC, BB, or FLB treatments, reflecting the fact that the SYS treatment consisted of each treatment on a rotating basis. Throughout the growth cycle beginning with the PT period, the greatest decrease in NSB was between F and SP, when moist conditions and insulating snow cover promoted rapid residue decomposition. Amounts of NSB in the MEC treatment decreased by an average of 2.1 Mg ha−1, between F and SP, significantly more than the decreases observed in FLB (0.8 Mg ha−1) or BB (1.0 Mg ha−1) (Figure 1a). Greater amounts of fall residue in the MEC treatment likely promoted over‐winter decomposition by providing a suitable local environment for decomposer communities. Likewise, first‐order decomposition rate equations as used by Berndt (2008) for thatch and Kopp and Guillard (2004) for grass clippings predict greater decomposition losses when greater amounts of residue are initially present. Although in situ decomposition of NSB as in the MEC treatment has long‐term benefits such as the return of nutrients to the soil, burning resulted in lower NSB during the critical F time period (Table 3), most certainly promoting seed yield.Standing biomass dynamicsStanding stubble biomass remaining after harvest (Thompson & Clark, 1989) and late summer tiller regrowth (Chastain et al., 1997) were both considered STB in this study. Standing stubble interferes with light and temperature induction stimuli and results in lower seed production. During the PT period, average STB was greater every year in the MEC treatment than in the FLB or BB treatments (Table 3). Average amounts of STB were 1.40 Mg ha−1 in MEC, 0.14 Mg ha−1 in SYS, 0.13 Mg ha−1 in BB, and 0.11 Mg ha−1 in FLB (Figure 1b). Lower amounts of STB in the burn treatments are a result of efficient burning (removal) of both growing and nongrowing (stubble) STB.Regrowth between the PT and F periods tended to be significantly greater in the SYS (1.12 Mg ha−1), FLB (0.76 Mg ha−1), and BB (0.77 Mg ha−1) treatments as compared with the MEC (0.31 Mg ha−1) treatment (Figure 1b). The growth rate and amount of STB in SYS varied by year and averaged between MEC and burn treatments, again reflecting the year‐to‐year variability when the treatment was burned or not. The regrowth in STB from PT to F periods highlights the transition from summer dormancy, induced by dry, hot conditions, into cool, sometimes moist, fall conditions. If moisture is not limiting, higher mid‐ to late‐fall soil temperature promoted by the blackened surface in the burn treatments may be a factor in determining the amount of regrowth. Greater regrowth of burned vs. unburned red fescue was attributed to higher November soil surface temperatures when burned (Chilcote et al., 1978). In addition to soil temperature, burning of excess tillers around the base of the plant may also have a stimulatory effect on new tiller production (Chilcote et al., 1978).Annual seed yieldMany factors influence annual seed yield of bluegrass including environment, cultivar, age, fertility, and moisture availability (Holman & Thill, 2005). Seed yield was affected by residue management treatment each year (Table 4) after establishment (2001). The MEC treatment yielded less than FLB and BB every year except the first year (Table 4). Previous studies also reported lower yields when mechanical methods of residue removal were compared with burning postharvest residue (Adams et al., 1976; Chastain et al., 1995; Ensign et al., 1983). Bale‐burn yielded less than FLB in 2006 and 2007, which was likely due to spotty burns caused by precipitation wetting postharvest residue and initiating some bluegrass regrowth prior to burning. Bluegrass yield might be particularly sensitive to spotty burns in BB or SYS as stands age, evident in Years 6 and 7 of this stand. Seed yields in the SYS treatment fluctuated from year to year depending on the residue management treatment implemented. When the SYS treatment was BB or FLB yields were high and similar to the FLB treatment, yet in years that the SYS treatment was MEC, yields were lower than those measured in FLB or BB. Across the life of the stand, yields tended to decrease as less burning was used FLB > BB > SYS > MEC (Table 4). The harvest index (seed yield/STB, Figure 2), was very similar to seed yield, indicating that residue management practice affected plant biomass production as well as seed yield. Seed yield in SYS was significantly less than that measured in the burn treatments in three of the study years. Although reduced yield affects profitability, alternatives to FLB may be viable and allow producers to sustain stand productivity while reducing negative impacts to air quality.4TABLEWithin‐year and average across‐year treatment effect on seed yieldStand age (harvest rear)Treatmenta1 (2001)2 (2002)3 (2003)4 (2004)5 (2005)6 (2006)7 (2007)Meankg ha−1FLB5041,087a846a710a734a595a545a774aBB5041,101a878a726a642a476b439b742bMEC504986b669b506b387b191d227c525dSYS504890c765ab685a369b302c489ab600cLSDNS6515095159917623aFLB, full load burn; BB, bale then burn; MEC, bale–mow–harrow; SYS, practice (MEC [2002]–BB [2003]–FLB [2004]–MEC [2005]–BB [2006]–FLB[2007]).2FIGUREHarvest index (HI) comparisons by year. Different letters indicate differences within a year (P ≤ .05). Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3). Treatments applied each year in the SYS treatment are shown. Harvest index = seed yield (kg ha−1)/standing biomass (STB) (kg ha−1), where STB is sampled pre‐swath (PS)Seed yield decline with stand ageAlthough seed yields typically decline as Kentucky bluegrass stands age (Canode & Law, 1975; Lamb & Murray, 1999), some residue removal strategies maintain annual yields longer than others, thereby increasing economic longevity of the stand (Evans, 1980; Holman & Thill, 2005; Holman, Hunt, Johnson‐Maynard, et al., 2007; Lamb & Murray, 1999). In this study, rate of yield decline was highly correlated with stand age (R2 = .92), and it was greater for MEC than for burn treatments (Figure 3). During the 7‐yr period (2001–2007), seed yield decreased an average of 98 kg ha−1 yr−1 in FLB, 106 kg ha−1 yr−1 in SYS, 132 kg ha−1 yr−1 in BB, and 153 kg ha−1 yr−1 MEC (Figure 3). The rate of decline in SYS may have been underestimated due to unexpectedly high yields in this treatment in 2007 (Figure 3), attributed to FLB management in fall 2006. The seed yield decline under FLB was comparable with that reported in a previous study that found seed production decreased 81 kg ha−1 yr−1 from the first through the fifth seed harvests when burned annually (Canode & Law, 1975). Decreased seed yield with burning was attributed to a decrease in panicles per plant (Canode & Law, 1975), and the formation of a sod‐bound stand (Canode & Law, 1979) over time. These are the first published rates of stand decline over the life of a stand in a long‐term, side‐by‐side comparisons of burn, reduced‐burn, and mechanical residue management practices.3FIGUREDeclining seed productivity of treatments between 2002 and 2007. Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3)Another factor that might have influenced the rate of yield decline across treatments is related to management effects on soil pH. Although soil pH was not significantly different across treatments, it did show an average decrease from 5.10 in the beginning of our study to 4.96 in Year 5 (data not presented). The availability of essential nutrients and potentially toxic elements such as Al, depend on soil pH. A decline in surface soil pH has occurred in other agricultural practices in the region that use reduced tillage and ammonium‐based fertilization (Brown et al., 2008; Umiker et al., 2009). Palazzo and Duell (1974) achieved maximum aboveground growth of Kentucky bluegrass when soil pH, at 0‐to‐15‐cm depth, ranged between 6.0 and 6.5, with lower pH resulting in reduced aboveground biomass production. At pH levels below 5.0 (0‐to‐20‐cm depth), poor seed and straw yields in Kentucky bluegrass were attributed to Al toxicity (Aamlid, 1991). Acidic pH in our study may have increased the availability of Al and contributed to decreases in yield and aboveground biomass as the stand aged. The lack of tillage with Kentucky bluegrass might also be causing stratification of low pH in the top 10 cm, similar to that noted in no‐till fields within the region (Brown et al., 2008).Factors affecting seed yieldNitrogen content in STB (STB %N) at PT and F were positively correlated to seed yield the following year, whereas STB and NSB were negatively correlated to seed yield the following year (Table 2). Measurements taken during SP indicated NSB was negatively correlated to seed yield that year, whereas STB and the amount of N in STB (STB%N) were positively correlated to seed yield (Table 2). Measurements taken PS had little to no impact on seed yield. These results show the importance of removing residue (both STB and NSB) after seed harvest, high N uptake of regrowth (STB %N) PT and F, good stand overwintering, and spring regrowth (STB in SP) on seed production. Yield correlation with STB %N in F indicates the importance of fall N uptake for seed yield the following year. The factor that tended to negatively affect seed yield the most was NSB in the fall (Table 5; Figure 4a). The amount of NSB in the F was not significantly correlated to yield in the FLB treatment but was in all the other treatments (Figure 4a), suggesting FLB reduced NSB levels across all years and to levels low enough to support high seed yields. The factor that most positively affected seed yield was N content of STB (STB %N) measured in the fall (Table 5; Figure 4b). This research, like previous studies, showed the importance of the fall period for determining seed production the following year (Holman, Hunt, Johnson‐Maynard, et al., 2007). Nonstanding biomass remaining in the field during late summer and fall have long been associated with decreased yields (Lamb & Murray, 1999). The data presented here, however, highlight the importance of linking residue and N management practices to reduce NSB and STB and increase N uptake between seed harvest and fall. Furthermore, through these findings and further testing, it might be possible to measure these variables the year prior to seed harvest and predict the seed yield potential of the next crop. The best N management practice might vary by residue management practice, and future research should test methods that would increase N uptake in the fall, particularly within reduced‐burn and no‐burn residue management practices. Yield may be predicted by STB %N, although more research is needed to determine the exact causes for yield relationships and if similar correlations are found for multiple cultivars and locations. If yield can be reasonably predicted in the fall long before seed harvest, then in years of low predicted seed yield, bluegrass biomass can be used as forage or the field can be taken out of production. Growers may also choose to alter fertilizer amounts and/or timing depending upon expected crop usage and seed yield.5TABLEAcross‐treatment correlation between seed yield and select residue parameters, standing biomass (STB), nonstanding residue (NSB), N concentration in standing biomass (STB %N), and amount of N in standing biomass (STBN) measured at sampling periods during the cropping years 2003–2007Correlation coefficient (r)Time periodaSTBNSBSTB %NSTBNPT−.42−.65.28−.39F−.35−.78.80.18SP.38−.38−.26.38PS.19−.18−.39−.03Note. Bold values indicate significant correlation (P < .05).aPT, post‐treatment; F, fall; SP, spring; PS, pre‐swath.4FIGUREThe relationship between (a) nonstanding residue (NSR) and (b) standing biomass %N (STB %N) to yield during the fall sampling period. Residue management treatments were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3)Economic assessmentWolfley et al. (2006) reported the profitability for the first 4 yr of this study (2001–2004), before seed yield declined in mechanical treatments. They reported BB as the most profitable system, with a NPV of US$867 ha−1, including the amortized cost of establishment, followed closely by FLB with a positive per‐acre NPV of $862. The NPV for MEC and SYS were $343 and $346 ha−1, respectively. Both MEC and SYS were hindered by lower seed yields and higher costs.This study presents economic returns for all residue management systems and years for the full stand life (2001–2007). Stand establishment was amortized over the 7‐yr stand life for each system. Net present value for each system was calculated annually, based on a discount rate of 5.75%, in order to determine the optimal stand life given current price assumptions and compare profitability across treatments. Input prices (Table 2) and machinery cost estimates were used to develop annual budgets. Together with crop price estimates, annual budgets for all practices and years were used to estimate profitability (Table 6). Straw price is dependent on market demand (regional drought and shortage of feed resources), and seed price is dependent on whether the seed is a nonprotected cultivar or proprietary cultivar grown under contract. Revenue is highly dependent on the market price of straw and seed, so a range of NPV was provided (Table 6). Total revenue varied by residue management system, seed yield, and stand life. Total and variable costs varied across production practices due to differences in management, inputs, and production. Variable costs were $486 ha−1 in FLB, varied between $482 and $706 ha−1 for BB, varied between $545 and $696 ha−1 in MEC, and varied between $486 and $594 ha−1 in SYS (Table 6). Harvesting costs were dependent on bluegrass yield (i.e., the greater the yield, the higher the harvest cost). Therefore, there was variation in harvest cost per year. Stand establishment was $723 ha−1, value of seed sold was $1.65 kg−1 (low seed price) and $2.78 kg−1 (high seed price), and value of straw sold was $0.06 kg−1 (low straw price) and $0.09 kg−1 (high straw price) (Table 6).6TABLESummary of economic returns: total revenue (TR), total costs (TC), returns over TC, variable costs (VC), returns over VC, and net present value (NPV) for Kentucky bluegrass production practicesTreatmentStand ageYearTRTCReturns over TCVCReturns over VCNPVLSLSLSHSHSLSHSHSUS$FLBEstablishment0723−723482−4820000120011,2561,144112486770−164−164100100220022,7071,5791,1284862,2212512511,0541,054320032,1071,3997084861,6214204201,6201,620420041,7701,2984714861,2834614611,9761,976520051,8311,3175144861,345521a5212,3442,344620061,4821,2122704869964684682,5272,527720071,3591,2171424868733543542,6182,618BBEstablishment0723−723482−4820000120011,2561,266−10542714−273−273−9−9220023,4111,8791,5327062,7054767591,2871,569320032,5161,6288896231,8947751,1891,9972,411420042,4051,5818246881,7171,0761,7142,6213,259520052,0401,4795616511,3891,2082,0043,0223,818620061,5081,3551536518581,1222,0273,1254,030720071,0811,271−1916514308641,8243,0033,963MECEstablishment0723−723000000120011,2561,308−52545711−311−311−46−46220022,9201,8091,1116592,2611403258941,079320031,8941,5003946011,2931404111,2081,479420041,8701,4943756961,1742006891,4921,982520051,2621,311−4961964335941,4582,049620066281,120−49258345−4052351,1251,766720076401,123−48356377−799−1368161,479SYSEstablishment0723−723000000120011,2561,368−112594662−364−369−100−104220022,6811,5961,0855832,0981113028171,008320031,9061,3395674861,4202063971,2711,46242004,2311,5037285941,6374538391,8212,207520051,1991,207−85836162937781,8162,30162006751992−242486265104951,6532,138 720071,3321,357−25594738−1903281,6362,154Note. Production practices were full load burn (FLB), bale then burn (BB), bale–mow–harrow (MEC), and a rotation system (SYS) (MEC in Year 1, BB in Year 2, and FLB in Year 3). Establishment costs were amortized over the 7‐yr stand life with a 5.75% discount rate. The NPV of each rotation was calculated annually in order to compare systems across time and determine the year in which net returns were maximized for each system. Net present value with low seed and low straw value (LSLS), low seed and low straw value (LSLS), high seed and low straw value (HSLS), and high seed and high straw value (HSHS).aHighest NPV in bold indicates the year in which the stand should be terminated in order to maximize profits.Net present value represents all previous cash flows less all costs, adjusted for the discount rate. Using these cost and price assumptions, the BB system was most profitable, maximizing returns with a NPV between $1,208 and $4,030 ha−1 in Year 6, or Year 4 when both seed and straw price were low (Table 6). The second most profitable treatment was FLB with a NPV between $521 and $2,618 ha−1 in Year 5 when seed price was low or Year 7 when seed price was high. The third most profitable system was SYS with a NPV between $453 and $2,301 ha−1 in Year 4, or in Year 5 when both seed and straw price were high. Lastly, MEC was the least profitable treatment, with a NPV between $200 and $2,049 ha−1 in Year 4, or Year 5 when both seed and straw price were high (Table 6).Revenue from bluegrass straw, particularly for the higher seed yielding BB system, explains higher profitability for systems with residue baling. The relative profitability of FLB vs. the other systems depends on the value and market for bluegrass straw. At times, the price for bluegrass straw may be too low to justify harvesting. In that case, only FLB would be profitable.CONCLUSIONResidue removal after seed harvest, particularly that portion of the residue that is not standing, was critical for obtaining high seed yield. Nonstanding residue removal is difficult to accomplish using mechanical methods, and FLB performed the best at removing this fraction of the residue. Reduced‐burn methods (BB and SYS), however, were able to keep this residue level lower than mechanical methods (MEC), and therefore also yielded more than MEC.Another factor that contributed to high seed yield was the percent N of regrowth in the fall. Future research on how to increase fall N uptake across burn, reduced burn, and mechanical production systems is needed. Additional research is also needed to determine the exact causes for these yield relationships and if similar relationships are found for multiple cultivars and locations. This study suggests it might be possible to predict yield potential by measuring the amount of nonstanding residue and N in the fall. Being able to estimate yield potential a year early would allow a producer to make an informed decision whether it would be most profitable to keep a stand in production or take it out and plant another crop. Making informed decisions on seed yield potential would result in significant economic improvements for the Kentucky bluegrass growers.Profitability, as determined by annual revenue less expenses and NPV over time, was followed the order of BB > FLB > SYS > MEC. These results are dependent upon the price assumptions used in this study. Without a bluegrass straw market, only the FLB system is profitable. If both seed and straw can be marketed above the cost of production, then BB was the most profitable treatment. Reduced‐burn systems have greater input costs and lower yields over time. This research suggests reduced burning can maintain productive stands in the first 4–5 yr of the stand life. The complete elimination of field burning in northern Idaho would result in lower seed yield and reduced stand longevity. Producers would need a stable bluegrass straw market in order to profitably raise bluegrass seed without the option of field burning. In order to maintain profitability when producing this seed crop, producers need to know market prices for both seed and straw. We concluded that profitability was highest with BB and FLB. Overall, understanding the factors that contribute to stand decline may help growers prolong stand life and increase profitability, while mitigating air quality issues.ACKNOWLEDGMENTSThe authors would like to thank Karl Umiker and Janice Reed for assistance with data collection.AUTHOR CONTRIBUTIONSJohn Holman: Conceptualization; Data curation; Funding acquisition; Investigation; Methodology; Project administration; Writing – original draft; Writing – review & editing. Jack D. Robertson: Data curation; Investigation; Methodology. Jodi Johnson‐Maynard: Conceptualization; Funding acquisition; Investigation; Methodology; Project administration; Writing – review & editing. Kathleen Painter: Formal analysis; Investigation; Methodology; Project administration; Writing – review & editing. Yared Assefa: Formal analysis; Validation; Visualization; Writing – review & editing.CONFLICT OF INTERESTThe authors declare no conflict of interest.REFERENCESAamlid, T. S. (1991). 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"Agrosystems, Geosciences & Environment"Wiley

Published: Jan 1, 2022

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