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ContextNitrate salts can be utilised by the rumen bacteria as a nitrogen source. Nitrate salts can induce a methaemoglobinaemia in cattle if consumed in sufficient quantities. Methaemoglobinaemia is the principal factor that leads to the onset of clinical signs for nitrate toxicity in cattle. A methaemoglobin concentration 20% is considered unsafe for cattle. There are, however, limited studies on the longer-term effects of nitrate supplementation on methaemoglobin formation in Bos indicus steers consuming forage that is reflective of northern Australias poor quality, native pasture in the dry season.AimsWe hypothesised that the Australian governments recommended daily dose of nitrate salts given to Bos indicus cattle would not cause a methaemoglobinaemia in the blood >20% throughout a 70 day treatment period.MethodsA 70 day study was conducted to determine the methaemoglobin, carboxyhaemoglobin, total haemoglobin, growth rate and forage intakes of cattle supplemented with a non-protein-nitrogen treatment containing nitrate (6.48 g NO3/kg dry matter intake (DMI) or no nitrate and consuming a chaffed Flinders grass hay (Iseilema spp.), a C4 species. The dose rate of nitrate was selected to match the Australian government guidelines. Ten 3-year-old fistulated Bos indicus steers (mean liveweight s.d., 400.7 26.2 kg) were randomly allocated into two groups (n = 5). Blood samples were collected at 0, 2, 4 and 6 h after treatment with nitrate or no nitrate on days 10, 30, 50 and 70 to measure haemoglobin fractions in the blood.Key ResultsNitrate treatment caused the mean methaemoglobin (P < 0.001), peak methaemoglobin (P < 0.001) and carboxyhaemoglobin (P = 0.008) concentration to be greater in the blood of steers compared with steers given no nitrate. Nitrate treatment had no general effect on the total haemoglobin, DMI or bodyweight of steers.ConclusionsBos indicus steers treated with 6.48 g NO3/kg DMI develop a methaemoglobinaemia that does not exceed 20% of total haemoglobin for 70 days. This data supports the Australian governments recommended feeding rate of nitrate to sedentary Bos indicus steers.ImplicationsThe Australian governments method for feeding nitrate to cattle is safe under the conditions of this study.
Animal Production Science – CSIRO Publishing
Published: Jul 8, 2021
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