The effects of feeding nitrate on the development of methaemoglobinaemia in sedentary Bos indicus cattle
I. Benu A B , M. J. Callaghan
C , N. Tomkins D , G. Hepworth E , L. A. Fitzpatrick A and A. J. Parker A F G
A College of Public Health, Medical and Veterinary Sciences, James Cook University, Townsville, Qld 4811, Australia.
B Faculty of Animal Science, University of Nusa Cendana, Kupang NTT, Indonesia.
C Ridley AgriProducts Pty Ltd, Toowong, Brisbane, Qld 4066, Australia.
D Meat & Livestock Australia, Fortitude Valley, Brisbane, Qld 4006, Australia.
E Statistical Consulting Centre, The University of Melbourne, Melbourne, Vic. 3010, Australia.
F Department of Animal Sciences, The Ohio State University, Wooster, Ohio, 44691, USA.
G Corresponding author. Email: parker.1203@osu.edu
Animal Production Science 61(16) 1680-1685 https://doi.org/10.1071/AN20148
Submitted: 28 April 2020 Accepted: 8 June 2021 Published: 8 July 2021
Abstract
Context: Nitrate 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 Australia’s poor quality, native pasture in the dry season.
Aims: We hypothesised that the Australian government’s 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.
Methods: A 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 Results: Nitrate 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.
Conclusions: Bos 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 government’s recommended feeding rate of nitrate to sedentary Bos indicus steers.
Implications: The Australian government’s method for feeding nitrate to cattle is safe under the conditions of this study.
Keywords: Bos indicus, methaemoglobin, MetHb, cattle, haemoglobin, nitrite, rumen bacteria, non-protein-nitrogen, NPN, beef cattle, supplements, rumen microflora.
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