Nitrate and Nitrite Metabolism in Ruminant Livestock

Abstract

<p>Global methane (CH₄) emissions from ruminant livestock have continued to increase in the last decade due to the global ruminant population increasing in response to growing demand for livestock products. Supplementing the diet of ruminants with nitrate (NO₃) is an effective strategy to reduce enteric CH₄ emissions and also provides additional non-protein nitrogen for the growth of rumen microorganisms. However, there is a risk of nitrite (NO₂) toxicity in ruminants after feeding NO₃ that is unpredictable because of the large between-animal variation in NO₃ metabolism. The main objective of this thesis was to provide new knowledge about the basic biology of NO₃ absorption and NO₂ formation in ruminants, in order to increase the safety of NO₃ supplementation. This thesis includes a review of the literature and five experimental chapters where NO₃ and NO₂ metabolism and the effects of dietary NO₃ on CH₄ emissions, together with ruminants and the physiological effects of dietary NO₃ on insulin sensitivity in sheep, were evaluated. The results obtained from this thesis indicate:</p> <p>Feeding dietary NO₃ in combination with canola oil has a more than additive effect on reducing methanogenesis in cattle.</p> <p>Dietary NO₃ provided to sheep fed an otherwise highly protein-deficient diet reduces CH₄ emissions and provides additional N for microbial growth, including through NO₃-N recycling via saliva into the gastrointestinal tract. </p> <p>Large between-animal variability in NO₃ metabolism in vivo and in vitro was observed throughout this thesis.</p> <p>The presence of protozoa in the rumen of sheep adapted to dietary NO₃ increases NO₃ reduction to NH3 without stimulating accumulation of NO₂ in the rumen; reducing the risk of NO₂ toxicity in ruminants.</p> <p>Faecal and urinary recovery of an intravenous dose of 15NO₃ was 64% after 6 days. Urinary urea-N was labelled confirming the passage of plasma NO₃ to the digestive tract of sheep had occurred</p> <p>This thesis found evidence of NO₃ accumulating in tissues of sheep for at least one hour after intravenous injection of 15NO₃ and 15NO₂.</p> <p>Evidence of NO₃ and NO₂ being rapidly absorbed from the rumen of sheep into the blood was found. Once in the bloodstream, NO₂ was oxidised to NO₃ which is recycled via enterosalivary circulation in like manner to urea</p> <p>Nitrate and NO₂ are highly absorbed from the small intestine into the bloodstream, with NO₂ absorption responsible for high levels of blood methaemoglobin. Absorption of NO₃ from the abomasum to the blood was evident, whereas it is hypothesised that NO₂ was converted to nitric oxide in the acidic conditions of the abomasum so did not lead to high blood NO₂.</p> <p>Nitrate is highly concentrated in saliva relative to blood NO₃ concentration</p> <p>We found no evidence that feeding a low dose of dietary NO₃ (18 g NO₃/kg DM) affects insulin sensitivity in sheep.</p>