Dietary nitrate metabolism and enteric methane mitigation in sheep consuming a protein-deficient diet

It was hypothesised that the inclusion of nitrate (NO₃^–) or cysteamine hydrochloride (CSH) in a protein deficient diet (4.8% crude protein; CP) would improve the productivity of sheep while reducing enteric methane (CH₄) emissions. A complete randomised designed experiment was conducted with yearling Merino sheep (<i>n</i> = 24) consuming a protein deficient wheaten chaff control diet (CON) alone or supplemented with 1.8% nitrate (NO₃^–; DM basis), 0.098% urea (Ur, DM basis) or 80 mg cysteamine hydrochloride/kg liveweight (CSH). Feed intake, CH₄ emissions, volatile fatty acids (VFA), digesta kinetics and NO₃^–, nitrite (NO₂^–) and urea concentrations in plasma, saliva and urine samples were measured. There was no dietary effect on animal performance or digesta kinetics (<i>P</i> > 0.05), but adding NO₃^– to the CON diet reduced methane yield (MY) by 26% (<i>P</i> = 0.01). Nitrate supplementation increased blood MetHb, plasma NO₃ ^– and NO₂^– concentrations (<i>P</i> < 0.05), but there was no indication of NO₂^– toxicity. Overall, salivary NO₃^– concentration was greater than plasma NO₃^– (<i>P</i> < 0.05), indicating that NO₃^– was concentrated into saliva. Our results confirm the role of NO₃^– as an effective additive to reduce CH₄ emissions, even in a highly protein-deficient diet and as a source of additional nitrogen (N) for microbial protein synthesis via N-recycling into saliva and the gut. The role of CSH as an additive in low quality diets for improving animal performance and reducing CH₄ emissions is still unclear.