LAUSR

Abstract Demand for livestock products and methane mitigation is increasingly stimulating a search for technologies capable of increasing animal productivity while lowering enteric methane emissions. Dietary nitrate (NO 3 − ) has shown this capability in sheep on low nitrogen diets. Cysteamine hydrochloride (CSH) has also been shown to have such dual efficacy, but whether it affects rumen fermentation directly or indirectly by modifying digesta kinetics is unknown. It was hypothesized that the administration of CSH to cattle would reduce in-vitro and in-vivo methane production and also increase their average daily liveweight gain (LWG) without affecting their DM intake (DMI). An in-vitro experiment was conducted to study the effects of CSH, NO 3 − , urea and nitrite, on methane and volatile fatty acid (VFA) production and on the protozoal population. Methane production, production of total VFA and acetate, and acetate:propionate ratio were not affected by CSH ( P >  0.05) relative to control incubations, however, pH was reduced while hydrogen accumulation was increased ( P   0.05) by CSH relative to control incubations. Subsequently, a 42-d in-vivo experiment was conducted using a completely randomized design with twelve yearling cattle (236 ± 49 kg liveweight; LW) to assess LWG, methane production and feed conversion ratio (FCR) on a basal roughage/concentrate diet containing either no additives, or 1% NO 3 − addition, or 80 mg/kg LW of CSH. Daily methane production rate (DMP; g methane/d) was measured over 2 × 24 h periods in open-circuit calorimetry chambers during both weeks 3 and 6 of the study, with nutrient digestibility determined by collecting faecal samples and using acid insoluble ash as an indigestible marker. Relative to cattle fed the control diet CSH supplemented cattle exhibited no change in LWG or FCR ( P >  0.05). While neither DMP nor methane yield (MY; methane/kg DMI) were reduced by CSH (mean 9.2% reduction), methane production rate was significantly reduced ( P   0.05.) for up to 6 h post-feeding relative to control animals. Nitrate reduced MY by 31.1% (15.7 g methane/kg DMI; P   0.01) relative to when the control diet was fed (22.8 g methane/kg DMI), and increased ( P   0.01) dietary crude protein digestibility. It was concluded that while NO 3 − can deliver greater methane mitigation than CSH, CSH has in some studies (though not this study) improved the efficiency of animal production, which, together with the observed short term efficacy in reducing methane emissions suggests CSH may have a role in enabling greater animal production at a reduced environmental cost.