Effect of dietary nitrate on enteric methane emissions, production performance and rumen fermentation of dairy cows grazing ryegrass pasture during spring

Limited studies investigated the effect of dietary nitrate addition as methane (CH4) mitigation strategy for dairy cows grazing pasture. This study aimed to investigate the effect of dietary nitrate addition on daily enteric CH4 emissions, production performance and rumen fermentation of multiparous Jersey cows grazing perennial ryegrass pasture (containing approximately 7.3 g of nitrate/kg of dry matter (DM)). Thirty-two intact and eight rumen-cannulated multiparous Jersey cows were subjected to a replicated 2 x 2 Latin square design with 16 intact cow replicates and four rumen-cannulated cow replicates supplemented with one of two concentrates containing either urea (urea treatment), or urea and nitrate (nitrate treatment) as nonprotein nitrogen source (NPN; containing 0.3 and 15.2 g of nitrate/kg of DM, respectively). Concentrates were formulated to be isonitrogenous and isoenergetic, and was fed at 5.4 kg of DM/cow per d along with a strict daily herbage allowance of 14 kg of DM/cow. Cows were gradually adapted to concentrates over a 3-wk period. Total nitrate intake was 5.2 and 9.7 g of nitrate/kg of DM for the urea and nitrate treatment groups, respectively. Daily enteric CH4 emissions of 28 cows were measured with the sulphur hexafluoride tracer gas technique for six consecutive days during each experimental period with parallel total DM intake (DMI) estimates. Pasture DMI was calculated from faecal output and pasture digestibility using TiO2 and indigestible neutral detergent, respectively. Total DMI (18.1 and 17.8 kg/d), milk yield (19.0 and 18.9 kg/cow per d) and daily CH4 emissions (400 and 405 g/d) were unaffected by dietary treatment for the urea and nitrate group, respectively. Total milk solids content (135 vs. 133 g/kg), milk lactose content (47.3 vs. 46.7 g/kg) and milk urea nitrogen concentration (MUN; 12.6 vs. 11.6 mg/dL) were higher for the nitrate group. Rumen fermentation parameters such as volatile fatty acid profile, ammonium nitrogen, and DM and fibre disappearance were unaffected by treatment. Minor effects on ruminal pH were observed with an increasing tendency towards the nitrate group. In this study, dietary nitrate supplementation was not an effective CH4 mitigation strategy for dairy cows grazing perennial ryegrass. This can be ascribed to the unforeseen high nitrate content of the grazed pasture causing a relative small margin in total nitrate intake between the urea and nitrate treatment diets.