Dietary nitrate reduces enteric methane production in sheep

Abstract

The chemical reduction of nitrate to ammonia in the rumen of livestock is thermodynamically more favourable than is reduction of carbon dioxide to methane, so nitrate supplementation may provide a means of decreasing enteric methane production from ruminants. The level of abatement achieved by providing dietary nitrate was evaluated in sheep using open circuit respiration chambers. Four (4) Merino sheep (59kg LW) were acclimated to a basal ration of 1kg/d of lucerne chaff fed in 8 equal portions at 3 hour intervals. The basal ration had been sprayed with 10g nitrate-N/kg DM, provided as hydrated calcium nitrate. After 10d of acclimation, sheep were placed in respiration chambers (n=4) and each sheep received 4 feeds/d of the basal ration interspersed with 4 feeds/d of a test ration containing 0.3, 0.5 or 0.7% added nitrate-N. The interspersing of test ration with basal ration minimised risk to the animals from nitrite toxicity and provided within-day replication of methane mitigation. On any test day, each sheep received only the basal plus one test ration, but each sheep was in the respiration chamber for 4 days so that all sheep received each test ration over the 4 days of respiration chamber studies. Methane production by sheep in each respiration chamber was monitored at 15 min intervals. The level of methane abatement achieved from nitrate inclusion was derived by comparing methane production in the 3h following the test ration with methane production in the 3h following the basal ration. A crossover statistical model including a 2 level factor (test v basal) and its interaction with the dietary nitrate treatment, a spline and individual sheep was used. There was a linear decline in methane production with increasing nitrate inclusion in the ration (L/d abatement = 0.65 – 7.7 x nitrate-N%; r² = 0.999), Graphing of methane production rate over time showed that while animals fed the basal ration increased methane production in the hour post-feeding, additional nitrate caused methane production rate to fall within the first hour post-feeding. The decrease in methane production from nitrate in test rations was approximately 70% of that predicted stoichiometrically, indicating that not all nitrate was fully reduced to ammonia within 3h of feeding. The data confirm that dietary nitrate is an effective feed additive to reduce enteric methane output but mitigation is less than expected and this may be due to both absorption and wash-out of nitrate and nitrite from the rumen.