An initial investigation on rumen fermentation pattern and methane emission of sheep offered diets containing urea or nitrate as the nitrogen source

The effects of dietary nitrate and of urea on rumen fermentation pattern and enteric methane production were investigated using 4-month-old ewe lambs. Ten lambs were allocated into two groups (n = 5) and each group was offered one of two isonitrogenous and isoenergetic diets containing either 1.5% urea (T1) or 3% calcium nitrate (T2). Methane production was estimated using open-circuit respiration chambers after 6 weeks of feeding. No difference in nitrogen (N) balance, apparent digestibility of N or microbial N outflow existed between treatments (P > 0.05). Animals offered the T2 diet lost less energy through methane than did those fed the T1 diet (P < 0.05). Total volatile fatty acid concentration, molar proportion of propionate, and the molar ratio of acetate to propionate in rumen fluid were not affected by dietary N source. Compared with urea inclusion, nitrate inclusion caused a significantly higher acetate and lower butyrate percentage in rumen volatile fatty acid. Nitrate supplementation tended to lower methane production by ~7.7 L/day relative to urea supplementation (P = 0.06). Methane yield (L/kg DM intake) was reduced (P < 0.05) by 35.4% when 1.5% urea was replaced by 3% calcium nitrate in the diet. Emission intensity (L methane/kg liveweight gain) was ~17.3% lower in the nitrate-supplemented sheep when compared with urea-fed sheep; however, the reduction was not statistically significant (P > 0.05). This study confirms that the presence of nitrate in the diet inhibits enteric methane production. As no clinical symptoms of nitrite toxicity were observed and sheep receiving nitrate-supplemented diet had similar growth to those consuming urea-supplemented diet, it is concluded that 3% calcium nitrate can replace 1.5% urea as a means of meeting ruminal N requirements and of reducing enteric methane emissions from sheep, provided animals are acclimated to nitrate gradually.