The combination of optimal nitrate and l-cysteine to safely mitigate rumen methane (CH4) emissions in ruminants was studied in an open-circuit respiration head-hood system using four rumen-fistulated Suffolk wethers in a 4 × 4 Latin square design. Four treatments were set up Control: fed on lucerne hay cubes without nitrate and l-cysteine, Nitrate: fed on lucerne hay cubes with 0.18 % NO3--N in dry matter (DM)), N + Cys-H: fed on lucerne hay cubes supplemented with 0.18 % NO3--N and 0.74 % l-cysteine (equivalent to half the upper limit of effective S requirement in DM), and N + Cys-Q: fed on lucerne hay cubes supplemented with 0.18 % NO3--N and 0.37 % l-cysteine (equivalent to 1/4 of the upper limit of effective S requirement in DM). In this experiment, the ingested nitrate at a subclinical concentration/s (0.18 % in DM) increased by 11.2 % mean methemoglobin value and alleviated rumen methanogenesis by 47 %. Administration of l-cysteine set at 0.74 % and 0.37 % in DM reduced by 68 % and 58 % methemoglobin formed by nitrate alone, respectively (P < 0.05). However, daily mitigation of CH4 emissions decreased by 35 % with the addition of l-cysteine at both addition concentration/s compared with Control. The results of this study suggest that mitigation of enteric methane emissions by the combination of nitrate and l-cysteine can be achieved by feeding diets in which the nitrate content is maintained at around 0.18 % NO3--N in DM and l-cysteine addition is adjusted to 0.37-0.74 % in DM. This method would be recommended as a safe, efficient, and practical way to mitigate enteric CH4 emissions leading to increased productivity while reducing the increased N excretion that causes N2O emissions.
Keywords: Enteric CH(4); Nitrate; Nitrite; Ruminants; l-Cysteine.
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