Nitrate supplementation has marginal effects on enteric methane production from Bos indicus steers fed Flinders grass (Iseilema spp.) hay, but elevates blood methaemoglobin concentrations

Abstract

This experiment has quantified the methane abatement potential of nitrate in the context of extensively managed cattle. The experimental protocol consisted of two, 4 × 4 Latin square design using eight rumen fistulated Bos indicus steers fed Flinders grass (Iseilema spp.) hay ad libitum. The treatments were Control (nil nitrogen supplement), urea (32.5 g/day urea) and two levels of calcium nitrate: CaN1 and CaN2 (to provide 4.6 g and 7.9 g NO3/kg DM equivalent to ~0.46% and 0.80% of DM, respectively). Complete supplement intake was ensured by dosing any supplement that had not been voluntarily consumed, through the rumen fistula, 1 h after feeding. Enteric methane production was measured using open circuit respiration chambers. Methane yield (g/kg DM intake) from the CaN2 treatment tended to be lower (P < 0.07) than either the Control or urea treatments. There were no significant differences in methane yield between Control, urea or CaN1 treatments. Mean blood methaemoglobin concentrations were significantly (P < 0.001) higher for CaN2 animals compared with the Control, urea or CaN1 treatments. In addition, a significant time effect after dosing (P < 0.001) and a significant interaction between treatment and time after dosing (P < 0.001) was apparent. Overall mean total volatile fatty acid concentration was 74.0 ± 1.53 mM with no significant treatment effect, but a significant effect for both time of sampling (3 h vs 6 h) within days and among 7 sampling days. The inclusion of calcium nitrate as a non-protein-N source significantly reduced the molar proportions of butyrate (P < 0.001), iso-butyrate (P < 0.05) and iso-valerate (P < 0.001) compared with the Control. The provision of nitrate supplements, providing both a NPN and an alternative sink for H that would otherwise support enteric methanogenesis, has some potential. In extensive grazing systems effective methane abatement strategies are required. The elevated concentration of MetHb using CaN2 suggests that the strategy of replacing urea with nitrate in supplements fed to extensively managed cattle in the northern rangelands may be inappropriate where supplement intake cannot be controlled on an individual animal basis and forage quality is seasonally variable.