Monensin supplementation and in vivo methane production by steers.

The effect of monensin on methanogenesis was examined. Growing steers were fed 4.1 kg daily of a low roughage diet (12.6% crude protein, 12% acid detergent fiber) or 5.4 kg daily of either a medium roughage (12.3% crude protein, 27% acid detergent fiber) or a high roughage diet (14.4% crude protein, 40% acid detergent fiber) with or without 200 mg of supplemental monensin. At the low and high roughage levels, monensin increased (P less than .05) the molar proportion of ruminal propionate and decreased acetate. Methane production and inhibition by monensin declined with time postprandially. Monensin decreased methane production by 16% (P less than .05) at the two lower roughage levels and 24% (P less than .01) at the high roughage level. Total heat production and respiratory quotient remained unaltered by monensin. Averaged across roughage levels, metabolizable energy was increased (P less than .01) 5.2% by monensin. Although dry matter digestibility and N retention were not statistically altered, both tended to increase with monensin. Reduced ruminal methane production with monensin feeding can explain a portion of the increased feed efficiency.