Effect of forage source and a supplementary methionine hydroxyl analogue on rumen fermentation parameters in lactating dairy cows offered a low crude protein diet

Abstract The objective of this experiment was to evaluate the effects of forage source (grass silage (GS) or maize silage (MS)) and a supplementary methionine hydroxyl analogue (HMBi) on rumen fermentation parameters in dairy cows offered a low-crude protein (CP) diet. Four lactating dairy cows permanently fitted with rumen cannulae were assigned to 1 of 4 diets in a 2 × 2 factorial, Latin square design. Diets were isoenergetic and isonitrogenous and were offered once daily as a total mixed ration. Rumen samples were taken every 2 h over 48 h for analysis of pH, volatile fatty acids (VFA), total lactic acid and NH 3 -N. Mean pH (6.21 ± 0.03) and nadir pH (5.64 ± 0.12) were not affected by dietary treatment. Animals offered GS based diets had higher VFA (113.69 vs. 107.55 ± 1.53 mmol/L) and NH 3 -N (62.82 vs. 55.54 ± 2.38 mg/L) concentrations vs. those offered MS diets. No effect of supplementary HMBi on rumen pH, VFA or NH 3 -N was detected in this study.