Effects of protected methionine and variable energy supply on lactational responses in dairy cows fed grass silage-based diets

Twelve multiparous Holstein cows (average body weight 610 kg; 56 - 84 d after calving at the start of the trial) in their second, third or fourth lactation were assigned to two-factorial (2 x 2) arrangement of treatments, in a balanced changeover design, involving two levels of energy (adequate, AE, 100% vs low, LE, 80% of INRA requirements) and two levels of ruminally-protected DL-methionine (Smartamine(TM) M: 0 vs 20 g/d). The treatments were M0-AE, M20-AE, M0-LE and M20-LE. The calculated intestinal concentrations of lysine and methionine (% PDI) were: 6.9 and 1.7, 6.9 and 2.1, 6.9 and 1.7, 6.9 and 2.2, respectively. The AE and LE diets contained (% DM): grass silage 44 and 50, and concentrates 56 and 50, respectively. Low energy intake was obtained by reducing the total amount of feed offered (from 19.6 kg DM in the AE diets to 16.2 kg in the LE diets). The diets provided 100% of requirements for protein digested in the small intestine (PDI). Average milk yield tended to be increased in the cows fed AE vs LE diets (averaging 27.4 vs 26.5 kg), but the differences were not significant. Milk fat, lactose and SNF contents did not respond to the treatments. Feeding ruminally protected methionine slightly, but significantly, increased milk protein content (2.91 vs 3.07%, for M0 vs M20 diets; P<0.01), with no effect on milk protein yield. Milk produced by the AE-fed cows contained significantly more casein-N (P<0.05) and less NPN (P<0.01) compared with the LE cows. Methionine supplementation resulted in significant increases in the contents in milk of total-N, protein-N, casein-N and whey-N (%), with no effect on NPN and urea-N (% in milk) and protein-N % of total N, casein-N % of total-N. The effect of methionine on the content of nitrogen fractions in milk was more apparent in the cows fed the LE diets. It is concluded that supplementing dairy cows fed grass silage-based diets with ruminally-protected methionine had no effect on milk yield but resulted in apparent changes in milk composition. The reaction of cows to additional supply of absorbable methionine was particularly evident in energy-underfed cows.