A trial assessing the production response of dairy cows to supplementation with methionine (Met) was conducted indoors over a 21-day period with 26 Friesian cows in mid-lactation and individually fed fresh perennial ryegrass/white clover pasture. Group 1 (n=9) received twice daily oral methionine (15 g/d) that was protected from rumen degradation (RPM; Lactet); Group 2 (n=9) was given a continuous intravenous infusion of free Met (15 g/d; MET-i.v.); and Group 3 (n=8) was given no methionine supplement (Control). Supplementary methionine tended to slightly depress feed intake but milk yield tended to be higher (P=0.08) for the MET-i.v. group compared with the Control and RPM groups (18.5 ± 0.48 vs 17.1 ± 0.45 and 17.2 ± 0.38 kg/d ± SE). Compared with the control group, there was a trend for Met (both MET-i.v. and RPM) to reduce milkfat concentration and to increase the concentration of casein. Neither lactose or whey nitrogen (N) concentrations were affected by the treatments. Methionine supplementation did not affect yields of total protein nor protein components; however, when protein and casein yields were adjusted to equal crude protein intake, Met supplemented cows produced significantly (P=0.05) more total protein and casein per unit of N ingested. These results suggest that post-ruminal Met supplementation improved the efficiency with which dietary protein was converted to casein in the pasture-fed dairy cow. More research is required to investigate which AA, individually or in combination, are likely to limit milk protein synthesis in the mammary gland of the pasture-fed dairy cow.
Proceedings of the New Zealand Society of Animal Production, Volume 57, , 147-150, 1997
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