Lactational responses of grazing dairy cows to rumen-protected methionine

Two groups of 18 Prim'Holstein cows grazed on a rye-grass meadow during the second and third cycles according to a strip grazing system, at a grass regrowth age of about 1 month. During each cycle, the cows were administered either 0 or 13 g/day rumen-protected methionine according to a reverse design. The response to the methionine supplementation was the same in both cycles. The methionine supplementation reduced milk yield slightly but significantly (26.0 vs 26.5 kg) and increased the milk protein content (31.4 vs 30.6 g/kg). Milk fat content, fat and protein yields were not significantly modified. A rumen protected methionine supplementation significantly increased plasma methionine concentration by 27%. Based on the fact that milk protein yield was unchanged, it was concluded that methionine is not the most limiting amino acid at grazing.