Ruminal digestion by dairy cows grazing winter oats pasture supplemented with different levels and sources of protein

Six Holstein cows fitted with ruminal cannulae were used in two simultaneou s 3 x 3 Latin squares to study the effects of protein supplements on rumina l fermentation and in situ crude protein degradability. Cows rotationally g razed a winter oats (Avena sativa L.) pasture and were supplemented with on e of three concentrate supplements: 1) low protein sunflower meal (L-SM); 2 ) high protein sunflower meal (H-SM); or 3) high protein feather meal (H-FM ). Concentrates (6.5 kg/d) were offered in equal portions twice daily durin g milking. Ruminal pH and total volatile fatty acids concentration were una ffected by treatments. Supplementation with L-SM and H-FM decreased ruminal NH3-N concentration compared with H-SM. The concentrate with feather meal had lower effective rumen degradability of crude protein than concentrates containing sunflower meal. Effective rumen degradability of crude protein o f pasture averaged 82.7%. Thirty-six multiparous Holstein cows (71 d in mil k) were used in a complementary experiment to study the effect of treatment s on intake, milk yield, and milk composition. Pasture (13.2 kg/d) and tota l (19.6 kg/d) dry matter intake (estimated using Cr2O3 as fecal marker) and milk yield (20.5 kg/d) were unaffected by level or source of protein suppl emented. Intake of rumen undegradable protein in grazing dairy cows was hig her when the amount of sunflower meal was increased or when feather meal wa s used in the supplement. However, higher rumen undegradable protein intake did not increase milk production, suggesting that rumen undegradable prote in was not limiting for cows on pasture producing less than 22 kg of milk.