Responses to graded postruminal doses of histidine in dairy cows fed grasssilage diets

Five Finnish Ayrshire cows were used in a 4 x 4 Latin square experiment des igned to study the effects of graded doses of postruminal His infusion on m ilk production, arterial concentrations, and mammary uptakes of plasma amin o acids (AA) as well as utilization of added His. Grass silage (16.9% CP) w as given ad libitum with 8 kg of cereal-based concentrate per day (11.3% CP ). Treatments were abomasal or duodenal infusions of 250 g of glucose/d in combination with 0, 2, 4, or 6 g of His/d. Infusions did not affect dry mat ter intake (mean 18 kg/d). Infusion of His increased milk yield linearly fr om 27.0 to 28.8 kg/d, protein yield from 861 to 919 g/d and lactose yield f rom 1345 to 1457 g/d. Milk fat yield and content changed in a cubic manner (1240, 1167, 1296, and 1177 g/d and 4.60, 4.16, 4.60, and 4.09). Infusion o f His had no influence on milk protein or lactose concentrations. Arterial Lys and His concentrations increased linearly, but other AA concentrations were unaffected as well as calculated arteriovenous differences and mammary AA uptakes. The extraction of His decreased linearly with an increasing am ount of His. The utilization of added His (28%) was not affected by the lev el of infusion, and mammary AA uptake seemed to be regulated by an inverse relationship between arteriovenous difference of essential AA and calculate d mammary plasma flow. This experiment confirmed that His is the first-limi ting AA on grass silage-cereal based diets.