Effects of supplemental fat and protein source on ruminal fermentation and nutrient flow to the duodenum in dairy cows.

Four lactating Holstein cows, fitted with ruminal and T-type duodenal cannulas, were used in a 4 x 4 Latin square design for four periods (14 d each). Treatments were 1) medium fat and low quality protein, 2) medium fat and high quality protein, 3) high fat and low quality protein, and 4) high fat and high quality protein. Fat was supplemented by addition of 2.5% prilled fatty acids to high fat diets. The low quality diets contained corn gluten meal and 0.47% Lys, and the high quality diets contained a mixture of fish, blood, and soybean meals and 0.77% Lys. The percentage of Met was similar for all diets (mean, 0.28%). Diets contained 35 to 38% steam-flaked sorghum and 32% chopped alfalfa hay. Dietary CP averaged 18%, and all diets were high in RUP (44% of CP). Yields of milk, milk protein, and SNF were increased by added fat and by high quality protein. Ruminal NH3 and butyrate were increased by supplemental fat. Cows fed high quality protein had increased ruminal VFA, propionate, butyrate, and valerate, but decreased ratios of acetate to propionate. Essential AA concentrations in coccygeal plasma and arteriovenous differences across the mammary gland were higher for Lys and lower for Leu in cows fed high quality protein than in those fed low quality protein.