Effects of feeding high forage diets and replacing nonstructural carbohydrate with supplemental fat in dairy cows

Fifty mid-lactation Holstein cows were used in a six-week feeding trial to study effects of high-forage, high-fat diets on dry matter intake, milk production, milk composition, and blood constituents. Cows were divided into 10 replicates, each consisting of five cows. Each cow was assigned to one of four experimental diets (high-forage (75%), high-fat (7.5%)(diet 1); high-forage, medium-fat (5.0%)(diet 2); medium forage (65%), high-fat (diet 3); medium-forage, medium-fat (diet 4)), or a control diet containing about 50% forage and 2% fat. All diets were isonitrogenous (17.7% crude protein). The forage mixture consisted of 20% alfalfa hay, 40% alfalfa haylage, and 40% corn silage. Supplemental fat included 80% rumen-protected fat and 20% yellow grease. Dry matter intake was decreased (P \u3c 0.01) in cows fed experimental diets (18.4, 20.9, 19.9, and 22.6 kg for cows fed diets 1-4, respectively vs. 27.5 kg for cows fed the control diet). Daily milk production was lower (P \u3c 0.05) for cows consuming experimental diets (30.5, 31.3, 31.0, and 32.5 kg for cows fed diets 1-4, respectively vs. 34.2 kg for cows fed control diet); however, milk output per unit of dry matter intake was greater for cows consuming experimental diets (1.74, 1.55, 1.60, and 1.53 kg milk/kg dry matter intake for cows fed diets 1-4, respectively, vs. 1.26 kg milk/kg dry matter intake for cows fed the control diet). Milk fat and total solids did not differ significantly; however, milk protein tended to be higher for cows consuming the medium-forage, high-fat diet. A non-significant difference was observed in concentrations of blood glucose for cows on different experimental and control diets. Plasma nonesterified fatty acids (NEFA) were higher in cows consuming experimental diets than those consuming the control diet. However, differences in NEFA concentrations in the plasma of cows consuming diets with different forage and fat levels were not significant. Rumen pH, concentration of volatile fatty acids (VFA) in rumen contents, and dry matter digestibility of control and experimental diets, and diets with different levels of forage and supplemental fat did not differ significantly