Effects of amount and ruminal degradability of protein on nutrient digestibility and production by cows fed tallow.

Five cows with ruminal cannulas were used in a 5 x 5 Latin square design to determine the effects of fat and amount and ruminal degradability of dietary crude protein (CP) on nutrient digestibility and production of milk and milk components. Treatments were 1) control; 2) 15% CP, soybean meal; 3) 15% CP, by-product proteins; 4) 18% CP, soybean meal; and 5) 18% CP, soybean meal and by-product proteins. Diets 2 through 5 contained 3.5% tallow. Diets consisted of 28% alfalfa haylage, 22% corn silage, and 50% concentrate on a dry matter (DM) basis. Fat did not affect dry matter intake or percentages and yields of fat and CP in milk but increased milk yield 2.5 kg/d. Fat did not affect N fractions in milk but decreased the percentages of short- and medium-chain fatty acids (C6:0 to C16:0) and increased the percentages of long-chain fatty acids (C18:0 and C18:1) in milk fat. Fat did not affect ruminal fermentation characteristics or the percentages of dietary DM, organic matter, CP, acid detergent fiber, neutral detergent fiber, starch, ether extract, and energy that were digested. An increase in dietary CP from 15 to 18% increased dry matter intake 1.7 kg/d; increased intake of gross energy 8 Mcal/d; increased the percentages and quantities of DM, organic matter, CP, and energy digested; increased the quantities of acid detergent fiber and neutral detergent fiber digested; decreased ruminal pH; increased concentrations of total volatile fatty acids; and increased NH3 N in ruminal fluid. However, the difference in dietary CP did not affect milk yield or composition. Replacement of soybean meal in the diet with a mixture of by-product proteins decreased NH3 N in ruminal fluid, tended to decrease concentrations of total volatile fatty acids and increase pH of ruminal fluid, but did not affect milk yield or percentages and yields of milk components.