RESPONSE OF EARLY LACTATION COWS TO DIETS THAT VARY IN RUMINAL DEGRADABILITY OF CARBOHYDRATES AND AMOUNT OF FAT

An experiment was conducted to test the hypothesis that increasing the amount of fermentable carbohydrates in a high fat diet may prevent the depression in milk protein content that usually occurs with high fat diets. Forty Holstein cows were randomly assigned in a 2 by 2 factorial design to evaluate two sources of carbohydrates (corn and dried whey) without or with added fat (soybean meal and extruded soybeans) in the concentrate mix during wk 4 through 16 postpartum. Total mixed diets, formulated to be isonitrogenous at 16% CP, contained (DM basis) 40% corn silage, 10% alfalfa hay, and 50% of the respective concentrate mix. Production of milk (32.7 and 33.6 kg/d), 4% FCM (28.3 and 29.4 kg/d), and SCM (28.3 and 29.2 kg/d) increased with dried whey in the diet. Percentages of fat (3.14 and 3.09) and protein (2.98 and 2.99) were similar for cows fed corn and dried whey diets. Production of milk (31.7 and 34.6 kg/d) increased when cows were fed extruded soybeans. However, production of 4% FCM (28.7 and 29.0 kg/d) and SCM (28.5 and 29.0 kg/d) were similar due to lower percentages of fat (3.26 and 2.98) and protein (3.03 and 2.95) for cows fed diets containing extruded soybeans. Feeding a readily fermentable carbohydrate source in a high fat diet (dried whey plus soybeans) did not increase the concentration of milk fat (2.91 and 3.05%) or protein (2.96 and 2.94%) relative to corn plus soybeans but did increased production of milk (35.3 and 33.8 kg/d). © 1990, American Dairy Science Association. All rights reserved.