PROTEIN VERSUS ENERGY SUPPLEMENTATION OF HIGH ALFALFA SILAGE DIETS FOR EARLY LACTATION COWS

An experiment was conducted to test the hypothesis that cows in early lactation are more likely to be limited by protein than by energy when fed diets containing high proportions of alfalfa silage. After a 2-wk covariate period, 26 multiparous cows were assigned to one of four treatments from wk 3 to 14 of lactation. Treatments were a positive control diet with equal amounts of forage and grain (DM basis), an all forage diet serving as negative control, and two test diets with 75% forage and 25% of either high moisture ear corn or expeller soybean meal. The latter two diets were equal in energy content but differed in the amount of protein made available for absorption in the small intestine. Alfalfa silage was the only forage in all diets. Milk yield with the four diets averaged 36.3, 25.5, 32.1, and 35.5 kg/d, and milk protein content was 2.93, 2.63, 2.77, and 2.75%. The negative control diet severely depressed milk protein content, which was not restored totally by either test diet. We concluded that absorbable protein supply to the intestine is first-limiting when high alfalfa silage diets are fed to cows in early lactation.