Effect of Dietary Nitrogen Solubility on Lactation Performance and Protein and Dry Matter Degradation In Situ

Abstract

Lactating Holstein cows (30) were fed 1 of 5 corn grain, corn silage based diets during a 20-wk trial to determine effects of dietary N solubility on milk yield, milk composition and apparent digestibility of the diets. Cows were fed soybean meal supplemented diets for 2 wk postcalving and then assigned randomly to 1 of 5 diets that ranged from 21.7-34.4% soluble N. Protein supplements used to alter N solubility of the diets were regular soyben meal and 4 defatted soybean flakes that received variable heat treatment. Diets fed provided 85% of the cow''s N requirements and > 100% of their energy requirements had no effects on milk yield, 4% FCM [fat corrected milk] yield or milk composition. In a 2nd experiment rumen degradability of protein and dry matter in protein supplements and complete diets was estimated by the dacron bag technique. Fractional disappearance rates of protein and dry matter from dacron bags were measured by N and dry matter disappearance after 1, 2, 4, 8, 12 and 16 h of incubation in the rumen. Complete diets had 2 rates of dry matter and N disappearance; protein supplements had 3. For supplements, a rapid disappearance rate occurred from 0-1 h of incubation, a slow rate during 1-4 h of incubation and an intermediate rate from 4-16 h of incubation. The 3 rates probably correspond to physical washout and solubilization, a lag phase and a degradation phase.