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

Thirty lactating Holstein cows were fed one of five corn grain, corn silage based diets during a 20-wk trial to determine effects of dietary nitrogen 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 one of five diets that ranged from 21.7 to 34.4% soluble nitrogen.Protein supplements used to alter nitrogen solubility of the diets were regular soybean meal and four defatted soybean flakes that received variable heat treatment.Diets fed to provide 85% of the cow's nitrogen requirements and greater than 100% of their energy requirements had no effects on milk yield, 4% FCM yield, or milk composition.tn a second 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 nitrogen and dry matter disappearance after 1, 2, 4, 8, 12, and 16 h of incubation in the rumen.Complete diets had two rates of dry matter and nitrogen disappearance; protein supplements had three.For supplements, a rapid disappearance rate occurred from 0 to 1 h of incubation, a slow rate during 1 to 4 h of incubation, and an intermediate rate from 4 to 16 h of incubation.The three rates probably correspond to physical washout and solubilization, a lag phase, and a degradation phase.