Varying protein content and nitrogen solubility for pluriparous, lactating Holstein cows: digestive performance during early lactation

Thirty-four pluriparous Holstein cows were used to examine effects of crude protein (15.3 vs. 13.6%, dry matter basis) and nitrogen solubility (39.7 vs. 47.9%) on digestibility, energy and protein balances, and related traits during early lactation. Cows were assigned randomly at parturition to treatments in a 2 X 2 factorial arrangement; measurements were during wk 6, 10, and 14 postpartum. Diets were protein supplements (varying in protein content and nitrogen solubility), low-protein concentrate, corn silage treated with urea at ensiling, and wilted grass silage fed individually for ad libitum intake. Reducing nitrogen solubility of diet improved energy intake and productive energy factors but resulted in no significant increase of milk yield or body tissue balances, although body fat balance tended to be higher for low-solubility diets. There was no effect on overall partition of nitrogen in the body. Digestibility of carbohydrates and protein was higher for 15.3 than for 13.6% crude protein diets, resulting in higher total digestible nutrients and digestible and metabolizable energy of dry matter. Raising crude protein in the ration decreased milk yield of cows in early lactation but generally increased body tissue balances. Concentrations of rumen ammonia and blood urea nitrogen were not influenced by nitrogen solubility; increasing dietary protein increased concentrations of both, although not enough to increase consumption of free water. Best digestive efficiency was for animals fed the ration of medium crude protein and low nitrogen solubility.