Protein degradability and calcium salts of long-chain fatty acids in the diets of lactating dairy cows: productive responses.

Our objective was to evaluate the effect of excessive intake of ruminally degradable crude protein [11.1 and 15.7% of dietary dry matter (DM)] and supplemental fat (Ca salts of long-chain fatty acids at 0 or 2.2% of dietary DM) on the productive performance of lactating Holstein cows (n = 45) during the first 120 d postpartum. The main N sources were soybean meal and urea in the diets with high concentrations of degradable protein versus a combination of vegetable and animal by-product feedstuffs in the diets with less degradable protein. Cows fed the diets with excess degradable protein had slower rates of increase in DM intake (DMI) and milk production, had lower plasma insulin and greater plasma glucose and urea concentrations, and lost more than twice the body weight of cows fed the diets with less degradable protein. Supplemental fat in the highly degradable protein diet reduced the loss of body condition, stimulated DMI, and reduced concentrations of plasma nonesterified fatty acids early postpartum compared with the highly degradable protein diet without added fat. Without affecting DMI, supplemental fat stimulated milk production (2 kg/d) starting at 3 wk postpartum. During early lactation, DMI and milk production were sensitive to the degree of ruminal degradability of protein and energy supplementation in the form of fat.