Effect of ruminal protein degradability on lactation performance, nitrogen metabolism and excretion, and the evaluation of feed formulation models to predict protein requirements of lactating dairy cows

Formulation of diets to optimize the production of microbial protein in the rumen is required to minimize nitrogen (N) losses to the environment without sacrificing milk production. An experiment was conducted to investigate the effect of crude protein concentration and rumen degradability of the dietary protein on milk production and composition of dairy cows at three different stages of lactation. Cows were fed one of four diets with varying crude protein (CP) and ruminally undegraded protein (RUP) concentrations. Cows in early lactation responded positively to the increase in CP provided in the control as well as increased RUP in the diets. Cows in mid and late lactation did not respond to changes in CP or RUP. Data from this experiment were used to evaluate the protein requirements predicted by the NRC Requirements for Dairy Cattle (NRC) and the Cornell Net Carbohydrate and Protein System (CNCPS). The CNCPS predicted deficient ruminally degraded protein (RDP) and excess RUP for all treatments, whereas the NRC predicted sufficient RDP for cows fed the high RDP diet, but deficient RDP for cows fed the medium RDP and low RDP diets. Across all stages of lactation and diets, NRC and CNCPS overestimated allowable milk by 9% and 35%, respectively.To further investigate the effects of RDP on lactation performance, ruminal fermentation, and N utilization, two studies were conducted to evaluate diets that provide increasing levels of RDP. Milk yield, fat yield, and protein yield all increased when cows were fed increasing amounts of RDP. Milk fat and protein concentration each increased by 0.16 percentage units for cows fed high RDP compared to low RDP. Lactation performance was improved with increasing levels of RDP up to 9.8% of diet DM. Additional RDP beyond 9.8% of diet DM did not significantly increase milk production but did increase predicted N excreted. In the metabolism study, total N flow to the small intestine increased, nonbacterial N tended to increase, and urinary N increased linearly as cows were fed an increasing amount of RDP. Bacterial N flow to the duodenum and microbial synthesis were unaffected by treatment.