Relationship between mammary gland metabolism and nitrogen efficiency in the lactating dairy cow

The objective of this research was to evaluate the responses in mammary metabolism and milk protein yield when the mammary gland is exposed to different nutritional scenarios and to correlate these with efficiency of nitrogen utilization. Two trials were conducted using Latin square designs. The first trial evaluated three dietary CP levels (16.1, 14.6 and 13.2%) with or without abomasal infusion of the predicted limiting essential amino acids (EAA) on 14.6 and 13.2% CP diets. Milk and milk protein yields were not affected by treatment (average 35.4 kg/d and 1.03 kg/d, respectively); efficiency of use of dietary CP and MP increased, and urinary N excretion decreased, as dietary CP decreased. Infusions of EAA increased mammary clearance rate for the non-infused EAA, allowing for the same yield of milk protein despite lower arterial concentrations of these AA in the infused treatments. Overall, efficiency of N utilization did not appear to be affected by changes in mammary metabolism and was probably driven by reduced extra-mammary catabolism as AA supply was reduced. The second trial evaluated post-ruminal infusions of glucose, acetate and casein, alone or in combination. Glucose increased milk protein yield through greater mammary AA clearance rate and reduced AA oxidation within the gland, achieving a 58% efficiency of MP utilization. When glucose was infused with casein, the recovery of infused casein in milk protein was 47%. Acetate had no effect on milk protein yield. Under the condition of this experiment, cell signaling proteins were not affected by treatment and therefore did not help explain the responses to infused nutrients. In conclusion, glucose has a clear stimulating effect on milk protein synthesis and the mammary gland has the ability to increase the supply of nutrients to match its synthetic capacity. Both experiments demonstrated that the NRC (2001) model does not have the ability to predict the responses observed on either low (13.2%) CP diet or with glucose infusion. This indicates that the single nutrient-limiting nature of the NRC (2001) model, which milk and milk protein secretion based only on MP limitation, is insensitive to the interaction of glucose and MP supplies.