Milk protein synthesis in response to the provision of an "ideal" amino acid profile at 2 levels of metabolizable protein supply in dairy cows.

Providing a well-balanced supply of essential AA (EAA) can serve as an opportunity to reduce the protein intake for dairy cows by increasing the efficiency of metabolizable protein (or PDIE, its equivalent in the INRA feeding system) utilization for milk protein yield. Our objectives were to compare the effect of supplying an "ideal" EAA profile (EAA+) with an imbalanced AA profile (control) at 2 levels of PDIE/NE(L) (net energy for lactation) supplies to study the interaction between PDIE and AA profiles, and to compare this ideal profile with a simple mixture of the 4 most deficient EAA (4 EAA) in the diets of dairy cows. Six lactating multiparous Holstein cows received 6 treatments with 2 different levels of PDIE supplied by diets and AA infusions in the duodenum according to a changeover design with 3-wk periods. Within each PDIE supply level, the cows received 3 different AA infusions in the duodenum according to a 3×3 Latin square design with 1-wk subperiods, which corresponded to the following treatment groups: control (Glu), 4EAA (Lys, Met, His, Leu), and EAA+ (4 EAA plus Ile, Val, Phe, Trp, and Tyr). In the low and high PDIE treatments, diets and infusions provided 54.7 and 64.0 g/Mcal of PDIE/NE(L), respectively, which corresponded to crude protein levels of 13.6 and 15.2%, respectively. High-PDIE supplies increased the milk protein yield by 163 g/d, the milk protein content by 1.4 g/kg, the milk yield by 4.1 kg/d, and the lactose yield by 178 g/d and decreased the PDIE efficiency of utilization by 12.4%, whereas the N efficiency of utilization remained unaffected. Supplying the 2 EAA profiles (4EAA and EAA+) increased the milk protein yield by 67 g/d, the milk protein content by 1.3g/kg, and the milk yield by 0.9 kg/d, whereas the milk fat and milk lactose contents were decreased by 2.4 and 1.6g/kg, respectively. The responses regarding milk yield and its composition were similar whether the cows received the 4 EAA or the EAA+ treatment. The responses were similar for the milk yield and composition whether the EAA were supplied by low- or high-PDIE supplies. In conclusion, the efficiency of PDIE utilization was improved by 6.6% and the N efficiency was improved by 7.0% by correcting the EAA profiles, independent of the level of PDIE supplied. In addition, the increased efficiency observed, associated with provision of the 4 EAA, was similar to the provision of all EAA (EAA+) in this experiment.