Changes in production and mammary metabolism of dairy cows in response to essential and nonessential amino acid infusions

This study was undertaken to increase our understanding of the need of the mammary gland for the different types of AA and how the mammary gland alters its metabolism in response to a variable AA supply. Eight lactating Holstein cows (61 ± 4 DIM) were used in a replicated 4 × 4 Latin square balanced for residual effects with 14-d periods. The diet was formulated to supply 100% of the net energy requirement and 72% of the metabolizable protein requirement. The 4 treatments were 1) abomasal infusions of water, 2) essential AA at 359 g/d, 3) nonessential AA at 356 g/d, and 4) essential AA at 359 g/d + nonessential AA at 356 g/d (total of 715 g/d). The infusates had the same AA profile as casein with the exception that Met was increased to maintain a 3:1 ratio of digestible Lys to Met and because of solubility limitations all the Tyr was replaced by Phe and part of the Glu was replaced by Gln. Milk yield and milk protein yield were increased by the essential AA treatments compared with the other treatments. Mammary uptake of β-hydroxybutyrate plus lactate tended to increase with the essential AA treatments, whereas glucose mammary uptake tended to be higher with the nonessential AA treatments. With the essential AA treatments, the mammary uptake:milk protein output ratio for the group 1 AA (His, Met, Phe, Trp, and Tyr) did not differ from 1 but tended to increase; the ratio for the group 2 AA (Ile, Leu, Lys, and Val) did increase, significantly exceeding unity when essential AA were infused. These results indicate that the mammary gland alters differently its uptake of both AA and energy-yielding nutrients in response to the amount and profile of AA presented to it and that even under situations of protein deficiency nonessential AA supplementation does not enhance milk and milk protein synthesis.