Production responses of high producing Holstein cows to ruminally protected phenylalanine and tyrosine supplemented to diets containing high levels of canola meal

Phenylalanine (Phe) was first identified as a potentially limiting amino acid (AA) in lactating dairy cows in 1974. There is the possibility that conversion of Phe to tyrosine (Tyr) is not efficient enough to supply all Tyr requirements for milk production in high producing dairy cows, thereby suggesting that Tyr itself could become a functionally limiting AA when it, and/or Phe, is not supplied to the mammary gland in adequate quantities. Our objective was to determine if previous positive responses of lactating cows to Phe supplementation when fed a diet containing high levels of canola meal (CM) could be improved by supplying Tyr in place of some Phe. The experimental design was a 4 × 4 Latin square using 4 pens of ∼315 early lactation multiparous cows/pen with four 21 d periods. Diets were formulated to include a low CM treatment (LCM) containing CM at ∼120 g CM/kg dry matter (DM) and a high CM treatment (HCM) with CM at ∼170 g CM/kg DM. The other two treatments were the HCM diet, but with Phe (HCM + P) or Phe plus Tyr (HCM + PT) added in a ruminally protected (RP) form. The DM intake tended (P = 0.10) to decline, but milk yield did not differ, between the LCM and HCM treatments, while milk fat, lactose and energy output was lower (P < 0.02) with the HCM treatment, and body condition score (BCS) gain was enhanced (P < 0.01). Adding Phe to the HCM diet had little impact on milk and component yields, but a positive BCS gain of 0.056 units/28 d decreased (P < 0.01) to a loss of 0.025 units. Substitution of half the added Phe with Tyr lowered (P < 0.05) milk and milk lactose yields while further lowering (P < 0.01) the BCS loss to 0.080 units/28 d. While differences among diets in plasma AA concentrations were small, adding Phe to the HCM diet increased (P = 0.05) plasma Phe and Ala (P = 0.02), but plasma Phe only tended (P = 0.09) to decline when Tyr partially replaced Phe to create the HCM+PT diet. Contrary to expectations based upon prior studies, increased CM inclusion from 120 to 170 g/kg DM only modestly decreased overall animal performance. While it is clear that supplementation of Phe, or Phe plus Tyr, to the HCM diet was not necessary, the unexpected and substantive negative effects of Phe or Phe plus Tyr on overall animal performance suggests that the bioactivity of these AA is highly important, while highlighting the risks of supplementing AA that are not required.