The objectives of this study were to evaluate the effects of supplementing three levels of rumen-protected methionine (RPM) at two levels of protein (14 vs. 16% CP) on reproductive and productive performance in primiparous lactating dairy cows during timed artificial insemination (TAI) protocol. Thirty-six lactating Holstein cows were synchronized by the Presynch-Ovsynch protocol. Cows were randomly assigned to 1 of 6 dietary treatments: 1) 14% CP and a control diet (14CP- CON; n = 6), 2) 14% CP and 15 g/head/d RPM (14CP-15RPM; n = 6), 3) 14%CP and 25 g/head/d RPM (14CP-25RPM; n = 6), 4) 16% CP and a control diet (16CP-CON; n = 6), 5) 16% CP and 15 g/head/d RPM (16CP-15RPM; n = 6), and 6) 16% CP and 25 g/head/d RPM (16CP-25RPM; n = 6). Treatments were initiated 30 ± 3 d and continued through 100 ± 3 d in milk. Progesterone (P4) concentrations and milk production and composition were evaluated weekly. Feeding 16CP-15RPM increased (P< 0.01) overall plasma P4. Furthermore, regardless of CP levels, cows supplemented with RPM decreased (P< 0.01) the calving interval. RPM diet had more (P<0.01) overall plasma P4 than cows fed the control diet. Feeding 16CP-25RPM or 16CP-15RPM increased (P< 0.01) milk yield, milk fat yield, and 4% fat corrected milk compared to other treatments. In conclusion, feeding 16CP-25RPM or 16CP-15RPM improved the productive and reproductive performance in primiparous lactating dairy cows.
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