Effect of nutrition on the LH response to calf removal and GnRH

Ten primiparous crossbred cows were assigned to two dietary groups at calving. One group received 120% and the other group received 80% of the National Research Council (NRC) recommended allowance of dietary energy for primiparous cows. At 60 days postpartum, calves were removed from their dams. Blood samples were collected from the cows at 15-min intervals for 8 hr beginning at the time of calf removal and again 24 hr, 48 hr and 72 hr after calf removal. At 72 hr after calf removal, all cows were given 200 ug GnRH intravenously. At calf removal, serum LH concentrations were higher (P<0.01) for cows on 120% (0.9 +/- 0.03 ng/ml) compared to cows on 80% (0.5 +/- 0.03 ng/ml) of recommendations. Serum LH concentrations increased (1.6 +/- 0.1 ng/ml, P<0.01) by 24 hr in cows on the highenergy diet. In contrast, a similar increase was not observed in cows on the low-energy diet until 48 hr after calf removal (1.4 +/- 0.2 ng/ml, P<0.01). These contrasting patterns in serum LH concentrations resulted in a diet by time interaction (P<0.01). Serum LH concentrations increased in both dietary energy groups following GnRH injection, but the response was greater (P<0.01) in cows on the low-energy diet compared to the cows fed the high-energy diet. These results indicate that inadequate dietary energy delays the LH response to calf removal and increases the LH response to exogenous GnRH.