Serum Progesterone and L U T E I N I Z I N G Hormone in D a I R Y C a T T L E Fed V a R Y I N G Levels of Crude Proteini,2, 3

Three levels of dietary crude protein (CP), provided by soybean meal, were fed to determine effects of protein intake on progesterone and luteinizing hormone (LH) concentrations in serum of early postpartum high-producing dairy cows. Serum LH increased between the first and second weeks of lactation in all groups. Cows fed 12.7% CP had decreased basal serum LH levels (1.1 + .04 ng/ml) as compared to those fed 16.3 and 19.3% CP (1.3 + .03 ng/ml). A linear decline in serum LH occurred in cows which became pregnant, whereas serum LH remained unchanged in non-pregnant cows. Serum LH on day 2 and 14 of the first postpartum estrous cycle, the preconception cycle and the conception cycle tended to be lower in cows fed 12.7% CP compared to those fed 16.3 and 19.3% CP, particularly on day 2 of the conception cycle (1.2 + .2 and 1.8 + .2 ng/ml). Cows fed 12.7 or 16.3% CP had a decreased response to 100 gtg gonadotropin releasing hormone (GnRH) compared to those fed 19.3% CP (1718 + 552 and 3,660 -+ 543 ng min/ml). Serum progesterone was significantly higher in cows fed 12.7% CP than in those fed 16.3 and 19.3% CP on day 14 of the first observed cycle and conception cycle. An interaction between percent CP fed and the change in progesterone from the first observed cycle to the cycle of