Effect of steroids and/or 48 HR calf removal on serum luteinizing hormone concentrations in anestrous beef cows.

Serum luteinizing hormone (LH) concentrations were quantified in 27 thin, anestrous cows with suckling calves in each of three treatment groups: Syncro-Mate-B (SMB), 48 hr calf removal (CR), and SMB plus CR (SMB + CR). The SMB treatment consisted of a 9 day ear implant containing 6 mg norgestomet and an intramuscular injection containing 3 mg norgestomet and 6 mg estradiol valerate given at the time of implant placement. In the SMB + CR group, CR began at the time of implant removal (0 hr). Blood samples were collected every 4 hr via puncture of a tail vessel beginning 12 hr prior to implant and/or CR and continued for 72 hr thereafter. Before implant and/or CR (-12 to 0 hr), LH concentrations were higher (P<.01) in the CR (1.1 ng/ml) group than in the SMB (.6 ng/ml) and SMB + CR (.8 ng/ml) groups. Following implant and/or CR (4 to 48 hr), LH concentrations increased (P<.01) in the CR (1.8 ng/ml) and SMB + CR (1.3 ng/ml) groups, but remained unchanged in the SMB (.7 ng/ml) group. Furthermore, LH concentrations were higher (P<.05) in the CR group compared to the SMB + CR group. Circulating concentrations of LH declined (P<.01) to 1.2 ng/ml in the CR group following calf return, but remained unchanged in the other two groups. Although more (P<.01) cows in the SMB and SMB + CR groups were detected in estrus than in the CR group, there was no difference (P.10) in the number of cows ovulating between the three treatment groups. These results suggest that CR will increase circulating LH concentrations by 24 hr post CR and that SMB may partially suppress the CR induced LH release following implant and calf removal.