LHRH antagonist decreases LH and progesterone secretion but does not alter length of estrous cycle in heifers.

The objective of this study was to evaluate the suppressive effect of an LHRH antagonist, Cetrorelix SB-75 (SB-75), on secretion of LH, FSH and ovarian function in beef heifers. In Exp. 1, heifers were treated with a single dose of 10 microg/kg body weight intramuscularly on d 3 of the estrous cycle. In Exp. 2, heifers received either a single injection (100 microg/kg) of SB-75 on d 3 of the estrous cycle or multiple injections of 20 microg/kg on d 3, 4, 5, 6, and 7. Serum LH, but not FSH, was suppressed from one to several days. However, neither FSH nor progesterone was significantly altered. In Exp. 3, heifers received an injection vehicle (5% mannitol) or 100 microg/kg BW of SB-75 on d 1 of the estrous cycle (30 h after first observed standing estrus). Injection of SB-75 suppressed LH pulse frequency on d 3, 5, and 7 (P < 0.001). The mean LH concentrations in the SB-75 treatment groups were lower on d 3 (P < 0.01) and 5 (P < 0.05). There were no differences (P > 0.1) between the two groups in the mean concentrations of LH on d 1, 7, or 14. Treatment did not affect the secretion pattern or concentration of FSH. Injection of SB-75 did not alter estradiol-173 concentrations (P > 0.1). Treatment reduced corpus luteum (CL) function as indicated by lower progesterone production. However, the length of the estrous cycle was not shortened. These data show that the CL can form and survive in the face of depressed LH concentrations during the early stages of the estrous cycle.