OVULATORY RESPONSES AND PLASMA LUTEINIZING-HORMONE CONCENTRATIONS IN DAIRY HEIFERS AFTER TREATMENT WITH EXOGENOUS PROGESTERONE AND ESTRADIOL BENZOATE

The objectives of this experiment were to determine the effects of 0.5 mg estradiol benzoate, administered intramuscularly 24 h after remova l of CIDR-B progesterone containing intravaginal devices, on the time to estrus, ovulation and peak LH concentration in dairy heifers. Ovula tory responses and plasma LH concentrations were examined using 14 Fri esian dairy heifers in 2 separate treatment periods. Al heifers receiv ed a CIDR-B progesterone-containing intravaginal device with an attach ed 10-mg estradiol benzoate capsule for 12 d. Within each period, 24 h after CIDR-B removal, 7 heifers received an intramuscular injection o f 0.5 mg estradiol benzoate while the remaining 7 heifers received an intramuscular injection of a placebo. Blood samples for LH assay were collected at 0, 6 and 12 h, and then every 4 h for 60 h after estradio l injection Detection of estrus was conducted at 4-h intervals, and ul trasonographical examination to detect ovulation was conducted every 8 h for 88 h after removal of the CIDR-B device. Treatment with estradi ol benzoate tended to reduce the time from device removal to the LH pe ak in Period 1 (median time to LH peak 40.1 vs 63.9 h; P=0.07). In Per iod 2, treatment with estradiol had no significant effect on the time to the LH peak, standing estrus or ovulation. We hypothesize that the period effect was due to the stage of cycle at the time of treatment. For heifers treated in Period 1, the stage of cycle was random. Howeve r, because of the prior synchronization of estrus, which was implicit in the experimental design, heifers in Period 2 tended to be in late d iestrus. The administration of estradiol benzoate after treatment with exogenous progesterone appears to overcome the variability in timing of LH peaks typically occurring in a herd of synchronized heifers due to different stages of follicular development. (C) 1997 by Elsevier Sc ience Inc.