Luteolysis during Two Stages of the Estrous Cycle: Subsequent Endocrine Profiles Associated with Radiotelemetrically Detected Estrus in Heifers

Our objective was to correlate hormonal changes with the timing and onset of estrus in heifers before and after luteolysis was induced with PGF_2α at two stages of the estrous cycle: d 6 to 9 (early; n = 10) or d 14 to 15 (late; n = 10). Blood was collected at intervals of 2 or 12 h to quantify serum concentrations of progesterone, estradiol-17β, and LH while heifers were observed visually for estrus and monitored for standing activity by pressure-sensitive, radiotelemetric devices. Although the concentrations of estradiol-17β that were associated with the putative appearance of the first dominant follicle declined before luteolysis was induced early in the cycle, some heifers that were given. PGF_2α were in estrus as early as 35 h. Compared with heifers treated late in the estrous cycle, heifers that were treated early in the cycle produced less progesterone before PGF_2α treatment and had greater peak concentrations of estradiol-17β at estrus. In addition, heifers that were treated early in the cycle had shorter intervals from PGF_2α treatment to estrus, to peak estradiol-17β, and to peak LH and to initiation of estrus after the peak in estradiol-17β than did heifers treated later in the cycle. The increase in estradiol-17β associated with the putative first-wave follicle of the subsequent cycle and the duration of that cycle in early cycle heifers was less than after cycle luteolysis. Results indicated that greater concentrations of estradiol-17β during estrus may be related to the durations of previous cycles and less progesterone exposure before luteolysis. The onset of estrus corresponded closely to, but preceded, the preovulatory LH surge by approximately 3 h.