LUTEOLYSIS DURING 2 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 on set of estrus in heifers before and after luteolysis was induced with PGF(2 alpha) 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- 17 beta, and LH while heifers were observed visually for estrus and mo nitored for standing activity by pressure-sensitive, radiotelemetric d evices. Although the concentrations of estradiol-17 beta that were ass ociated with the putative appearance of the first dominant follicle de clined before luteolysis was induced early in the cycle, some heifers that were given PGF(2 alpha) were in estrus as early as 35 h. Compared with heifers treated late in the estrous cycle, heifers that were tre ated early in the cycle produced less progesterone before PGF(2 alpha) treatment and had greater peak concentrations of estradiol-17 beta at estrus. In addition, heifers that were treated early in the cycle had shorter intervals from PGF(2 alpha) treatment to estrus, to peak estr adiol-17 beta, and to peak LH and to initiation of estrus after the pe ak in estradiol-17 beta than did heifers treated later in the cycle. T he increase in estradiol-17 beta associated with the putative first-wa ve follicle of the subsequent cycle and the duration of that cycle in early cycle heifers was less than after late cycle luteolysis. Results indicated that greater concentrations of estradiol-17 beta during est rus may be related to the durations of previous cycles and less proges terone exposure before luteolysis. The onset of estrus corresponded cl osely to, hut preceded, the preovulatory LH surge by approximately 3 h .