PREOVULATORY PATTERNS OF ESTRADIOL-17-BETA IN RELATION TO THE SUPEROVULATORY RESPONSE IN COWS AND HEIFERS

A study was undertaken to determine the relationship between the profiles of preovulatory estradiol concentrations in blood, the number of preovulatory follicles, and the number of collected embryos in superovulated cows and heifers. Superovulation (SO) was FSH-induced in 23 cows and 12 heifers of dairy breeds. Luteolysis was induced with Cloprostenol, given on day 3 of the FSH treatment. The animals were observed twice daily for signs of estrus and inseminated at fixed times after injection of prostaglandin. On day 7 (estrus = day 0) embryos were collected non-surgically. The animals were clinically examined (rectal palpation and ultrasonography of the ovaries) and plasma estradiol and progesterone levels were determined. Signs of estrus were observed in all animals 48 h after PG injection. Cows had more (p < 0.05) pre ovulatory follicles (27.5 +/- 2.4, mean +/- SEM) than heifers (14.8 +/- 1.7). The mean number (+/- SEM) of preovulatory follicles correlated positively with the preovulatory peak concentration of estradiol. The correlation was significant in cows (r = 0.50, p < 0.005) but not in heifers (r = 0.50, p = 0.094). The mean number (+/- SEM) of day-7 embryos collected nonsurgically, was significantly higher (p < 0.05) in cows (8.5 +/- 1.0) than in heifers (4.6 +/- 1.0), and also correlated positively with the mean (+/- SEM) preovulatory-peak estradiol concentration in cows (r = 0.49, p < 0.05) and in heifers (r = 0.44, p = 0.14). The patterns for estradiol and progesterone during the overall superovulatory treatment and up to 4 days after embryo collection in four animals (three cows, one heifer) selected at random, showed that estradiol decreased for 3-4 days after the peak concentrations were attained. This was followed by a tendency to increase on days 5-6 (1-2 days prior to embryo collection), which was followed by decreasing concentrations. Concentrations of progesterone were within expected concentrations throughout the experimental period.