Abstract
Forty-nine gilts (average live weight 232 lb.) of mixed breeding were divided into five experimental groups. The experimental period began on the 12th day of an estrual cycle with all treatments being timed from that stage. Group I received no injections; groups II, III and IV received progesterone (0.4 mg. per lb. of body weight, subcutaneously) from day one through day 17 of the experiment; group IV also received 20 mg. estradiol and group V 20 mg. estradiol only on day 14. Group II was slaughtered on day 25 of the experiment while all other groups were killed on day 18. The average number of follicles 3 mm. or larger on the ovaries of gilts in the five treatment groups was 19.9, 7.6, 36.8, 9.2, 3.7, respectively. Estradiol suppressed the development and maintenance of follicles beyond the 3 mm. stage. Prolongation of the cycle by progesterone allowed a significantly greater number of follicles to develop. Pituitary glands were tested for gonadotrophic potency using five-day-old, White Leghorn cockerels. Chick testis weights for the five treatment groups were 51, 54, 64, 64 and 60 mg. and comb weights were 96, 86, 134, 140 and 107 mg. Significant differences were found only between comb weights. These differences between comb weights indicate that the action of progesterone in delaying heat and ovulation is to block the release of luteinizer from the pituitary rather than to suppress its production. Withdrawing progesterone led to LH release but a single injection of 20 mg. estradiol in the presence of functional corpora lutea or of the exogenous progesterone did not. Seven of 11 gilts in group II had ovulated at time of slaughter with an average of 14.1 ova. These differed (P<0,05) from the averages of groups land V (11.3 and 11.7). Copyright © . .