Thirty-One mature ewes of mixed breeding were hysterectomized on day 8 of the cycle (day 0=day of heat) and assigned at random to a 2 µ 2 µ 3 factorial experiment. Treatments consisted of 0 or 0.5 mg bovine LH per day for 6 days beginning on the 15th (23-day-old CL) or 30th (38-day-old CL) day after hysterectomy. Ewes were slaughtered 1, 10 or 20 days after LH withdrawal and the corpora lutea incubated in vitro and extracted for progesterone. New ovulations were induced in five of 13 ewes given LH. The 23-day-old CL were significantly (P<.01) heavier than the 38-day-old CL. There was a significant interaction (P<.05) of interval to incubation with age of CL. Total progesterone was greater (P<.10) in the 23-day-old CL than in the 38-day-old CL. Net progesterone synthesis was significantly (P<.05) greater in the 38-day- than 23-day-old CL. There was a significant interaction (P<.05) of age of CL with LH or saline injection and interval to incubation. There was an apparent “depression of synthesis” or net loss of progesterone, in response to LH added in vitro, by the 23-day-old CL removed 1 day after 6 days of LH treatment in vivo. The previous administration of LH in vivo did not cause a shortening of the life span of the CL.

The effect of small amounts of estradiol-17-β on early reproductive performance in 68 gilts fed low and high fiber rations was studied. Estradiol-17-β given in doses of 20 and 100 mcg. daily from the 9th to the 24th day of gestation did not affect embryo survival. The average individual corpus luteum weight of the treated gilts was less than that of the control gilts. Breed differences in corpus luteum weight also were significant, the Poland China having the heaviest followed by the Chester White and Hampshire. Gilts fed on a low fiber ration had significantly heavier average individual corpus luteum weights than those fed on a high fiber ration. Total progesterone content of the corpora lutea was not affected significantly by breed, fiber content of ration or treatment with estradiol-17-β.

Forty-five Chester White and 40 Poland China gilts were used to study physiological variations in ovulation rates, presumably brought about by variations in the amount of pituitary gonadotropins secreted and released, and by the ability of the ovaries to respond to the gonadotropins. Experimentally, variations in ovulation rates were brought about by using animals on different energy intakes and of different breeds. Supplementing the basal ration with lard for approximately 14 days increased the potential ovulation rates significantly by 3.3 ova. The Chester White gilts had a greater potential by 3.9 ova than Poland China gilts of the same sexual age. There appeared to be no difference in the proportion of mature follicles that were ovulated as an effect of feed or breed. Total follicular fluid weight was greater by 0.63 gm. in the high-energy than low-energy-fed gilts. Similarly, the Chester White gilts showed greater follicular development than Poland China gilts. Progesterone inhibition of follicular development was greater in the low-energy than high-energy-fed gilts. Follicular development in the Chester White and Poland China breeds was equal under progesterone inhibition, but the Chester Whites showed greater response to PMS than the Poland Chinas. The ration treatments to which the gilts were subjected failed to show any differences in unit concentrations of gonadotropins in the anterior pituitary gland weight. The Poland China breed had higher unit concentrations of FSH and LH than the Chester White breed. The release of FSH and LH was at least partially inhibited by progesterone. The ratio of FSH to LH was wider in progesterone-injected than in estrous gilts.

Five trials involving 193 sows and gilts indicate that 6-methyl-17-acetoxyprogesterone doses of 100 mg. or greater are required to inhibit estrus and ovulation in the gilt. One or more cystic follicles developed in 58% of the females after withdrawal of eight different hormone doses ranging from 50 to 400 mg. per head per day. Cystic follicles developed during treatment when a low dose of 60 mg. was fed and after treatment when the dose was 240 mg. Gilts fed 75 mg. developed fewer cystic follicles after hormone withdrawal if treatment was started during the follicular rather than the luteal phase of the estrous cycle. Sows maintained preovulatory size follicles for at least 8 days after the withdrawal of 200 mg. or more, and gilts maintained pre-ovulatory size follicles for 8 days or more after the withdrawal of 400 mg. Sows also reached other dose response end points at a lower level than gilts. Increasing the duration of hormone feeding from 10 to 20 days resulted in more corpora lutea or follicles>6 mm. in diameter, fewer cystic follicles and more preovulatory size follicles at 8 days after hormone withdrawal. After withdrawal of a 240 mg. dose, Yorkshire sows had developed cystic follicles; whereas Duroc sows had developed preovulatory follicles. Neither frequency of feeding the progestational hormone, the energy content nor the fiber content of the ration affected the probability of ovulation or cystic follicle formation after hormone withdrawal.

The effect of restricted feeding, crossbreeding and season of birth on age at puberty in swine was studied in six trials involving 368 gilts. Full-fed gilts grew more rapidly and attained puberty at an earlier age than restricted-fed gilts. Their earlier age at puberty appeared to be explainable on the basis of their greater growth rates. Poland China gilts grew more rapidly than Chester White gilts but no consistent differences in age at puberty were found for the two breeds. Reciprocal crossbred gilts of the two breeds attained puberty significantly earlier than either parent breed. The effect of season of birth on age at puberty was found to differ for the two breeds; spring-born gilts attained puberty earlier than fall-born gilts in the Chester White breed, while in the Poland China breed the fall-born gilts attained puberty earlier.