Environmental and neuroendocrine control of puberty and ovarian function in gilts and beef heifers

Abstract

<p>Experiments were conducted to investigate the influence of environment and neuroendocrine factors on puberty in Yorkshire gilts and ovarian function in hypophysial stalk-transected (HST) heifers;Thirty-six, 3-month-old gilts were reared in isolation under 3 photoperiodic regimens of total darkness, constant light and normal summer daylight and their growth and ages at puberty compared to littermates reared in groups under normal summer daylight. The growth rates, percentages of animals cycling at 8 months and the average ages at puberty were similar for all treatment groups. The olfactory bulbs of 12 Yorkshire gilts (OB) 3-5 months old were removed and their growth and ages at puberty compared to sham-operated littermates (SC). Growth rates in both groups were similar, but the percentage of OB gilts cycling at 9 months (33) was significantly less (P < 0.05) than SC gilts (80). These results indicate that isolation, total darkness or constant light do not influence significantly growth rate or time of puberty in Yorkshire gilts. Prepubertal olfactory bulbectomy, however, delays puberty but does not result in permanent loss of gonadal function;The hypothalamic regulation of luteinizing hormone (LH) secretion and ovarian function were also studied in beef heifers by infusing LHRH in a pulsatile manner (1-4 (mu)g/ml; 1 ml each hour) into 10 hypophysial stalk-transected (HST) beef heifers. The preinfusion levels of LH were low (< 1.0 ng/ml) and nonepisodic in these heifers. In 7 of 10 HST heifers, episodic LH secretion occurred in response to the LHRH infusions. Low amplitude LH spikes (5-10 ng/ml) occurred during the first 15 days of LHRH infusions. Between the 20th and 30th day of infusions, higher amplitude LH spikes (10-30 ng/ml) occurred. The LHRH infusions induced follicular growth in 4 heifers. In 2 of these heifers, follicular development to preovulatory diameters of 12-15 mm occurred. One heifer ovulated after the frequency of infusion had been increased to 1 (mu)g every 20 min for 8 h. This heifer was inseminated but failed to conceive. These results indicated that pulsatile infusion of LHRH (1 (mu)g each hour) induces LH secretion and initiates ovarian follicular growth in HST beef heifers.</p>