The effect of photoperiod on serum concentrations of luteinizing and follicle stimulating hormones in prepubertal heifers following ovariectomy and estradiol injection

Eleven heifers, between 63 and 197 days of age, were exposed to 18 hr light/day (L) or natural photoperiods (N), beginning October 19, 1979. They were ovariectomized 8 weeks later. LH concentrations after ovariectomy were not affected by photoperiod, but the rate of increase of FSH after ovariectomy was greater (P<0.10) for group L than for group N. Three weeks after ovariectomy, heiters were injected, IV, with 0.1 mug/kg estradiol-17beta. LH concentrations initially decreased after injection. This was followed by a series of pulses larger than those prior to injection. FSH concentrations declined after injection and remained low throughout the sampling period. The net response of LH concentrations to estradiol (mean post-injection concentration minus mean pre-injection concentration) was greater (P=0.05) for group L (4.7 +/- 0.49 ng/ml) than for group N (2.9 +/- 0.37 ng/ml). Photoperiod did not affect the net response of FSH concentrations to estradiol. We concluded that exposing prepubertal heifers to 18 hr light/day during the winter resulted in a greater rate of increase of FSH after ovariectomy and greater estrogen-induced LH release. Because the response of LH to estradiol-17beta differed from the response of FSH, these hormones may be regulated differently.