Serum luteinizing hormone, follicle stimulating hormone and prolactin response to photoperiod during the estrous cycle of Holstein heifers.

The objectives were to determine if photoperiods of 8 h light:16 h dark (8L:16D) and 16L:8D affect time of day when preovulatory surges of luteinizing hormone (LH) and follicle stimulating hormone (FSH) occur and if photoperiods affect diestrus and estrous concentrations of LH, FSH and prolactin (PRL). Postpubertal Holstein heifers were assigned to receive either 8L:16D (n - 9) or 16L:8D (n = 9) from September 21 through January 31. Hormone concentrations were determined in serum collected at 2-h intervals during diestrus and estrus of a single estrous cycle in autumn and again 4 mo later, during a single estrous cycle in winter. Neither photoperiod nor season affected time of day at which preovulatory LH and FSH surges occurred in heifers. The LH surges did not differ in amplitude and duration within and between photoperiods and within individual animals. In 29 of 33 surges, peak concentrations of FSH coincided with peaks of LH. Peak concentrations of FSH were greater during the winter than during autumn in heifers exposed to 8L:16D, but were not different between autumn and winter in heifers exposed to 16L:8D. Between autumn and winter, surges of FSH were similar in magnitude and duration within individual animals. In comparison with 8L:16D, PRL was increased in heifers that received 16L:8D in autumn, but photoperiod had no effect on PRL during winter. Concentrations of PRL were increased at estrus relative to diestrus in autumn and winter. It is concluded that photoperiods of 8 and 16 h do not affect the timing, amplitude or pattern of preovulatory surges of LH or FSH, the rise in PRL associated with estrus, or length of estrous cycles in postpubertal Holstein heifers.