Duration of the reproductive period decreases the frequency of preovulatory luteinizing hormone surges in heavy weight-sire line turkey hens.

The frequency of preovulatory luteinizing hormone (LH) surges is an important determinant of ovulation and oviposition rates in turkeys. Egg production rate is relatively poor in heavy weight-sire line type turkey hens and declines with advancing duration of the reproductive period. The purpose of this study was to measure frequency and characteristics of preovulatory LH surges in turkey hens of a heavy weight-sire line type early, at peak of egg production (Early), and late, after egg production rate had declined (Late), in a reproductive period. The Early hens were photostimulated with a continuous photoperiod [24 h light (L):0 h dark (D)] at 40 weeks of age and sampled during peak egg production at about 47 weeks of age. The Late hens were photostimulated at 40 weeks of age with a long day photoperiod (14L:10D). After a 27-week egg production period, the Late hens were switched to the 24L:0D photoperiod and sampled at 74 weeks of age. Continuous lighting was used during blood sampling to allow the rhythm of preovulatory LH surges to free run. All hens were cannulated 3-5 days before starting sampling and hourly blood samples were collected for 200 h. All hens were necropsied and ovarian and oviductal morphologies were measured after serial bleeding. The Late hens had a longer interval between intra-clutch preovulatory LH surges than the Early hens, and a higher incidence of atretic ovarian follicles. The Early hens had higher baseline and surge amplitude LH concentrations but lower progesterone (P4) surge amplitude concentrations than the Late hens. The duration of preovulatory LH surges, incidence of "blind" preovulatory LH surges, baseline P4 concentrations, and overall estradiol-17beta (E2) concentrations were not different between Early and Late hens. In conclusion, a longer interval between preovulatory LH surges, lower LH baseline and surge amplitude concentrations, a higher incidence of atretic follicles, and higher P4 surge amplitude concentration were associated with the decline in egg production late in the reproductive period in a heavy weight-sire line of turkey hens.