Poor reproductive response of anestrous suffolk ewes to ram exposure is not due to failure to secrete luteinizing hormone acutely

Twenty Polypay-sired ewes (Group P) and 14 Suffolk ewes (Group S) were bled at 48-h intervals for 10 d beginning on April 6, 1989, and the serum was assayed for progesterone to determine which ewes were anestrous; 9/20 Group P ewes were anestrous, compared with 14/14 Group S ewes (P < .001). Catheters were placed into the jugular vein of anestrous ewes from both breed groups (eight of Group P, seven of Group S), and samples of serum were collected at 12-min intervals for 4 h. Then, the ewes were exposed to mature, intact rams, and additional samples were taken at 12-min intervals for 4 h after ram exposure. The serum was later analyzed to determine the secretion of LH in response to ram introduction. After the acute bleeding period, all Group P and Group S ewes were commingled and exposed to a ram continuously for 42 d. Samples of serum were collected thrice weekly and analyzed for progesterone to monitor ovulatory response to ram introduction through the 42-d period. In addition, breeding activity and lambing data were recorded. When all Group P ewes were compared to Group S ewes, a greater proportion (P < .001) of Group P ewes were mated (20/20 vs 3/13; one Group S ewe died during the 42-d mating period) by the end of the 42-d period and more (P <.001) ewes lambed in the fall (17/20 Group P vs 2/13 Group S). Furthermore, when the reproductive responses of only anestrous ewes at ram introduction were compared, a greater proportion (P < .001) of anestrous Group P ewes were mated and lambed (9/9 and 8/9, respectively) than Group S ewes (3/13 and 2/13, respectively). The average concentration of LH was increased (P = .07), and the frequency of pulses of LH tended to be increased (P < .13) in both breeds of ewes after introduction of the ram, and these increases did not differ between the two breed groups. Finally, the profiles of progesterone during the period of ram exposure indicated that a greater proportion (P < .001) of anestrous Group P ewes had consecutive ovulations (more than one full-length luteal phase) after ram introduction (9/9 anestrous Group P) than Group S ewes (4/13). Thus, most Group S ewes either ovulated and had only one full-length luteal phase (6/13) or failed to have a full-length luteal phase (3/13) after ram introduction. Results of the current study do not support the hypothesis that ewes of highly seasonal genotypes respond more poorly to ram introduction than ewes of less seasonal genotypes because they fail to release LH acutely upon ram introduction. Rather, anestrous ewes of more highly seasonal genotypes seem to be less likely to have more than one full-length luteal phase after ram introduction; therefore, they are less likely to be mated in spring for fall lambing.