Does introduction of rams during the late luteal phase promote the estrus response in cyclic ewes?

The administration of two doses of PGF2α is widely used for estrous synchronization in cyclic ewes. The introduction of rams to previously isolated ewes (ram effect) induces an increase in LH pulsatility which stimulates estradiol secretion. Consequently, the introduction of the rams may trigger the luteolytic process through the increase of estradiol. Therefore, the aim of our first experiment was to determine if the second dose of a two PGF2α treatment can be substituted by half PGF2α dose plus the ram effect. Ewes were randomly assigned to 3 experimental groups. Ewes from 2 of those groups (E1-PGFD: n = 91, and PGHD: n = 110) remained in permanent contact with 16 vasectomized males since Day-40 (Day 0 = introduction of the rams). Both groups received a dose of PGF2α on Day-13. On Day 0, E1-PGFD ewes received a second dose of PGF2α, and PGHD ewes received a half-dose. Ewes from the third group (HD + RE; n = 95) remained isolated from males until Day 0, and received a dose of PGF2α on Day-13, and a half-dose on Day 0. On Day 0, the three groups were joined. The introduction of the rams increased the number of ewes that came into estrus early after the second dose, but this difference was not maintained the following days. Thus, we designed another experiment to determine if the day of the late luteal phase in which ewes are stimulated by the rams (12-15 days after a dose of PGF2α dose) influences the response. The ewes were randomly assigned to five groups, from which only the control group (n = 44) remained in contact with males; the other 174 ewes remained isolated from males since Day-30. Estrous cycles were presynchronized with 2 PGF2α doses separated 7 days, and received a third dose 13 days later (Day 0) (control group), or a half-dose plus the introduction of rams 12, 13, 14 or 15 days later (Day 0). However, we did not observe any differences in the estrus response according to the estrous cycle day. In conclusion, the ram effect was effective substituting half PGF2α in part of the flock, but the response was unrelated to the day of the late luteal phase in which the ewes are stimulated.