Effect of rams on temporal hormone concentrations in Targhee ewes during the transition into the breeding season
McCosh, Richard Bryan
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The biostimulatory effect in sheep is known to cause a relatively rapid increase in LH pulse frequency in ewes that accelerates resumption of ovulatory activity during the transition into the breeding season. In addition there is the possibility that the biostimulatory effect involves changes in the metabolic status of ewes. Experiment 1, the hypotheses were that exposing seasonally anovular ewes to rams would not alter patterns of cortisol concentrations and that these changes are not associated with changes in temporal characteristics of LH concentrations. Cortisol pulse duration was longer in ewes exposed to rams (RE) than in ewes exposed to wethers (NE). The number of LH pulses, and LH pulse frequency was greater in RE ewes than in NE ewes. In RE ewes, as the pulse frequency and number of cortisol pulses increased there was a linear decrease in LH pulse frequency and number of LH pulses. The hypotheses in experiment 2 were that were that resumption of luteal activity and temporal patterns of cortisol, leptin, prolactin, IGF-1, T3 and T4 do not differ among virgin ewes exposed to rams during the transition into the breeding season. Resumption of luteal activity began earlier (P < 0.05) in RE than in NE ewes. Concentrations of T4 in RE ewes decreased less rapidly and over a longer interval before increasing by the end of the sampling period than those in NE ewes. Concentrations of PRL were greater in RE than in NE ewes 4 d after exposure but decreased over the next 12 d; whereas, PRL decreased in NE ewes during the first 6 d then increased over the next 14 d. In conclusion, ram exposure during the transition into the breeding season alters hypothalamic-pituitary-adrenal axis activity, which is related to an increase in LH pulse frequency, that hastens luteal activity. In addition, the onset of luteal activity is associated with alterations in metabolic status in ewes exposed to rams during the transition in to the breeding season.