Two methods of induction of ovulation were examined in Manchega ewes that were in postpartum anestrus during the nonbreeding season. The experiment was of 2 × 2 × 3 factorial design. The methods compared were introduction of rams and this treatment plus an im injection of 50 µg luteinizing hormone releasing hormone (LHRH) at the time rams were introduced. Variations in response due to month of treatment (April, May or June) and interval from lambing to treatment (1, 2 or 3 mo) and their interactions with type of treatment were examined. Responses studied were proportions of ewes showing increases in plasma progesterone at (a) 10 d or (b) 17 or 24 d post-treatment, or lambing by 200 d post-treatment, and interval from treatment to lambing for ewes that did lamb by 200 d. The formation of a corpus luteum was determined by concentrations of progesterone in plasma; a positive response was considered to be a concentration greater than .5 ng/ml (baseline values averaged .1 ng/ml). Overall, there was no benefit of LHRH above the response to introduction of males only, in any trait examined. There was a significant interaction of treatment with month of treatment on the proportion of ewes forming corpora lutea by 17 or 24 d after initiation of treatment. This proportion was lower in June (38 vs 66% in April and 82% in May) for ewes receiving LHRH, but did not differ among months (61 to 68%) for ewes exposed to males only. The higher estrous response in ewes receiving LHRH in May was reflected in a shorter interval from treatment to lambing in that group (168 vs 174 to 185 d in the other groups), but only 37.5% of the ewes lambed in that group. Intervals to lambing were shorter when treatments were given 2 or 3 mo after lambing, especially for ewes treated in May.
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López-Sebastián et al. (1984) studied this question.