The induction of oestrus during the early post partum period was attempted in 172 Merino ewes which lactated during autumn. Treatment was initiated on day 15 post partum and included groups receiving (i) a single 50 μg injection of GnRH, (ii) two 50 μg injections of GnRH 16 days apart, (iii) 600 IU PMSG plus a progesterone implant for 16 days followed by a further 600 IU PMSG, (iv) 600 IU PMSG and 50 μg oestradiol benzoate (ODB) followed by aprogesterone implant for 16 days and a final injection of 600 IU PMSG on day 31, and (v) a control group. Half the ewes were run with fertile rams, while the remaining ewes were associated with teaser rams to detect oestrus. Compared to untreated ewes the interval to first oestrus (mean 49,3 days) was significantly reduced (P<O,O1) in all of the hormonally treated groups, except those which received two injections of GnRH. Overt oestrus within 40 days after lambing was more effectively induced by treatments which included PMSG and a progesterone implant than GnRH injection only (63,4 vs 36,2 per cent; P <0,01). In contrast, the lambing rates for these treatments exhibited the opposite trend (56,7 vs 65,7 per cent), while 93,7 per cent of the control group lambed. Overall, no significant differences in the mean lambing intervals were obtained, suggesting that the use of exogenous hormones was of little value during the autumn period
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