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Comparison of LH release and luteal function in cyclic and LH-RH-treated anoestrous ewes pretreated with PMSG or oestrogen

Summary. Pretreatment of seasonally anoestrous Clun Forest ewes with 750 i.u. PMSG or 50 µg oestradiol benzoate 24 or 7 h respectively before a single injection of 150 µg synthetic LH-RH significantly increased the release of LH compared to that after injection of 150 µg LH-RH alone. Total LH release in the two 'combined' treatments was approximately 70% of that found at a natural oestrus, compared to 25% for LH-RH alone. All but one of the treated ewes ovulated, but only those pretreated with PMSG consistently produced corpora lutea capable of elevating peripheral plasma progesterone concentrations although these were lower than those at natural mid-cycle. These progesterone concentrations were, however, comparable to those during the natural cycle when corrected for the higher metabolic clearance rate found during anoestrus.