THE EFFECT OF LACTATION ON THE RESUMPTION OF REPRODUCTIVE ACTIVITY AND THE PREOVULATORY RELEASE OF LH IN FINN × DORSET EWES
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Summary.
The resumption of cyclic reproductive activity after lambing was studied in a total of fifty Finn × Dorset ewes which lactated for 1, 24 or 56 days. The ewes lambed in December 1971 and were maintained on a high plane of nutrition throughout pregnancy. Eight of the seventeen ewes which suckled for 1 day showed spontaneous oestrus between 2 and 20 days after lambing compared to only one of the thirty-three lactating ewes. Following treatment with `progestagen' vaginal sponges for 12 days from 20 to 24 days post partum, all the ewes showed oestrus. The mean interval between sponge withdrawal and the onset of oestrus was 35·3±1·5 and 33·2±1·6 hr for ewes which suckled lambs for 1 and 24 days respectively, and 41·0±1·5 hr for the lactating ewes (P<0·01). Thirty-one of thirty-three ewes ovulated. No significant differences were found between the numbers of corpora lutea observed for the three treatments: 3·09±0·3, 2·4±0·3 and 3·09±0·3, respectively. A preovulatory LH release was detected in the peripheral plasma of thirty-four of the thirty-five ewes studied. The mean intervals from the onset of oestrus to the start of the LH discharge were 9·1±1·2 and 6·1±1·3 hr for the ewes which suckled lambs for 1 or 24 days and 12·6±1·3 hr for the lactating ewes (P<0·01). The magnitude and duration of the LH peak were not influenced by lactation.
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