RESUMPTION OF ESTROUS AND OVARIAN CYCLICITY DURING THE POSTPARTUM PERIOD IN AUTUMN-LAMBING EWES IS NOT INFLUENCED BY AGE OR DIETARY-PROTEINCONTENT

The objective was to determine the effect of age and dietary protein c ontent on post-partum oestrous and ovarian cyclicity in autumn-lambing ewes. Twenty-four multiparous (age 4.5 (s.e. 0.61) years) and 24 prim iparous (age 1.5 years) ewes which lambed on 28 October (s.e. 0.2) and were rearing 2.0 and 2.7 (s.e. 0.13) lambs, respectively, were equall y allocated to two dietary treatments in a 2 X 2 factorial design. Die ts comprised ad libitum hay supplemented with concentrate (12.5 MJ met abolizable energy per kg dry matter (DM) and either 210 or 140 g crude protein per kg DM), offered at a daily allowance of 1.2 kg per ewe fr om lambing to weaning (18 December) and 0.4 kg per ewe thereafter. Thr oughout the study (3 November to 24 May) eves were group-housed in str aw-bedded pens under natural photoperiod at 57 degrees N and their liv e weights and body condition scores were assessed at 2-week intervals. A raddled vasectomized ram was continually present with each group an d ewes marked by the ram were recorded thrice weekly. Blood samples we re obtained thrice weekly for measurement of peripheral progesterone c oncentrations and ovulation rate was determined by laparoscopy during January and March. There was no significant effect of dietary protein content on any of the variables measured. For multiparous and primipar ous ewes, mean live weights and body condition scares at lambing were 79.5 (s.e. 1.41) v. 58.4 (s.e. 1.54) kg (P < 0.001) and 2.5 (s.e. 0.05 ) v. 2.2 (s.e. 0.05) (P < 0.001) and daily live-weight losses over the suckling period were 153 (s.e. 19.3) v. 85 (s.e. 21.2) g/day (P < 0.0 5). Intervals from parturition to first overt oestrus were 39 (s.e. 3. 2) v. 33 (s.e. 2.3) days and to the onset of ovarian cyclicity were 29 (s.e. 4.9) v. 25 (s.e. 1.5) days. Durations of ovarian cyclicity were 133 (s.e. 27.1) and 110 (s.e. 5.7) days (P < 0.01) during which 7.6 ( s.e. 0.32) and 6.5 (s.e. 0.34) (P < 0.05) ovarian cycles were recorded respectively. Fifteen aves had initial short cycles (less than or equ al to 10 days), six had prolonged cycles (greater than or equal to 22 days) and 11 had extended inter-luteal phase intervals (greater than o r equal to 8 days). All ewes had recent ovulations at the January lapa roscopy but only 19 multiparous and nine primiparous ewes had ovulatio ns in March. Mean ovulation rates were: January 2 2 (s.e. 0.14) and 1. 4 (s.e. 0.13) (P < 0.001), and March 1.6 (s.e. 0.23) and 1.2 (s.e. 0.1 5) for multiparous and primiparous ewes, respectively. It is concluded that, for aves lambing at their normal time of mating, (i) the strong seasonal drive for reproduction results in the resumption of oestrous and ovarian cyclicity during the early post-partum period; (ii) initi al luteal phases may be of short or extended duration; (iii) there is no effect of age or dietary protein content, within the range studied, on the onset of ovarian cyclicity; and (iv) multiparous ewes experien ce a greater number of ovarian cycles and achieve higher ovulation rat es than primiparous ewes.