The resumption of cyclic reproductive activity after lambing was studied in a total of fifty Finn \m=x\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`p rogestagen' 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\m=.\3\m=+-\1\m=.\5and 33\m=.\2\m=+-\1\m=.\6hr for ewes which suckled lambs for 1 and 24 days respectively, and 41\m=.\0\m=+-\1\m=.\5hr for the lactating ewes (P<0\m=.\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\m=.\09\m=+-\0\m=.\3,2\m=.\4\m=+-\0\m=.\3 and 3\m=.\09\m=+-\0\m=.\3,respectively. A pr e\ x=r eq-\ ovulatory LH release was detected in the peripheral plasma of thi rty\x=req-\ four of the thirty-five ewes studied. The mean intervals from the onset of oestrus to the start of the LH discharge were 9\m=.\1\m=+-\1\m=.\2and 6\m=.\1\m=+-\1\m=.\3hr for the ewes which suckled lambs for 1 or 24 days and 12\ m=. \ 6\ m=+-\ 1\ m=. \ 3hr for the lactating ewes (P<0\m=.\01). The magnitude and duration of the LH peak were not influenced by lactation.

SUMMARYDexamethasone (16 mg) induced parturition in sheep when injected i n the dam on day 141 of gestation. The mean time of parturition was 49·0 ±2·18 hr after the injection, with 90% of the injected ewes lambed by the 143rd day of gestation compared with the 147th day in the control group. There was no ill effect on the ewe or the lamb born.

In two experiments carried out in the autumn of 1971 and 1972 (treatment prefixes 1 and 2 respectively), 84 Finn x Dorset ewes found by X-ray to carry from 1 to 4 foetuses were individually fed during the last 6 weeks of gestation either: 1A, ad libitum; 1H and 2H, 33 kcal M/kg ewe live weight+365 kcal per ME/kg foetus (anticipated birth weight); 1M, 80%; and 2L, 50% of the total energy fed to treatments 1H and 2H. The amounts given remained constant during the experimental period. The method of feeding according to foetal number within treatments, attempted to standardize the nutritional state within treatments. All ewes (except those on 2L which remained constant) gained 6 to 8 kg live weight during the last 6 weeks of gestation and lost 8 to 10 kg at parturition. The birth weight of lambs was not affected by the range of energy (2-3 to 4-4 Meal ME/ewe per day) consumed during the last 6 weeks of gestation. Plasma concentrations of free fatty acids (FFA), glucose and ketone bodies were determined weekly. The mean FFA levels were significantly higher in the low energy treatment groups than the high ones. The present data indicated that if there is little change in body weight in the early stages of pregnancy a 65 kg ewe bearing twin lambs, will require about 4 Meal ME/day during the last 50 days of gestation to prevent a rise in plasma FFA concentration or loss of weight over the gestation period.

Progesterone and LH concentrations were measured in the plasma of blood samples taken from forty-eight pregnant ewes on Days 100, 120 and 134 of gestation. The ewes, in two groups of twenty-four were maintained from Day 100 until parturition on two planes of nutrition which supplied daily energy and protein intakes of about 4-1 or 2-3 Mcal metabolizable energy and either 192 or 111 g digestible crude protein per ewe. Within the groups, the ewes carried one, two or three fetuses and the feed intake was adjusted according to litter size to produce a uniform nutritional state within the group. On Day 100, litter size affected the concentration of plasma progesterone (P less than 0-001), but had no effect on Days 120 or 134 when the ewes were fed according to litter size. The low feed intake however caused a significant increase in plasma progesterone concentrations. The LH concentrations showed no major changes during late pregnancy and no effect of nutrition or little size on the plasma hormone concentration was observed. It was concluded that the effect of litter size on plasma progesterone concentration recorded on Day 100 or gestation was partly mediated by level of nutrition.

After lambing forty-five ewes were allocated to three groups, two of sixteen and one of thirteen ewes. The lambs of the two groups of sixteen ewes were weaned on Day 1 after lambing and the ewes were fed a diet of 100% (Group H) or 50% (Group R) of maintenance energy requirements. The thirteen ewes in the third group (Group L) suckled twin lambs and were fed freely. During the first 3 weeks after lambing, oestrus was observed for 11/16 (Group H) and 8/16 (Group R) ewes; of the ewes which had shown oestrus in the two groups, ovulation occurred in 5/8 and 5/7 respectively. Only 1/13 Group-L ewes showed oestrus and ovulated during the same period. The mean plasma concentrations of progesterone and LH were unaffected by the treatments and were around 0-4 and 1-5 ng/ml, respectively. Restricted feeding had no effect on oestrus, ovulation or the hormone levels during the oestrus cycle following synchronization. The onset of oestrus and the start of the preovulatory discharge of LH were 3 and 6 hr later, respectively, in the lactating ewes (Group L) than in those in Groups H and R. Ewes in Group L also had a higher ovulation rate, 2-8 +/- 0-2 versus 2-1 +/- 0-2 (P less than 0-05). Restricted feeding reduced the number of ewes lambing; only 1/11 ewes in Group R, considered to have conceived because of the presence of high progesterone levels 17 days after mating, subsequently lambed compared with 6/12 in Group H and 5/9 in Group L.