EFFECT OF AGE AND DIETARY-PROTEIN CONTENT ON POSTPARTUM ESTROUS CYCLICITY IN EWES LAMBING WITHIN THEIR NATURAL BREEDING-SEASON

The objective was to determine the effect of age and dietary protein c ontent on onset and pattern of oestrous cyclicity in ewes lambing with in their breeding season. Twenty-four multiparous (age 4.5 yr, se = 0. 61) and 24 primiparous (age 1.5 yr, se = 0.00) ewes that had lambed on 28 Oct (se 0.2 d) and were rearing 2.0 and 1.9 (se 0.13) lambs respec tively were allocated equally to 4 groups. From 3 Nov, each group was housed with a raddled vasectomised ram in a strawbedded pen under natu ral photoperiod at 57 degrees N and fed ad libitum hay plus 1.2 kg/hea d/d concentrate containing 12.5 MJ ME/kg DM and 180 or 120 g/kg CP. Bl ood samples were obtained 3 times/week from all ewes for determination of plasma progesterone (P4). Data were analysed using a General Linea r Model. Mean liveweights for multiparous vs primiparous ewes at alloc ation were 80.0 vs 58.8 kg (se 1.46) and at weaning (Is Dec) were 72.7 vs 55.1 kg (se 1.65). Liveweight loss during lactation was affected b y age (P < 0.01) but not diet. There was no effect of age or diet on t he onset of ovarian activity (P4 > 1 ng/ml in at least 2 consecutive s amples), which commenced 14 to 43 days post-partum. Mean +/- se days t o onset of ovarian activity were 28.0 +/- 1.53 vs 25.0 +/- 1.50 for mu ltiparous vs primiparous ewes and 26.5 +/- 1.53 vs 26.6 +/- 1.50 for e wes fed the Is vs 12% CP concentrate. Within each group, 2 to 5 ewes e xperienced initial short luteal phases and I farmed a persistent corpu s luteum. Results demonstrate that (i) the strong seasonal drive for r eproduction in sheep overrides the inhibitory effects of lactation; an d ii) post-partum oestrous cyclicity is not influenced by age or dieta ry protein content within the range studied. Published by Elsevier Sci ence B.V.