Effect of age and dietary protein content on post-partum oestrous cyclicity in ewes lambing within their natural breeding season
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AbstractThe objective was to determine the effect of age and dietary protein content on onset and pattern of oestrous cyclicity in ewes lambing within 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 respectively were allocated equally to 4 groups. From 3 Nov, each group was housed with a raddled vasectomised ram in a strawbedded pen under natural photoperiod at 57 °N and fed ad libitum hay plus 1.2 kg/head/d concentrate containing 12.5 MJ ME/kg DM and 180 or 120 g/kg CP. Blood samples were obtained 3 times/week from all ewes for determination of plasma progesterone (P4). Data were analysed using a General Linear Model. Mean liveweights for multiparous vs primiparous ewes at allocation were 80.0 vs 58.8 kg (se 1.46) and at weaning (18 Dec) were 72.7 vs 55.1 kg (se 1.65). Liveweight loss during lactation was affected by age (P < 0.01) but not diet. There was no effect of age or diet on the onset of ovarian activity (P4 > 1 ng/ml in at least 2 consecutive samples), which commenced 14 to 43 days post-partum. Mean ± se days to onset of ovarian activity were 28.0 ± 1.53 vs 25.0 ± 1.50 for multiparous vs primiparous ewes and 26.5 ± 1.53 vs 26.6 ± 1.50 for ewes fed the 18 vs 12% CP concentrate. Within each group, 2 to 5 ewes experienced initial short luteal phases and 1 formed a persistent corpus luteum. Results demonstrate that (i) the strong seasonal drive for reproduction in sheep overrides the inhibitory effects of lactation; and ii) post-partum oestrous cyclicity is not influenced by age or dietary protein content within the range studied.

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