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INFLUENCE OF AGE OF EWE AND PHOTOPERIOD ON THE INTERVALS BETWEEN PARTURITION AND FIRST OESTRUS IN LACTATING AND NONLACTATING EWES AT DIFFERENT NUTRITIONAL LEVELS

Summary.: A total of 131 2- to 6-year-old Mutton Merino ewes in two experiments were maintained in pens during the post-partum period and were exposed either to an artificial photoperiod of decreasing daylength, or to the naturally increasing photoperiod. After lambing in July, the ewes either were deprived of their lambs or they reared one lamb for 40 days. In both groups, the ewes were fed at one of four nutritional levels until the first post-partum oestrus had been recorded. The proportion of ewes showing first oestrus within 90 days of lambing was 44% for those subjected to the artificial photoperiod compared with 67% for those exposed to the natural photoperiod. First oestrus in the ewes which showed first oestrus more than 90 days after lambing was less delayed in those exposed to the artificial photoperiod. Nutritional level had no effect on the time of resumption of oestrus. In the natural photoperiod, 67% of lactating ewes and 68% of non-lactating ewes showed oestrus in 56·3 and 42·1 days, respectively, and a similar effect was observed in those exposed to the artificial photoperiod. In the natural photoperiod, 33% of 2-year-old, 53% of 3-year-old and 84% of mature ewes showed first oestrus within 90 days of lambing. It was concluded that, in most periods of the year, many ewes of this breed can be remated within 2 to 3 months of lambing and may thus be able to lamb three times in 2 years.