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INFLUENCE OF SEASON OF LAMBING ON POST-PARTUM INTERVALS TO OVULATION AND OESTRUS IN LACTATING AND DRY EWES AT DIFFERENT NUTRITIONAL LEVELS

Summary.: Seventy-two Mutton Merino ewes in a 3x2³-factorial experiment lambed in July, November or March/April, suckled a lamb for 1 or 40 days, were fed after lambing at the rate of 110% or 60% of the estimated daily nutrient requirements of non-lactating and lactating ewes, respectively, and were run with (`joined') vasectomized rams from 20 or 30 days post partum. After the July lambing, the date of the first post-partum ovulation (estimated at laparotomy) was significantly earlier for ewes on the high plane of nutrition (48·2 versus 88·5 days after lambing), but the post-partum interval to the first oestrus was very variable (32 to 198 days, mean 102·9 days) and was not significantly influenced by the plane of nutrition. Following lambing in November and more especially after lambing in March/April, both ovulation (47·3 days and 27·6 days, respectively) and oestrus (65·5 days and 50·7 days, respectively) recurred more quickly than following lambing in July, but the difference in feeding treatment was not effective after these lambings. The resumption of sexual activity was not influenced by suckling or by the stage at which the ewes joined the rams. It is suggested that, if lactating and non-lactating ewes are fed to meet their respective nutritional requirements, the length of their post-partum anoestrous periods will not differ.