SUMMARYOver a 15-year period nearly 2000 young females were exposed to the ram at 6 to 9 months of age. These Clun Forest females were born between January and April in the Cockle Park experimental flock, and mated between October and December of the same year. On average nearly 42% of those present at lambing did not produce a lamb. Analyses are presented of the reproductive efficiency of these animals and of some of the possible causes of their failure to lamb. Observations in 10 of these years suggested that only 12% had not been mated and might therefore be assumed to be prepuberal. Age at first oestrus for those animals achieving puberty in their first autumn was inversely related to date of birth. The number which lambed increased by 1·1% per kg increase in live weight before mating.In 3 years simple nutritional and housing treatments were applied to groups of female lambs. Feeding a cereal supplement to animals at pasture before, during and after mating, and housing 1 to 2 weeks after mating, appeared to increase the percentage which lambed. It is suggested that the effect was to reduce the amount of early embryo death caused by nutritional and climatic stresses on the normal grazing animal.

SUMMARYPhenotypic and genetic parameters for reproductive performance in a flock of Clun Forest ewes have been estimated. Average litter size (ALS) appeared to be more repeatable than the number of lambs born per ewe mated and present at lambing (NLBEM), and the estimates obtained by regression (0·15 and 0·14) were slightly higher than those from the analyses of variance (0·12 and 0·09). The repeatability of barrenness was very low (0·05). It was concluded that little scope existed for the improvement of reproductive performance from a regular policy of culling.Estimates of heritability by analysis of variance and a method for all-or-none traits were 0·12 and 0·11 for ALS, and 0·15 and 0·10 for NLBEM, whereas barrenness was less heritable (0·07). An estimate of 016 was found for the heritability of ALS performance over two years. The phenotypic correlation between body weight and reproductive performance were positive yet small (ALS:+0·16; NLBEM:+0·10), while at the genetic level ALS was more strongly related to body weight (+0·21) than NLBEM (-0·04). The genetic regressions indicated that an increase of 5 lambs per 100 ewes lambing would accompany genetic gain of 5 kg in ewe body weight, while an increase of 1 lamb per ewe lambing would lead to a correlated response in body weight of 5 kg.Selection for ALS on the basis of dams' performance, within the flock studied, would be expected to yield a rate of progress of 2% per annum.

SUMMARYThe reproductive performance of a lowland flock of Clun Forest ewes is reported. During the period from 1955 to 1966, over 1300 ewes entered the flock and more than 4800 lambing records were obtained. The effects of culling and death on the records available were investigated and it was found that the reduction of the material caused by death was random, while that caused by culling was related to previous performance.Barrenness decreased with age, and for ewes aged 1, 2 and more than 2 years was 44, 7 and 5%. Average litter sizes (ALS) over the same ages were 1·15, 1·55 and 1·75. Accordingly the number of lambs born per ewe mated and present at lambing also increased with age of ewe, and values for the same ages were 0·64, 1·44 and 1·68.An analysis of over 170 ram mating groups, consisting of ewes older than 1 year, revealed no differences between rams on the ALS of their mates. The results of a similar smaller analysis of barrenness among 1-year-old ewes also showed no evidence of a ram effect.