Relationship between spontaneous, out-of-season ovulatory activity and other reproductive characteristics of Chios sheep

Spontaneous, out-of-season ovulatory activity (SOA) may contribute to the improvement of the reproductive capacity and management of sheep. The relationship between SOA and other important traits becomes an issue in the development of a breeding goal. In this article, the relationship of SOA with fertility, measured as success at conception, prolificacy, measured as number of lambs born, and stillbirth, measured as number of lambs born dead, was assessed. Data included 662 reproductive records of 391 Chios ewes kept at the Agricultural Research Station, Chalkidiki, Greece. These ewes were daughters of 140 rams. Blood progesterone levels were used to determine SOA status. Measurements took place prior to the onset of the breeding season, which is marked by the introduction of rams to the flock. Data were collected over two consecutive years. The frequency of SOA was 29.9%, the average success rate at conception was 81.7% and the mean litter size was 2.14 lambs, while 0.24 lambs, on average, were born dead. Linear and non-linear mixed models were used for the statistical analysis of these data. Both ewes that did and did not exhibit SOA had similar success at conceiving in the subsequent mating season. However, ewes that had exhibited SOA gave birth to 0.22±0.08 fewer lambs than ewes from the other group. This estimate was adjusted for the effects of season of lambing, year, age and status of the ewe, and sire of the ewe, and was significant (P<0.05). The effect of SOA on stillbirths was not significant. Additive genetic and residual correlation estimates of SOA with fertility and stillbirth were not different from zero. The additive genetic correlation between SOA and prolificacy was not different from zero. A residual correlation estimate of −0.15±0.05 was found between these two traits, associating the presence of SOA with smaller litter size. This residual correlation is due to environmental and non-additive, non-heritable genetic factors. The results of this study imply that development of a breeding program including SOA would not be expected to affect fertility, litter size and stillbirth rate in the Chios breed.