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Efficiency of Small Ruminant Reproductive Management
Productivity of the female small ruminant is below its true potential in part due to sub-optimal reproductive performance. Lifetime reproductive performance of the ewe is influenced by age at first lambing, frequency of lambing and prolificacy. Replacement females comprise 20-30% of the breeding flock but their pregnancy rates, embryonic survival and prolificacy are lower than that of their adult flock mates, which reduces overall reproductive efficiency. Strong correlations exist between weight at first breeding and variables contributing to lambing rate. A positive correlation also has been reported between fertility and genetic potential for growth of the ewe lamb. These findings indicate that fertility of replacements can be improved through nutritional management and selection without delaying the age at first breeding. Additionally, pre-treatment with progestogen and gonadotropins enhances fertility and should be integrated in management of replacements. Seasonality of reproduction restricts lambing to once per year in temperate latitudes. Frequency of lambing can be accelerated by increasing the proportion of females bred out-of-season through light management, selection and breeding, and the use of the male effect in combination with pre-treatment with progestogens. These approaches are less effective in fall-born ewe lambs and lactating ewes, but genetic selection may improve fertility in these categories. Development of efficient, low cost weaning systems, and approaches that increase the fertility of the lactating ewe during seasonal anestrous are needed to increase lifetime reproductive performance. Maintaining an appropriate ratio of parous to non-parous breeding females, synchronization of estrus, and pregnancy diagnosis will increase the proportion of ewes lambing and assist in controlling the timing of births to facilitate accelerated lambing programs. Wide variation in ovulation rate and prolificacy exists between and within breeds and across parity, and, lower prolificacy is observed in ewe lambs and ewes bred out-of-season. Optimum prolificacy can be achieved through a combination of selection and breeding, nutritional management and by maintaining an appropriate parity distribution in the breeding flock. Pharmacological treatments, including use of gonadotrophic hormones and immunization against protein and steroid hormones can increase prolificacy, but the response is limited by increasing prenatal losses as ovulation rate is increased. Cost effectiveness of such treatments needs to be evaluated further. In conclusion, our current understanding of reproduction provides an opportunity to make incremental changes to age at first lambing, pregnancy rate, frequency of lambing and prolificacy and thereby increase lifetime reproductive performance of the ewe.
Keywords: Reproductive efficiency small ruminants