New technologies for manipulating reproductive efficiency of sheep

Abstract

The reproductive efficiency of sheep is limited by periods of seasonal and post-partum anœstrous and by the low ov&jadnr;lation rate of most breeds. This paper reviews recent research directed towards the control of reproduction in different systems of flock management. Several methods are available for the synchronisation of œstrus and the method employed will depend mainly of mat ng method used and prolificacy level needed. Advantages offered by the use of the progestagen-PMSG method are presented. This method is satisfactory but further reduction in the variability of ovulation is desirable using cervical insemination. To overcome poor transport of spermatozoa through the cervix, intra-uterine insemination is proposed chiefly when frozen semen is used. The induction of fertile ovulation during seasonal anœstrus can be achieved by the use of the "ram effect" associated with a progestagen and photoperiodic (or melatonin) treatment. Prolificacy can be improved by different hormonal methods including the use of gonadotrophins (PMSG and pFSH) and immunization against steroids. Immunization against androstenedione has been extensively tested in a wide variety of genotypes and n anagement systems and has been shown to improve lambing performance by between 18 and 40 %. A significant improvement of overall production of lambs weaned (+ 31 %) was obtained in Charmoise ewes repeatedly immunized in the autumn over 4 years Recent research has been directed to progestagen treatments and to the use of gonadotrophic hormones to stimulate luteal function in order to reduce the level of non-genetic embryonic wastage. Early lambs mortality is also an important determinant of r eproductive efficiency and can be reduced using scanning diagnosis of litter size to improve the management of mothers at the end of pregnancy.