Induction of pregnancy in lactating anoestrous ewes

Abstract

Efforts have been made to control the oestrous cycle of ewes so that breeding can be induced at most times of the year (Gordon, 1963, 1975; Cognie, Hernandez-Barreto & Saumande, 1975; Smith, Boland & Gordon, 1981). Despite a high oestrous response, conception rates have generally been low in lactating anoestrous ewes (Hunter, 1968; Land & McClelland, 1971; Cognie et al. 1975; Shevah, Black & Land, 1975). The ability to achieve conception in post-partum ewes appears to be related to season, breed and lactational status (Cognie et al. 1975; Evans & Robinson, 1980; Rhind et al. 1980). Early research has shown that conception rates of 90% can be achieved in Finnish Landrace × Dorset Horn ewes using exogenous hormones, early weaning and artificial lighting (Robinson, Fraser & McHattie, 1975). Subsequent studies have shown that similar results can be obtained under natural daylight conditions provided that each ewe received pregnant mare serum gonadotrophin (PMSG) at the time of sponge withdrawal (Fraser el al. 1976). Some breeds of sheep may be induced to lamb at 6-month intervals, when maintained under ideal conditions including artificial lighting (Evans & Robinson, 1980). There seems to be some conflict in the literature as to factors affecting conception rate in ewes. Kann & Martinet (1975) have shown that it is the problem of suckling rather than lactation that is responsible for the delay in return to normal reproductive activity. It has been shown that luteinizing hormone (LH) release is quantitatively less in lactating than in cyclic ewes (Cognie & Pelletier, 1976; Evans, 1980). Also the pattern of LH production has been shown to differ in ewes conceiving and those which fail to conceive (Rhind et al. 1980). Cognie & Pelletier (1976) have suggested that in lactating ewes, a proportion of eggs are shed abnormally late in relation to the LH surge and may not be fertilized. The purpose of the present experiment was to investigate some of the factors which may influence the induction of pregnancy in nursing ewes in the non-breeding season.