Numbers of spermatozoa required to effect a normal rate of conception in naturally mated Merino ewes

Abstract

Summary. The semen deposited by rams into ewes at a single insemination was estimated from the semen samples collected into intravaginal devices before and after the rams had been mated to oestrous ewes. Of 109 naturally cyclic ewes receiving >60 × 106 spermatozoa (from 5 rams with ejaculates containing <10× 106– 1000 × 106 spermatozoa) 95% became pregnant compared with 31% (P < 0·001) of ewes receiving <60 × 106 spermatozoa. It is concluded that the number of spermatozoa deposited at a single insemination by continuously mated rams may be insufficient to ensure good fertility, and that more than one insemination may be necessary.