Oestrus and ovulation can be controlled in ewes during the breeding season by administering progestagens (Lamond, 1964). However, fertility at the controlled oestrus (40 to 60%) is generally low, regardless of the particular progestagen or the mode of administration (Robinson, 1968). The physiological basis for this subfertility at the controlled oestrus is not clear at present.
Fertilized eggs transferred to ewes previously treated with progesterone developed normally (Schmidt, 1961; Shelton & Moore, 1966), indicating that the capability of such uteri to sustain normal growth and development of the embryo was not adversely affected. Quinlivan & Robinson (1969) found that the pattern of distribution and survival rate of spermatozoa became progressively altered at the first oestrus following progestagen withdrawal when compared with the pattern in non-treated control ewes. A significant decline in the cervical population of spermatozoa occurred