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J. F. ROCHE and J. P. CROWLEY

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

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J. F. ROCHE and J. P. CROWLEY

Summary.

Oestrus and the time of ovulation were controlled in eighty-seven mature non-pregnant heifers by using silastic implants of MGA followed by an injection of HCG 48 hr after removal of the implants. The heifers were allocated at random to three groups and inseminated with frozen semen before the estimated time of ovulation. Animals in Groups 1 and 2 were inseminated once at 14 and 24 hr, respectively, after HCG while animals in Group 3 were inseminated twice 14 and 24 hr after HCG. All heats observed after insemination were recorded until the animals were killed 55 days later, when 23% of them were found to be pregnant. The conception rate was not affected by changing the time or the frequency of insemination. Possible reasons for the low fertility are discussed.