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To be ultimately successful, techniques used to control the oestrous cycle of the cow will have to embody methods which allow the time of insemination to be arranged without reference to behavioural oestrus. This is particularly import- ant in the case of beef suckler cows as it is difficult to detect heat in these animals probably owing to the suckling stimulus. This means that treatments used to synchronize the oestrous cycle willa lso have to give precise control of the time of ovulation or that other hormones will have to be given to control ovula- tion time. The aim of the experiment described in this paper was to obtain hormonal control of the time of ovulation following synchronization of oestrus with implants of progesterone (Roche, 1974a). This would allow all animals to be inseminated on a fixed time basis. Eighty-four mature Hereford cross heifers fed on grass silage during the winter of 1972 were randomly assigned to different treatments. Twenty-four untreated control heifers were inseminated as they came into oestrus. All the remaining animals received implants of progesterone for 21 days (Roche, 1974a) and twelve were inseminated as they came into oestrus following removal of the implants. Twelve more heifers, given 400 µg oestradiol benzoate intramuscularly in oil 20 hr after removal of the implants, were inseminated at 18 hr and at 24 hr after the oestrogen injection. Twenty-three heifers were given 1500 i.u. HCG intramuscularly 24 hr after removal of the implants and were inseminated at 16 and 22 hr after the HCG. Finally, twenty-five heifers were given 100 µg gonadotrophin-releasing factor (Gn-RH) intramuscularly 30 hr after removal of the implants and were inseminated at 6 and 12 hr after Gn-RH injection. The heifers were inseminated with frozen semen by inseminators from a commercial A.I. station. Inseminations were carried out according to the schedule outlined previously (Roche, 1974b). Twelve of the controls and the heifers which received Gn-RN were inseminated with semen from the same ejaculate (Semen A), while the remainder were inseminated with semen from a different bull (Semen B). All animals were slaughtered 30 to 40 days after insemination and the reproductive tracts were examined to determine the numbers which were pregnant based on the presence of live embryos. The conception rates obtained following the various treatments are shown in Table 1. Since the conception rate in the control heifers inseminated with semen from different bulls was similar, the conception rates for the different treatments were compared. The fertility obtained following all synchronizing

ATTEMPTS TO DETERMINE THE OPTIMAL TIME OF ARTIFICIAL INSEMINATION IN HEIFERS