Sixty-three dairy and beef heifers were given single intravenous (i.v.) injections of 2, 5 or 10 mg of oestradiol cypionate (ECP®) during, on the last day, or on Day 1 or Day 2 after last feeding of melengestrol acetate (MGA®). Thirty-five heifers, which were used as controls, were fed MGA for intervals corresponding to those given ECP. The dairy heifers were checked for pregnancy by rectal palpation 35 to 45 days after service, but the beef heifers were slaughtered following breeding. Reproductive tracts were grossly examined for abnormalities and the oviducts were flushed for ova. Data were collected from both groups on the occurrence of oestrus, ovulation and on conception rate.
ECP increased the overall incidence of oestrus. Ovulation, however, did not appear to be greatly affected by ECP, although there was a slight tendency for ovulation to occur sooner after oestrus in the heifers given ECP. ECP appeared to have a detrimental effect on the conception rate at the first controlled breeding, but the conception rate at subsequent breedings appeared normal. The reduced conception rate at first service following MGA plus ECP treatment appeared to be due to multiple causes: ovulation failure, ovum loss, fertilization failure and embryonic death. ECP would appear to have a favourable effect only when oestrus and/or ovulation failure was the primary cause of reduced conception rate.
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