Synchronization of Estrus and Conception in the Beef Heifer

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Abstract
Three trials were conducted with 80 yearling Hereford heifers to study the effect of two synthetic steroids on estrus and conception. In trial I, a mixture consisting of 98.5% 17-α-ethynyl-17-hydroxy-estr-5(10)-en-3-one (norethynodrel) and 1.5% 17-α-ethynyl-3, 17-estradiol-methyl ether was administered daily for 10 days starting at day-15. This mixture is referred to as NE. Estrus and ovulation were inhibited during the treatment period by subcutaneous injections or oral administration. The animals in the injected groups returned to estrus 3 to 7 days post-treatment, with 44% conceiving from inseminations at this adjusted estrus. In the orally treated groups estrus was observed 3 to 24 days post-treatment. Conception rate in the latter group was 75% at first service. A single injection of NE in repositol form did not effectively synchronize estrus when administered either at estrus or day-11 of the cycle (trial II), although 70% of the animals treated at day-11 returned to estrus 9 to 11 days later. Conception rate at the first estrus following treatment was 35% in treated groups. The control group had a conception rate of 70%. The highest incidence of ovarian abnormalities and infertility was observed in the heifers treated while in estrus. In the third trial oral administration of 17-α-hydroxy-6-α-methylpregn-4-en-3-one-17-acetate (6-α-methyl-17-α-acetoxyprogesterone) at various stages of the estrous cycle completely inhibited estrus during the 20-day feeding period. The first estrous periods following treatment were effectively synchronized, all animals exhibiting estrus 3 to 6 days after end of treatment. Conception rate at the controlled estrus was similar to that observed in the control group (55% vs. 60%).

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