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This study was conducted to investigate the optimal physiologic mating time in Hanwoo for protection to decrease of reproductivity and improvement of production of offspring. We observed 32 cows that were devide into 4 parts of treatment : T1(12 months of age and 0.5kg daily gain), T2(12 months of age and 0.8kg daily gain), T3(15 months of age and 0.5kg daily gain) and T4(18 months and 0.5kg daily gain). The first heat of treated cows was 263.3pm6.4 days and average weight was 181.1pm11.3kg. It was revealed the conception rates of first insemination were 25%(T1), 75%(T4) and number of insemination of T3 and T4(both 1.5) was lower than T1 and T2(2.3 and 2.4). In return of estrus after heifer′s first parturition, they(T1, T2, T3 and T4) showed 66.2 days, 76.7 days, 62.4 days and 68.5 days respectively and the average was 65.7 days. Plasma progesterone(P4) concentration was nearly the same during the observation periods of treated cows and P4 was released just after 12 months. Only 5 cows (15.6%) in 32 were showed normal estrus cycle and ovulation before 12 months. Before and after parturition, P4 concentration was decreased fastly and then there was no detection of P4 from after parturition to 40 days after milking. P4 would be released again on 45 day after parturition. The results were summarized as that the optimal mating time of Hanwoo heifers was decided by the 14 months of age, 110 cm height and 265kg weight.
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