In order to synchronize estrus and ovulation, 55 Holstein heifers were divided into four groups, I to IV which were injected intramuscularly with two doses of 0.4, 0.5, 0.8 and 1.0 mg, respectively, of the prostaglandin F
2α (PGF
2α) analogue, ONO-1052, at a 10-day interval.
The rate of heifers which began to show standing estrus within four days after the second injection was 83.3%(l0/12), 100%(07/17) 100%(11/11), and 60%(9/15) for groups I, II, III and IV, respec-tively. It was significantly lower in group IV treated in July than in any of the other groups treated in December or February (P<0.05). The onset of estrous behavior was significantly earlier and more concentrated after the second injection than after the first.
The rate of heifers in wihch ovulation occurred within five days after the second injection was 100% for groups I to III, and 86.7% for group IV. There were no statistical differences in this rate among the four groups.
Conception rate at the first insemination after the second injection was 45.5, 41.2, 45.5 and 35.7% for groups I, II, III and IV, respectively. No statistical differences were observed in this rate among the four groups.
These resluts indicate that treatment with the PGF
2α analogue by intramuscular injection of 0.4, 0.5, 0.8 or 1.0mg of ONO-1052 twice at a 10-day interval was effective enough to induce estrus and that it was applicable to the synchronization of estrus in heifers.
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