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黒毛和種の種雄牛1頭と繁殖雌牛14頭を用いて，自然発情（自然発情区）後の自然交配と発情同期化処理に基づく発情誘起（発情誘起区）による自然交配における繁殖成績を調べ，人工授精における繁殖成績と比較検討した．発情同期化処理としてCIDR（イージーブリード）を利用したオブシンク（Ovsynch : Ovary Synchronization）プログラムを用いた．それぞれの延受胎率は自然交配の自然発情区9...3.3％，発情誘起区50.0％，人工授精の自然発情区42.4％，発情誘起区40.0％を示し，受胎した繁殖牛1頭当たりの授精回数はそれぞれ1.1回，2.0回，2.4回，および2.5回であった．自然交配の自然発情区では14頭中13頭が初回授精にて受胎した（92.9％）．また，自然交配の受胎間隔は人工授精のそれより有意に短縮され（83日短縮），345日であった（P＜0.01）．黒毛和種において種雄牛による自然交配により繁殖成績の向上，繁殖管理の省力化が図られ，同一種雄牛で同一月齢の子牛生産が可能になると推察された．黒毛和種種雄牛の飼養管理にかかる年間経費は，年間40頭の子牛を受胎させる繁殖経費とほぼ等しく，種雄牛による自然交配の導入は大規模繁殖農家にとって有効であると考えられる．今後は黒毛和種による自然交配で生産された子牛に対する市場評価を高めることが課題であろう．
Reproductive efficiency of a Japanese Black bull to 14 cows was examined on the mating in natural estrus and introduced estrus caused by Ovsynch (Ovary Synchronization) program with CIDR (EAZI-BREED) for synchronization of estrus. The efficiency of the mating was compared with it of artificial insemination (AI) service. Conception rate on the mating to natural estrous cows was 93.3%, the number of fertilization per one conception was 1.1 times, and the scores on the mating to Ovsynch-treated cows were 50.0% and 2.0 services respectively. On AI service, conception rate in natural estrus was 42.4%, the number of fertilization per one conception was 2.4, and the scores in introduced estrus were 40.0% and 2.5 services respectively. 13 of 14 cows were conceived by the first mating in natural estrus. Moreover, conception interval on natural mating (345 days) was shorter than on AI services (428 days). From these result, it is suggested that natural mating by bull is quite useful to improvement of reproductive efficiency and labor-saving management of reproduction and make it possible to breed calves whose father is the same at the same season. On the reproductive cost, the introduction of bull is effective for farmers who raise more than 40 calves a year. To realize this natural mating in Japanese Black cows, the estimate of its produced calf would be essential on the market.show more
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