広大農場乳牛における除籍状況からみた牛群管理の方向性

近年日本の乳牛の平均除籍産次数が3.4産程度に低下しており，対策が必要な現状にある．本研究の目的は，本学農場の乳牛除籍の傾向を把握して，対策について考察することである．2000～2015年度の本農場の記録をもとに解析した．本農場では常時20～25頭程度の搾乳牛が飼養され，2011年3月までは1日2回のパーラー搾乳，それ以降は24時間自動搾乳が行われた．期間内に83頭の乳牛が除籍になり，うち67頭が肉畜として出荷され，16頭が農場内で死亡した．除籍頭数割合は産次とともに増加し，6産時が最も多かった．除籍率は，初産時が最も低く，徐々に増加して4産，5産時に40%を超え，6産時に急激に増加して77.8%となった．除籍理由として，繁殖障害，乳房炎等の疾病が推察された．一般的な繁殖成績改善と疾病予防を進めるとともに，6産以降は繁殖と疾病の履歴に基づき，飼養継続の判断基準確立が必要と考えられた．

In Japan, mean value for parity of dairy cows at culling has decreased to 3.4; measures to improve this are required. The aim of the present study is to understand the trends of culling of dairy cows at the Hiroshima University Farm. For that purpose, the farm’s records for dairy cow culling during the period of 2000–2015 were analyzed. At the farm, 20–27 dairy cows are regularly being milked. In March 2011, the milking system at the farm was changed from a milking parlor system to an automatic milking system. During the 16 years analyzed, 67 cows sold for meat and 16 cows that died at the farm totaled to 83 culled cows. The proportion of culled cows for each parity to total culled cows increased with increasing parity, and the number of cows culled at their 6th parity was the highest. The proportion of culled cows to cows that delivered at each parity gradually increased to more than 40% at the 5th parity, and drastically increased to more than 77.8% at the 6th parity. The reasons for culling were reproductive difficulty and diseases, such as mastitis. Therefore, general measures for improving reproductive performance and disease prevention should be taken for the cows at the 4th or lower parity. In addition, for dairy cows at the 5th or higher parity, a diagnostic decision on whether they are artificially inseminated for further delivery or not might be required depending on the individual cow’s record of reproduction and diseases.