We used test‐day records and daily records from provincial weather stations in Japan to evaluate heat tolerance (HT) in Holstein cows according to a random regression test‐day model. Data were a total of 1,641,952 test‐day records for heritability estimates and 17,245,694 test‐day records for genetic evaluation of HT by using milk yield and somatic cell score (SCS) in Holstein cows that had calved for the first time in 2000 through 2015. Temperature–humidity index (THI) values were estimated by using average daily temperature and average daily relative humidity records from 60 provincial Japanese weather stations. The model contained herd–test‐day, with lactation curves on days in milk within month–age group as a fixed effect. General additive genetic effect and HT of additive genetic effect were included as random effects. The threshold value of THI was set to 60. For milk yield, estimated mean heritabilities were lower during heat stress (THI = 78; 0.20 and 0.28) than when below the heat stress threshold (THI ≤ 60; 0.26 and 0.31). For SCS, heritability estimates (range 0.08–0.10) were similar under all heat stress conditions. Genetic trends of HT indicated that EBVs of HT are changing in an undesirable direction.

中文翻译：

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日本荷斯坦奶牛耐热性的遗传评价。

我们使用日本省级气象站的测试日记录和每日记录，根据随机回归测试日模型评估荷斯坦奶牛的耐热性 (HT)。数据是 2000 年至 2015 年首次产犊的荷斯坦奶牛的遗传力估计的 1,641,952 个测试日记录和 HT 遗传评估的 17,245,694 个测试日记录，使用产奶量和体细胞评分 (SCS)。温湿度指数 (THI) 值是根据日本 60 个省级气象站的日平均温度和日平均相对湿度记录估算的。该模型包含牛群测试日，月龄组内牛奶中天数的泌乳曲线作为固定效应。一般加性遗传效应和加性遗传效应的HT作为随机效应包括在内。THI 的阈值设置为 60。对于产奶量，估计的平均遗传力在热应激期间（THI = 78；0.20 和 0.28）低于热应激阈值（THI ≤ 60；0.26 和 0.31）。对于 SCS，遗传力估计值（范围 0.08–0.10）在所有热应激条件下都相似。HT 的遗传趋势表明 HT 的 EBV 正在朝着不希望的方向变化。