Fertility reduction in summer poses a considerable problem in dairy cattle. The present study performed a genetic analysis of heat tolerance regarding the conception rate (CR) during lactation, based on temperatures recorded at weather stations close to Holstein cow herds. The records of artificial insemination between 22 to 315 days in milk from 2012 to 2016 were obtained from the Livestock Improvement Association of Japan. Insemination data included 245,876 records (2,212 herds, 124,407 cows) for the first lactation, 206,875 records (2,222 herds, 100,922 cows) for the second lactation, and 140,474 records (2,194 herds, 69,000 cows) for the third lactation. The daily average dry-bulb temperature was recorded by the nearest weather station to cow herds (843 weather stations of Japan Meteorological Agency). Data were analyzed using the linear or the threshold random regression sire model based on temperature. The CR was 0.47 at 17 ºC, and decreased as the temperature at insemination increased from 20 ºC to 32 ºC. In the linear model, the heritability estimates of CR were 0.02–0.03 in the mid-temperature range (-10 ºC to 25 ºC). The heritability estimates of CR at -22 ºC were 0.03 in the first lactation and 0.08 in the second and third lactations; at 32 ºC the CR heritability estimates were 0.04 in the first and second lactations and 0.06 in the third lactation. The trend observed in heritability estimates with temperature changes was similar in both models, although the heritability estimates obtained by the threshold model were higher than those obtained by the linear model. Spearman’s rank correlation coefficients between the transmitting ability of sires with 25 or more recorded daughters estimated by the linear and threshold models were high: 0.997, 0.995, and 0.988 in the first, second, and third lactation, respectively. These results indicated that the genetic estimates of CR did not differ between the models. Because the genetic correlation of CR between mid- and high-temperature conditions was low and that between low- and high-temperature conditions was high, the few genes controlling CR might be common under heat stress and low temperature, but might differ under moderate temperature. Keywords: dairy cattle, heat stress, genetic analysis, reproduction
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume Electronic Poster Session - Biology - Reproduction 1, , 265, 2018
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