The purpose of this study was to clarify the effect of heat stress on the days open(DO)from the change of environmental and reproductive performance. DO was investigated from calving records and pedigree records of Japanese Black cattle collected from improvement organization in Okinawa. In data editing, DO of 21 d or less was excluded, 22 d to 50 d was set to 50 d, and 250 d or more was set to 250 d. Records of DO with imperfect record was set to 250 d. Year of calving was from 2008 to 2012, and the parity was from first to sixth. In addition, we restricted the data to farmer records producing more than 150 calves from 1990 to 2012. After editing, the data set consisted of 33,777 records of 13,611 cows in 194 farm.
Four models were used for the analysis, including the season of calving (model 1), the month of calving(model 2), covariates of the temperature and humidity index(THI)at a day of calving(model 3), and the fixed effect of THI(model 4). Analysis of model 1 showed the highest value in summer(123.85 d)and the lowest value in autumn(119.26 d). The difference was about 4.59 d. Analysis of model 2 showed the highest value in June(127.58 d)and the lowest value in April(115.65 d). The difference was about 11.93 d. In analysis of model 3, THI showed a quadratic curve, the intercept was 145.02, and the slope was 0.029. In addition, the lowest value(144.86 d)was shown at THI 70 and 71. Analysis of model 4 showed that the highest value was 124.13 d from THI 56 to 60 and the lowest value was 118.83 d from THI 61 to 65. In all models, DO increased during summer heat season. Using THI as an indicator, there was an increasing trend with THI 70 as the border. As DO decreased after autumn and spring calving,these two seasons are considered to be most adaptable breeding season. In each models, the effect of heat stress was confirmed in June and at THI 70 or higher which is known to be hot season.
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