Effect of Acclimatization on Sensible Heat Stress for Days Open of Japanese Black Cattle in Okinawa

Abstract

In this study, we examined days open（DO）of Japanese Black cattle in order to examine the hypothesis that animals reduce DO due to acclimatization to sensible heat stress. DO was calculated based on calving records of heifers and cows from 2008 to 2016 in Okinawa. DO of 22 d to 50 d was set to 50 d, DO of 250 d or more was set to 250 d and DO of 21 d or less was excluded from the data set. Target areas were Okinawa Island and Yaeyama Islands. The hot season with high sensible temperatures was divided into two: a rising period and a falling period, and the rest of the season was set as a thermal neutral period. This classification was implemented with reference to the temperature humidity index（THI）at each location. Assuming that DO during the falling period was a reaction to acclimatization after the rising period, DO of the rising period was compared with that of the falling period. In the linear model analysis, root mean square error（RMSE）was calculated to compare linear models and to estimate deviations from the rising period to the falling period. As a result, animals in the rising period had approximately seven THI higher than those in the falling period. This study indicates that further research focusing on the rising period of the heat stress is needed to improve DO to accomplish reproductive cycle with single birth per year.