Suitable indicator of heat stress for genetic evaluation of heat tolerance in Holstein cows in Japan.

Only a few, principal, weather stations in Japanese prefectures have the daily humidity records required to calculate the temperature-humidity index (THI) as a dairy cow heat-stress indicator. We compared three heat-stress indices: (1) THI calculated from daily average temperature and daily relative humidity at a principal weather station (PTHI); (2) daily average temperature at each herd's closest local weather station (TEMP); and (3) THI calculated from daily average temperature at each herd's closest local weather station and daily relative humidity at the principal weather station (HTHI). We used daily records from 532 provincial weather stations and test-day records of milk production from Days 6 to 305 post-first-calving in Holsteins to compare the indices as indicators of heat-stress effects on milk yield and somatic cell score (SCS). The models used the BLUPF90 package to analyze the effects of herd-year, calving age, days in milk, and PTHI, TEMP, or HTHI. We estimated each model's mean square error (MSE) and compared suitabilities among indices for each trait. TEMP heat-stress thresholds were ~18°C (milk yield) and 15-20°C (SCS). The MSE of the HTHI model was the smallest, but no significant differences were found among the indices for milk yield.