Heat stress effects on somatic cell score of Holstein cattle in tropical environment

ABSTRACT Considering the importance of dairy farming and negative effects of heat stress, the objective of this study was to investigate the effect of heat stress via temperature-humidity index (THI) and diurnal temperature variation (DTV) for somatic cell score (SCS) of Holstein dairy cattle, using random regression models. Data were a total of 52,012 test-day records for SCS of 9,765 first parity Holstein cows from Brazil, collected from 1997 to 2013, along with weather records (THI and DTV) from 18 weather stations. Least square linear regression models were used to determine THI and DTV thresholds for SCS increase caused by heat stress. In addition to the standard model (SM; without bioclimatic variables), THI and DTV were combined in various ways and tested for different days, totaling 21 models. Thresholds of THI and DTV for SCS increase was 70 (0.09 score unit/THI) and 9 (0.03 score unit/DTV), respectively. The model that included THI and DTV as fixed effects, considering the two days average, presented better fit (AIC, BIC and -2logL). Estimated breeding values (EBVs) and reliability of EBVs improved when using this model. Changes on SCS may be an early indicator of heat stress in Holstein cattle reared in tropical conditions. The sires are re-ranked when bioclimatic variables are included in the model. More significant reclassifications of sires were observed when we increased selection pressure. This study provides strong evidence of a genotype by environment interaction on SCS. Genetic evaluation using average of two days of THI and DTV as fixed effects, improves EBVs and reliability of EBVs.