The relationship between temperature-humidity index and test-day milk yield of Iranian Holstein dairy cattle using random regression model

The aim of this study was to assess the impact of heat stress on milk yield of Iranian Holstein dairy cattle. The national data set consisted of 840,834 first lactation test day records on 103,624 cows between 2005 and 2010. Additionally, the climatic data were obtained from 198 public weather stations. The average of daily temperature-humidity index (THI) for 3 days before test date was used as an indicator of heat stress. The (co)variance components were estimated using Random Regression Model (RRM) by Bayesian's Gibbs sampling method. Posterior means of additive genetic and permanent environment estimated for THI and days in milk (DIM) combinations with Legendre polynomials. The additive genetic variances for yield as a function of DIM decreased from 5.2 to 4.0 in early month then increased to 9.9 on DIM=305. As a function of THI, the highest additive genetic variance was 5.4 for THI=32 and the lowest variance was 1.26 for THI=71. Estimated heritabilities ranged from 0.14 to 0.31 that the lowest heritability was estimated in the highest THI and in the beginning of lactation; and the highest heritability was estimated in the lowest THI and in the end of lactation. The genetic correlations gradually decline as the distance between THI values increases; and correlations across THI were generally medium. It is evidence that different records along the different THI such as lactation curve are affected by different genes. Therefore, substantial indication for genotype by environment interaction was found for milk yield. The overall idea of this study on heat stress indicates the need to model an additive genetic random regression effect of heat tolerance in national genetic evaluations of dairy cattle herds.