Relationship between temperature-humidity index and test-day milk and fat percentage milk of Holstein dairy cattle in Mediterranean climate of Iran

In this regard, the objective of this research was to investigate the impact of temperature-humidity index (THI) as an environmental descriptor for heat stress on milk yield (MY) and fat percentage (FP) of Holstein dairy cattle in Mediterranean climate of Iran. The first lactation information between 2001 and 2016 from 42781 and 38829 cows with 348868 and 302851 records, for MY and FP were considered, respectively. The average of daily THI for 3 days before test date was used as an environmental factor which calculated using information of the nearest weather stations to each herd. (Co) variance components were estimated using Bayesian methodology via random regression model (RRM) for different combinations of THI and days in milk (DIM). As a function of DIM at the end of the lactation increases the heritability and decreases the heritability as a function of the THI with increasing THI.For both DIM and THI values, the genetic correlations gradually decline as the distance between values increases. It is evidence that different records along the different THI values like DIM are affected by different genes. In this regard, the estimated genetic correlations for FP were lower than corresponding correlations for MY. The overall idea of this research on heat stress indicates the need to model an additive genetic effect for heat tolerance of dairy cattle in Mediterranean climate of Iran and the genetic potential of the animal can play an important role in controlling the reduction in the production of herd milk in stressful weather conditions.