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Investigation of Genetic Trend of Holstein Dairy Cattle Production in a Function of Temperature-Humidity Index using Random Regression Model

The objective of this research was to investigate the impact of temperature-humidity Index (THI) on milk yield (MY) and fat percentage (FP) trend of Holstein dairy cattle in Mediterranean climate of Iran. Data included 348,868 and 302,851 test-day records, respectively for milk yield and fat percentage from 42,781 and 38,829 cows, respectively. Data were collected from 9 provinces located in the Mediterranean region of Iran between 2001 and 2016. The climate information from the nearest weather stations to each herd was used to calculate THI. Then the average of daily THI for 3 days before test date was considered as an environmental descriptor for milk production traits. Significance test using GLM procedure was applied to consider fixed effects in the model. Random regression model using Bayesian methodology was applied to estimate breeding values (EBV) across different combinations of days in milk (DIM) and THI via and subsequently, genetic trends for production traits were estimated along year of birth and THI values. The results showed that in addition to re-ranking of sires in different DIM, the different ranks were observed across different THI. In the other word, animals had different responses along THI and sires with higher EBVs for lower THI had lower EBVs for higher THI. Across THI, the re-ranking of sires for FP was more considerable than for MY. In other words, animal for FP were more sensitive in response to climate changes. For both traits, negative genetic trends were observed with increasing THI. In addition, in contrast with MY, a negative trend for FP was obtained based on year of birth. It could be concluded that THI has substantial effect on milk production traits and including this factor with applying random regression model could be useful to investigate heat tolerance of dairy cattle herds in Mediterranean climate.