Determination of environmental factors affecting milk yield ofholstein friesian cattle breeding in ceylanpinar agriculturalenterprise and estimation of genetic parameter

The first aim of this study is to determine the effect of calving year, calving season, lactation order and lactation period, as well as environmental factors such as temperature and humidity, on the milk yield level of Holstein Friesian dairy cattle bred in the Ceylanpinar agricultural enterprise. The second aim of the study is to determine, with the findings obtained here, how genetic parameters change according to comfort conditions when animals are exposed to heat stress, and to determine the optimal selection time for both heat stress and comfort conditions, with the help of a random regression model. In the research, a total of 207 003 daily milk yield records of 690 dairy cows belonging to the first, second, third, fourth and above lactation periods and pedigree information about the animals were used. Meteorological data regarding temperature and humidity values were obtained from the public meteorological station approximately 15 km away from the farm. Maximum air temperature and minimum relative humidity values were used for the temperature-humidity index (THI). As a result of the research, the effects of calving year and season, lactation period and parity, calving age, and THI on milk yield were found to be highly significant (P<0.0001). It was observed that when the THI value exceeds 77, milk yield starts to decrease, and when it reaches 92, milk yield decreases by 1.09 kg compared to an SNI value of 77. Heat stress generally leads to an increase in genotypic and permanent environmental variances in the herd, resulting in increased heritability levels. The average heritability estimates predicted daily during the periods defined as comfort period (CP) and heat stress period (HSP) were found to be 0.13±0.007 and 0.18±0.010, respectively. Furthermore, it was determined that the most suitable selection times in both the CP and HSP in this enterprise are approximately around the 120th and 60th days of lactation, respectively. In the study, it has been concluded that the inclusion of climate factors such as temperature and humidity in genetic evaluation studies through statistical models will assist in making more accurate genetic parameter estimations and enhancing herd productivity. As a result, it will provide opportunities for the development of breeding programs that will lead to the acquisition of dairy cattle genotypes with higher heat tolerance.