The effect of animal-related and some environmental effects on daily milk production of dairy cows under the heat stress conditions

The study aimed to determine the effect of different fixed factors (breed, region, calving year, calving season, parity and lactation stage) on daily milk production (yield and content) of four dairy cattle breeds under the heat stress conditions (THI≥72). The research was conducted on a set of test-day records of four dairy cattle breeds (Simmental, Holstein-Friesian, Red Holstein and Brown Swiss) reared in the Republic of Serbia (Region of Mačva, Podunavlje and Šumadija). The statistical analysis included test-day records collected in the period from the beginning of the year 2012 to the end of the year 2019. Based on the performed analysis, it could be demonstrated that all the effects included in the statistical model (breed, region, calving year, calving season, parity and lactation stage) statistically highly significant (p<0.001) affected daily milk production (yield and content) of cows reared in the terms of the heat stress conditions. The highest daily milk production was determined in the region of Podunavlje, and by the Holstein-Friesian breed. Furthermore, dairy cows calved during the spring season, in the third lactation and from the 61st to the 120th day of lactation, had the highest milk production during the summer period in an environment characterised by heat stress. Also, daily milk yield of cows in a heat stress environment decreased in the period from June to September. The results obtained indicate different animals' responses to heat stress concerning the animal's breed, rearing region, calving year, calving season, parity and lactation stage.