Heat stress has a negative impact on the performance of dairy cows as it affects feed consumption, milk production, fertility and animal welfare, all of which lead to poor economics. Due to global warming, the prevention of heat stress becomes more important from year to year. During heat stress, milk production decreases. In addition, heat stress during the dry period also affects milk yield in the next lactation. We analysed data on the milk yield of cows that calved in Slovenia in the period from 2010 to 2022. Milk yield in the standard lactation is presented for Brown Swiss, Simmental and Holstein dairy cows depending on the month of calving. Cows that calved in the summer months, from June to August, which means that they were dry beforehand, had a lower milk yield in standard lactation than cows that calved in winter, months from December to February. The month of calving and thus the dry period had a greater influence on milk yield in the multiparous dairy cows than in the primiparous dairy cows. The differences in milk yield between the individual calving months were greater in Brown Swiss cows than in Holstein dairy cows. It is therefore important to avoid heat stress in all production periods, especially during the dry period, as the negative consequences of heat stress only become noticeable with a delay in the next lactation.
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