Heat stress has a significant impact on the cattle fertility, especially on high-producing
dairy cows. However, as fertility of dairy cows correlates well with optimum
maximum milk production, we first conducted a literature review on the effects of heat
stress on fertility of three breed Brown Swiss, Simmental and Holstein dairy cows and
then showed the effects of calving month on calving interval (CI) in dairy cows in
Slovenia. Heat stress affects hormone concentration and thus oestrus expression,
folliculogenesis, premature atresia of ovarian follicles, oocyte maturation, premature
embryonic death and implantation success. Since heat stress reduces the consumption
of dry matter, the energy balance in the body is disturbed, which provokes metabolic
diseases and fertility. The effects of heat stress can be mitigated by nutrition, genetic
improvement, embryo transfer, planned ovulation, hormone administration and the
construction of barns with an outdoor climate, ventilation or spraying. We analysed
the calving interval (CI) for dairy cows that calved between 2010 and 2022 in
Slovenia. The CI for Brown Swiss, Simmental and Holstein dairy cows is presented
according to the month of calving. Cows that calved in the summer months, from June
to August, had the shortest CI and cows that calved in winter, from December to
February, had the longest CI. The differences in CI between the individual calving
months were greater in the Brown Swiss and Holstein cows and lower in the
Simmental cows. The calving month has a greater influence on the CI in the first
lactation than in the second and subsequent lactations in Brown Swiss and Simmental
cows.
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