Examining the effect of summer heat stress on the reproductive performance of dairy cows

Background: The Hungarian climate has continental character, the average temperature is 10 to 11 degrees C and the annual average temperature is in line with global trends. The warmest period of the year is the end of July and the beginning of August. According to the data of the National Meteorological Service year 2015 was significantly warmer than usual. Objectives: Heat stress is a major contributing factor to the low fertility of dairy cows inseminated in the late summer months. Materials and methods: We examined the reproductive performances of cows and heifers on Holstein-Friesian farms, with particular regard on hot summer months. We focused mostly on the year 2015, which was the warmest globally since 1850. Results and discussion: Based on our results the conception rate of lactating cows in summer months was poor. However the fertility index of the heifers was equally good throughout the year (between 1.6-2.5), the cow's fertility index increased from May, and the worst was in August (8.8). Examining the effect of heat stress on gestation length we observed that parallel with the monthly average temperature rising, calving began earlier. In summer period the calving may start even on day 273 of gestation. There were up to 13 more day differences in gestation length between the winter and summer calving animals. The calving interval can be 450 days of animals which service period is in the hottest summer time, because of the poor fertility. Our results demonstrated in the hottest months of the year that the conception rate was very low. Therefore, we suggest not to do the insemination process routinely, but paying particular attention on physiological condition and age of the animals, on weather (temperature, front-effects), and so on. Insofar the circumstances are inadequate we may decide to postpone the insemination, not to waste time, money and energy.