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The overall objective of this study was to determine the incidence of heat stress in dairy livestock in the moderate climate and to examine the impact of heat stress on the reproductive performance of dairy cows.
The first study investigated if the climate data obtained from a meteorological station located in the vicinity of the experimental barn can be used to consider heat stress inside dairy livestock. Climate conditions in all barns differed significantly from the climate recorded at the corresponding meteorological stations. The ambient temperature and THI were significantly higher in all barns compared to the corresponding meteorological stations. Furthermore, the number of days with heat stress (THI ≥ 72) was higher in all experimental barns. The results of this study indicated that inside dairy barns microclimates are generated that differ significantly from the climate conditions at the meteorological station. Therefore, heat stress is underestimated in its magnitude and duration when climate data is obtained from a meteorological station in the vicinity of the study site. Especially the holding pen is a location of intensive heat stress that requires effective heat abatement strategies.
The first study provided evidence that heat stress occurs in dairy livestock in the moderate climate, however, the effect of heat stress on the reproductive performance remained unclear. Therefore, a second study was conducted 1) to investigate the impact of heat stress on the conception rate of dairy cows, 2) to determine a threshold for the impact of heat stress on the conception rate of dairy cows and 3) to detect the period around the day of breeding in which the conception rate of dairy cows is most sensitive to heat stress. The results of this study demonstrated that heat stress has a major impact on the conception rate of dairy cows in the moderate climate. The THI of 73 was determined as threshold for the influence of heat stress on conception rate in dairy cows. Negative effects of heat stress were apparent already at lower levels of THI, and 1 h of mean THI ≥ 73 at the day of breeding decreased the conception rate significantly. In the current study, cows exposed to a high but constant level of heatstress showed lower decrease in conception rate than cows exposed to a lower but fluctuating level of heat stress. Thus, heat abatement strategies should maintain low and constant THI to avoid a major decrease in the conception rate during the summer. The conception rate of lactating dairy cows was negatively affected by heat stress both before and after the day of breeding. The period from d 21 to d 1 before the day of breeding was most sensitive to heat stress. Thus, heat abatement strategies should maintain low and constant THI to avoid fluctuations in heat stress over a period of at least 3 weeks before to 3 weeks after the day of service.
To avoid a depressed reproductive performance during summer months, the use of timed artificial insemination programs has become popular because of their predictability and the elimination of estrus detection. Therefore, the objectives of the third study were 1) to examine the effects of heat stress on the conception rate of dairy cows under natural and artificial insemination breeding programs and 2) to determine the effects of short and long term heat stress on the conception rate under different breeding programs. The results of this study indicate that even under short and long term heat stress, breeding at natural estrus was the most effective breeding program compared to an Ovsynch or modified Ovsynch program. These findings provide evidence that breeding by timed artificial insemination after Ovsynch programs lead to a decrease in the conception rate in addition to heat stress. Breeding by timed artificial insemination after Ovsynch programs cannot be recommended in general to compensate poor reproductive performance during periods of heat stress. Especially under short and long term heat stress, breeding at natural estrus can be an approach to avoid a major decrease in the conception rate in the summer months.
Overall, the 3 studies clearly demonstrated that heat stress in dairy cows plays a great role in the moderate climates of the temperate latitude. The location of climate data collection in dairy livestock is an important factor to determine heat stress appropriately, and therefore, heat stress is underestimated in its duration and magnitude. Heat stress significantly depresses the reproductive performance of dairy cows in the moderate climate, and already low duration and magnitude of heat stress can significantly decrease the conception rate of dairy cows. The use of timed AI programs is not a general solution to compensate low summer fertility. Therefore, further research is necessary to investigate in approaches to avoid a major decrease in conception rates during periods of heat stress in the moderate climate.