Effects of Heat Stress on Reproduction

When dairy cattle are subjected to heat stress, reproductive efficiency declines. Cows under heat stress have reduced duration and intensity of estrus, altered follicular development, and impaired embryonic development. However, the extent of reduction in reproductive efficiency during summer in the United States or elsewhere is difficult to assess unless reproductive measures are calculated monthly based on results of actual inseminations rather than as rolling annual averages. Common methods to ameliorate effects of heat stress have been to provide cooling in the form of shades, soakers, fans, or evaporative coolers. Because negative effects of heat stress have been identified from 42 d before to 40 d after insemination, continual cooling must be provided. Tools for synchronization of estrus have been developed that greatly reduce the need for detection of estrus. Pregnancy rates were more consistent over season when timed artificial insemination programs were used compared with artificial insemination after detected estrus, although negative impacts of heat stress were still observed. Recently, calving in summer has been reported to reduce the success of a timed first insemination between 60 and 66 d postpartum, although some researchers have found calving in the summer to have a positive impact on days open. Other techniques that have been investigated to reduce the negative impact of heat stress on reproduction include embryo transfer, induction of accessory corpus lutea, and crossbreeding. Each technique has potential advantages but not without limitations or costs. Comparative economic evaluations of various combinations of strategies to attenuate negative effects of heat stress on both milk production and reproduction are needed.