Getting lactating dairy cows bred to maintain a reasonable calving interval can be challenging any time of year but especially during the hot summer months. Heat stress can occur in lactating cows at temperatures of 80°F or higher. Several factors contribute to reduce fertility in lactating cows during summer breeding. Even during cooler times of the year the percentage of estrus periods detected is often around 50%. During summer, cows may have less mounting activity, exhibit estrus for shorter periods of time and may not exhibit estrus until after the temperature cools at night. As a result, the per - centage of cows detected in estrus during summer may only be about half of that percentage detected during the cooler months.

Heat stress can affect the viability of oocytes, reducing their ability to fertilize, and/or the ability of the resulting embryos to develop into viable offspring. From initial growth of a follicle within the ovary of a cow, until it is fully mature and ready to release the oocyte at ovulation, is about a 100-day process. Severe heat stress during this follicle maturation period can reduce the developing oocyte’s viability. Due to the length of this follicle/oocyte maturation period, there can be a lag period of a few months after the heat stress ends before normal fertility returns.

Heat stress during fertilization and the first day or two of embryo development can be very detrimental. After this initial period, the developing embryo becomes more tolerant of heat stress, until pregnancy recognition begins about 15 days after estrus. Heat stress may reduce production or alter metabolism of progesterone, which is necessary for pregnancy maintenance and the uterine secretions that nourish the developing embryo. This altered uterine environment can result in retarded embryo development. Heat stress also can increase production of prostaglandin F2alpha by the uterus, which in turn can cause regression of the corpus luteum, leading to loss of progesterone production and pregnancy failure.

Overall, the reduced fertility during summer can be con - sidered to be the cumulative effect of poor estrus detection, reduced oocyte viability, retarded embryo development and altered uterine function. Semen quality would also contribute to lower fertility if natural mating were used during summer but should not be a concern when using frozen-thawed semen purchased from the various semen companies.

The percentage of cows detected in estrus might be improved by the use of mount detectors such as the Estrotect heat detection patch. The need for estrus detection can be eliminated altogether by use of estrous synchronization protocols developed for use with timed insemination (TAI). Various recommended protocols can be found at the Dairy Cattle Reproduction Council website ( These protocols are designed to control when follicles undergo final growth and maturation, and the time of ovulation. It is important to closely follow the recommended timing of the various hormone treatments.

Dairy cows have a very limited ability to sweat and naturally cool down by evaporative cooling. A common method to reduce heat stress is to provide housing or shade, and evaporative cool ing with fans and misters or foggers. In countries with high summer temperatures, repeated cooling cycles with sprinkling followed by ventilation during the hottest times of the day has been shown to be effective in maintaining milk production and improving conception rates. Of course, high humidity reduces the effectiveness of evaporative cooling.

Based on what is known about the effects of heat stress on fertilization and early embryo development, the most critical time to cool cows would be the first two or three days after onset of estrus and during pregnancy recognition (days 15 to 20 after estrus). Progesterone supplementation (via CIDR insert) during the second and third week after estrus has been evaluated as a method for improving pregnancy rates, but with varying results. Although it may not be practical for most producers, collection and cryopreservation of embryos during the cooler times of the year and transfer of those embryos during hot weather has been shown to increase pregnancy rate over what can be achieved by AI of heat-stressed cows.

Forages are known to generate more heat during digestion than high grain rations. Feeding high quality forages during summer reduces the amount needed to meet requirements, producing less heat during digestion. Feeding the majority of the ration when temperatures are cooler (8 p.m. to 8 a.m.) might also be beneficial in reducing heat stress. Heat stress increases free radical production that can damage oocytes and embryos. Feeding anti oxidants to reduce free radical pro duc tion during heat stress periods may improve pregnancy rates. One study has reported that feeding 400 mg per day of beta-carotene starting 15 days after calving and continuing 90 days improved pregnancy rate during hot weather breeding.

In summary, the conception rate of lactating dairy cows in summer might be increased by using estrous synchronization protocols that allow for timed insemination, providing shade and evaporative cooling, altering the ration to reduce heat generated during digestion and feeding the majority of the ration during the cooler times of the day.