Limiting inseminations to the cooler part of the day is not enough to solve the effects of heat stress. By the time of insemination, the follicle has already been damaged, and if conception does occur, the resulting embryo is sensitive to heat stress for the first two to three days of gestation.
However, you can reduce the magnitude of heat stress on fertility by providing cooling for a few days around ovulation, according to a cooling strategies paper at eXtension.org
The advice is to cool cows subjected to timed artificial insemination protocols from about three days before insemination (when cows on Ovsynch receive prostaglandin) until three to four days after insemination.
The eXtension.org paper also discusses the mechanisms that cows use to regulate body temperature, how to measure heat stress, the importance of cooling heifers and dry cows, and emerging ideas in cow cooling.