Dairy farmers have made great strides in reducing heat stress in the last 20 years. However, the average cow produced about 1.4% more milk per year over that time period, which means higher dry matter intake and heat production.
    The summer of 2019 did not seem to have extended periods of heat and humidity resulting in widespread dairy cow heat stress, and most farms did not see huge drops in production in our area; however, examination of pregnancy and conception rates on most high producing dairies we see tells a different story.
    It was not uncommon to see a 5 or 10 percentage point drop in conception rates, e.g., from 45% to 35%, for the warmest month or two. While this does not sound like a lot, the result is significantly fewer pregnant cows for the period and a corresponding increase in cows eligible for insemination in subsequent months, eventually resulting in a slug of pregnancies later. Thus, there is a continued need for better heat abatement strategies.
    If you are not convinced your herd suffers from heat stress or would like to know how you compare to other farms, consider checking your summer to winter ratio. The S/W ratio was developed in Israel and is described in a recent paper (Guinn: Journal of Dairy Science, 2019). The concept is to look at performance as defined by several indices for the summer (June 21 to Sept. 21) and the winter (Dec. 21 to March 19). Indices included were energy corrected milk, somatic cell count, percentage fat, percentage protein, heat detection rate, conception rate and pregnancy rate.
    The S/W ratio is the summer average divided by the winter average. For example, for energy corrected milk, Midwest herds had an S/W ratio of 0.94. The top 25th percentile had a 0.99 ratio, and the bottom 25th percentile had a ratio of 0.89. So, if your ratio is 0.89, it means your summer production was 89% of your winter production, and 75% of Midwestern herds had a higher S/W ratio than yours. And, your herd likely suffered more severe heat stress than most.
    Not surprisingly, pregnancy rate shows the lowest average S/W ratio in the study. Pregnancy rate is dependent on conception rate and heat detection rate, both of which typically decline in the summer, so pregnancy rate will decline more than heat detection or conception rate. The average S/W ratio for pregnancy rate was 0.82, meaning average pregnancy rate in the summer is only 82% of winter. The top 25% of herds had a S/W ratio of 1.04, meaning they actually did slightly better in the summer than winter, and the lowest 25% had a ratio of 0.63. The average S/W ratios for SCC, fat, protein and heat detection rate are 1.05, 0.95, 0.96 and 0.97, respectively. DHI test day data was used in this study, but one could calculate ratios using DC 305 or other on farm data sources.
    A recent literature review regarding heat stress management (Negron-Perez: Journal of Dairy Science, 2019) made a few interesting points regarding causes of decreased milk production and poorer reproductive performance.
    One, only about one-third of reduction of milk production is due to decreased dry matter intake during heat stress. The rest is due to increased maintenance requirements to dissipate heat and changes in metabolism of the cow. Thus, strategies focused on maintaining dry matter intake alone will not control heat stress.
    Two, impaired fertility during heat stress is mainly due to reduced estrus detection, altered follicular function and early embryonic death. Ovarian follicle size is 0.1 mm less for each additional point of temperature-humidity index on the day of estrus. The composition of follicular fluids changes during heat stress too, and there are a variety of direct damages to the oocytes themselves. Oocyte damage also explains the carryover effect, where one sees approximately two months of impaired fertility in the cooler fall. It takes 40 to 50 days for follicles to develop into large, dominant follicles and ovulate, so if insemination occurs just before ovulation of a damaged oocyte, fertilization is less likely, and embryonic death is more likely should fertilization occur. Heat stress in dry cows near the end of gestation can reduce fertility after calving for the same reasons.
    Third, cooling cows at night is more effective than during the hottest part of the day. Milk production dropped by 1.66% when cows were cooled at night only and by 6.81% when cooled only during the day. Of course, the correct strategy is to cool during day and night.
    Fourth, strategies to reduce heat related reproductive losses might include strategic use of GnRH injections. Timed A.I. systems that rely on use of GnRH force follicular turnover and clear the ovary of damaged oocytes. Herds that do not use timed A.I. might consider GnRH use during and at the end of the summer by injecting GnRH near to or at the end of the voluntary waiting period. GnRH may also increase conception rates when given at the time of insemination on hot days.
    Dairy farmers have done an excellent job addressing heat stress in their cows as evidenced by high summer milk production and very good reproductive performance. However, heat stress is still a significant problem in most high producing herds in the Midwest. Unless improvements are made in heat abatement systems, the problem will only get worse. Heat stress is not going away any time soon.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.