by Tamara Scully
The Northeast may not be the hottest region in the country, yet it has seen an increasing amount of hot days, where the temperature-humidity index (THI) rises above 72 points, the level at which thermal neutrality is breeched and dairy cows begin to be negatively impacted by heat stress. Climate change predictions for the region indicate that an increase in the amount of days where a heat stress event occurs, as well as an increase in the duration and intensity of such events, is all but inevitable.
The current methods of reducing heat stress include shade structures, fans and sprinklers. Their effectiveness is limited, at best, and high humidity conditions, common in the region, lessen the ability for these methods to reduce the impact during days of elevated THI.
Experiments in conductive cooling, including those researched at Cornell University by Kristen Perano, a Ph.D student in the Department of Biological and Environmental Engineering, are showing promise. Using cooled water flowing through modified waterbeds has been demonstrated to be an effective tool in reducing the rectal temperature and respiration rate of dairy cows, and increasing their milk production. These results indicate that conductive cooling is a useful tool in mitigating the impact of heat stress.
Reproduction and heat stress
Lactating dairy cows are “the group that we are finding will suffer the most,” when a heat stress event occurs, Julio Giordano, DVM, Assistant Professor, Dairy Cow Biology and Management, Cornell University, said.
It has been known, since the 1950s, that cows exposed to heat stress experience reductions in pregnancy rates. In dairy cows, the 21-day pregnancy rate is a measure of “how fast cows get pregnant during lactation,” Giordano said. “There are so many things affecting the cow.” The mechanisms which cause reduced fertility are not completely understood.
It does not matter whether bulls, artificial insemination (A.I.), or a mix of the two is utilized: the reproduction rate suffers, with up to a 50 percent reduction in pregnancy rates from winter to summer. Bulls, too, suffer from the impact of heat, with decreased sperm count and decreased interest in breeding, A.I. may be more successful than breeding via natural methods during periods of elevated THI.
The duration of the heat stress event has an impact, too. In studies where cows were exposed to various duration of heat stress — day of breeding only, three days before plus the day of breeding, and seven days prior plus the day of breeding — the cows with the shortest exposure exhibited a three percent drop in the pregnancy rate. The moderate exposure saw a nine percent decrease, and the long duration exposure showed approximately a 10 percent decrease.
“Holstein cows in pretty well-managed herds with fans and sprinklers also used in some of them,” have been shown to suffer these reductions in pregnancy rates, Giordano said.
Even more alarming, studies show that it takes some time for a full recovery of pregnancy rates after heat stress has abated. Heat stress impacts the estrous cycle of the cows by changing the ability of the follicles to produce hormones. The cows exhibit an increase in the number of follicles and a reduction in follicle size. In addition, their levels of estradiol are decreased, resulting in a shorter, less intense fertile period.
The oocytes are also impacted during heat stress events, with consequences both acute and chronic in nature. Embryo quality, quantity and the fertilization rate all decrease. When temperatures rise, blood goes to the peripheries, and the uterus doesn’t get as much blood as it should. Any embryo fertilized won’t be receiving the proper amount of nutrients to thrive.
“Heifers under heat stress conditions: their embryos are just very, very poor,” Giordano said. “The oocyte is one of the most affected players important for reproductive performance.”
Because cows do not demonstrate they are in heat as clearly when the THI is elevated, it may be prudent to monitor their estrous cycles with some type of automated device. This would increase the chances of impregnation.
“If the cows have shorter estrous cycles, and the intensity is less,” it can take constant monitoring to catch that fertile window, Giordano said.
Synchronizing A.I., so that all cows ovulate and are bred simultaneously, “so you don’t have to find cows in heat” can also increase the pregnancy rate in heat stress conditions. “At least, you’re putting semen into the cow.”
In vitro fertilization (IVF) is another option. In a study comparing the use of IVF to A.I., using IVF and transferring fresh embryos increased the pregnancy rate by more than 50 percent over the A.I. rates. One option may be to utilize IVF during the summer months, when heat stress is extreme.
But even when fertilization occurs, protecting the embryo itself from heat stress is critical. It is known that embryos are most susceptible to heat stress in the first three days. One possible method of guarding against embryo decline due to heat stress is embyro transfer. Embryos can be transplanted at seven days, sheltering them from the most damaging impacts from heat.
Not only does keeping cows cool keep them comfortable and increase their milk production, it also mitigates the consequences of heat on the reproductive system. But cooling down heat stressed cows does not restore fertility completely.
Keeping cows cool during periods of elevated THI will become more important and challenging as climate change impacts are felt. Periods of heat stress, as well as their intensity and duration. are predicted to increase. While the true economic impact of heat stress in dairy cows is not known, it is clear that heat stress events impact not only today’s milk production, but tomorrow’s as well.
Heat abatement has been “shown to have the greatest benefit on reproductive performance,” Giordano said.
Dr. Giordano presented this workshop at the Dairy Environmental Systems and Climate Adaptation Conference, held in July at Cornell University.