Thiago Nogueira Marins, Ph.D. Candidate
Sha Tao, Ph.D., Associate Professor, stao@uga.edu/706-542-0658
Department of Animal and Dairy Science, UGA
Heat stress causes a tremendous negative economic impact on U.S. livestock industries. Heat stress negatively affects the productive and reproductive performance, health, and welfare of dairy cattle; therefore, good understanding of environmental conditions that cause heat stress and the adoption of cost-effective heat abatement strategies are crucial to maintain productivity and positive economic returns in a dairy farm. The decline in milk production is linearly associated with the rise in the temperature-humidity index (THI), which is an index calculated based on air temperature and relative humidity and commonly used to characterize thermal environment surrounding cows. The critical threshold of THI = 72 has been used for a long time to identify heat stress, above which the cow’s productive performance begins to drop. Higher producing cows have a lower THI threshold for heat stress. As the average milk production per cow increases, the THI threshold for heat stress decreases. Lately, it was reported that when daily average THI equals to or above 68, the milk production of high-producing multiparous Holstein cows starts to decline. It is important to mention that the THI threshold of heat stress varies with different traits examined. For example, Campos et al. (2022) reported that daily maximum THI of 68, 57, and 60, and daily average THI of 64, 50, and 58 are THI thresholds for milk, fat, and protein yields, respectively, in Canadian Holstein cows, indicating that milk component yields are more sensitive to heat stress. Gernand et al. (2019) reported that the rates of insemination and pregnancy per artificial insemination began to decrease when average THI on the day of insemination exceeded 57 and 65, respectively.
The use of THI can be an effective tool for assessing the environmental conditions that cows are exposed to, providing valuable insights guiding decision-making in enhancing animal comfort. The public weather station data is readily available that can be used to evaluate heat stress. For instance, we calculated monthly THI data using air temperature and relative humidity measurements from the National Oceanic and Atmospheric Administration (NOAA) in Valdosta, in southeast Georgia (Figure 1), and Athens, in northeast Georgia (Figure 2). We calculated the monthly average of daily maximal and mean THI and the length within a day when THI was greater than 68.
In both locations, the average THI was greater than 68 from May to September. During this period, both locations experienced significant heat stress, with maximum THI values exceeding 80. Cows in both locations experienced days when THI equaled to or exceeded 68 for the entire 24-hour period, indicating a continuous heat stress condition throughout the day without night cooling. This information highlights significant heat stress risks in these regions during the warmer months due to the unique climate conditions in Georgia characterized by high temperature and relative humidity. In southeast GA, the average time with THI ≥ 68 within a day varying between 89% and 100% from May to September, meaning cows were exposed to heat stress condition for 21.4 to 24 hours each day. While in the northeast GA, the average time with THI greater than 68 accounted for 58 – 99% (13.9-23.8 h/d) of the time within a day from May to September. These suggest that cows in southeast GA experience more persistent heat stress than those in northeast GA during the warmer month.
Figure 1. Monthly THI data in Valdosta (southeast Georgia). The bars represent the monthly average of daily mean THI and the top of the error bars represents monthly average of daily maximal THI. The % time represents monthly average of the length within a day when THI was greater than 68. Data obtained from NOAA (VALDOSTA REGIONAL AIRPORT, GA)
Figure 2. Monthly THI data in Athens (northeast Georgia). The bars represent the monthly average of daily mean THI and the top of the error bars represents monthly average of daily maximal THI. The % time represents monthly average of the length within a day when THI was greater than 68. Data obtained from NOAA (ATHENS BEN EPPS AIRPORT, GA)
During the cooler month (October to April), although the average THI in both locations was below 68, the max THI exceeded 68, suggesting the presence of heat stress. In southeast GA, the maximal THI in cooler month was consistently higher than 72, and the percentage of time with THI greater than 68 within a day ranged from 11-54% (2.6 -13.0 h/d). While in northeast GA, the percentage of time within a day with THI greater than 68 was 0-27% (0-6.5 h/d). These data suggest that cows in southeast GA constantly experience heat stress throughout a year. Although cooler months had lower temperatures compared to warm months, the sporadic high temperature and constant high relative humidity result in heat stress for dairy cows, particular in the southeast GA.
The data from these two locations in different regions of Georgia highlight the importance of carefully analyzing environmental factors at a specific region to fully understand heat stress risk. It is essential for producers to monitor environmental conditions year-round and provide appropriate management adjustment to ensure cow comfort. During the winter, dairy producers should remain vigilant about potential heat stress, as high THI events can still occur. Additionally, winter provides an opportunity to focus on facilities and equipment maintenance such as shade, ventilation, and cooling system in the barn, holding area, and milking parlor. By addressing any issues during the winter, producers can be better prepared to handle heat stress when summer arrives.
