Heat Stress in 2021: Will it be Intense?

Mr. Jason Hartschuh, Extension Educator, Crawford County, Agriculture and Natural Resources, Ohio State University Extension, and Dr. Aaron Wilson, Climate Research Specialist, Ohio State University Extension

Heat stress can lead to major challenges for lactating cows during peak milk production and dry cows during the transition period. It can even cause challenges for calves before and after birth. In the U.S., heat stress is estimated to generate between $1.69 and $2.36 billion in total annual loss to the livestock industry.

While the upper limit of the thermal neutral zone for dairy cows is 68°F and for calves at one month of age is 77°F, heat stress includes more than just temperature. Heat stress factors in the humidity as well. For this reason, we utilize a temperature-humidity index (THI). While cooling should begin as soon as we leave the thermal neutral zone, heat stress for cows begins around a THI of 72, with 72 to 79 causing mild stress. At 79 to 88 THI, cows enter moderate heat stress which occurs at 80°F and 90% relative humidity (RH) or 85°F and 60% RH. Low end mild heat stress results in a milk production loss of 2.5 lb/cow/day, while the upper end of mild to moderate heat stress leads to 6 lb/cow/day of lost production. Figure one can be a useful tool for operating your cooling system. In the Stress Threshold (tan zone), fans should be operating. As the stress increases, more heat mitigation practices are needed. Besides monitoring environmental conditions, signs of heat stress can include increased breaths per minute, which should be between 40 to 60 under normal conditions. While not convenient, rectal temperature may also be used, which under normal conditions is between 101.2 to 102.5°F. If over 5 to 10% of your herd exceeds 105°F, your herd is in the emergency level of heat stress and additional cooling is definitely needed.

Figure 1. Temperature Humidity Index (THI) for cattle. Lactating dairy cows are at greater risk for heat stress when the THI exceeds 68.

The latest summer outlooks from the Climate Prediction Center call for an elevated probability of above average temperatures in Ohio for the month of June, as well as the June-August period (Figure 2). There are also indications that precipitation is likely to be above average throughout the period as well. While predicting THI for individual days is not possible this far in advance, the outlooks suggest a tendency toward higher values (warm temperatures and more moisture), especially during the first half of the summer.

To mitigate heat stress, cattle respond in a few natural ways and we can assist in these efforts. First, providing shade cloth will assist cattle as they naturally seek shade, especially for calves living in hutches. Second, cattle increase their sweating. This can lead to dehydration even as it provides evaporative cooling. We can supplement this response with a well-designed soaker system. They will also increase water intake by 1.2 to 2 times normal rates, leading to water needs per cow of up to 52 gal/day. Frequent checking to ensure adequate available water is critical, including drinking space, well-capacity, and water flow rates to keep your cows cool this year. Also, work with your nutritionist to make plans for ration adjustments before heat stress sets in, not after the effects are felt in your bulk tank. Lastly, walk your barns on a regular basis to check fan performance. The ideal wind speed in your barn should be around 5 mph.

Figure 2. Probabilistic temperature (left) and precipitation (right) outlooks from the NOAA Climate Prediction Center. Colors represent the probability of above, below, and near normal conditions for June-August 2021.