Keeping a close eye on weather conditions and your animals can be helpful in monitoring heat stress and taking action early, before animals are severely stressed. Be aware of changing weather conditions that are likely to make heat stress worse. The Temperature Humidity Index (THI) is a tool that combines the effects of heat and humidity into one number. At a constant temperature, THI goes up as relative humidity increases. Generally, mild heat stress is considered to begin at a THI of 72 for cattle with stress increasing to moderate levels at 79 and severe levels at 89; however, there is very little research data available that is specific to dairy calves and heifers. THI tables are available on the internet, or can be calculated using the equation: THI = dry bulb temperature + (0.36 × dew point temperature) + 41.2, with temperatures in degrees C.
Signs of heat stress include: crowding under shade or around water sources, increased water intake, reduced feed intake, panting or open-mouthed breathing, increased salivation (drooling), and increased respiration rate. According to USDA, more than 90 breaths per minute is an indication of heat stress, and respiration rates over 110 indicate a dangerous heat stress level. Body temperature is also elevated. In severe cases animals may become lethargic or lose coordination. Cattle sweat at a lower rate than humans, and it is generally not enough to remove large amounts of body heat. However, sweating and panting can lead to dehydration. Although we can’t see it, maintenance energy requirements increase in heat-stressed animals as a result of working harder to cool their bodies. This can depress weight gain, feed efficiency, and immunity.
Keep in mind that some animals are at a greater risk for heat stress than others, and take extra precautions to help them cope as well as possible. Animals at higher risk include: newly weaned calves, animals that have been transported, heifers grazing endophyte-infested fescue pasture, heifers with a history of respiratory disease that may have lung damage, predominantly black heifers, springing heifers due to calve during summer months, and heifers that have been underfed or stressed by other illnesses.