What is Heat Stress?

Heat stress in the cattle herd may be a concern during the warmest months of the year in the Southeast USA (June through early September; when the temperature is above 85°F and the humidity is high). Unlike humans, cattle get rid of excess heat in their system through breathing and panting more than sweating. The combination of high temperatures and high relative humidity can influence animal performance, and decrease production potential in the cow herd. The combination of these factors along with animal age, hair coat color and length, and nutritional status may influence how animals respond to heat stress.

Potential Impacts of Heat Stress

• Loss in animal performance – body condition
• Less time spent grazing
• Lengthens the estrus cycle
• Weaning and shrink loss among calves

Learning to deal with the heat is a given part of living in the Southeast. However, the following provides some preventive measures we can take to minimize effects on the cattle herd:

Ways to beat the HEAT:

1) Handle cattle early in the morning

• When working cattle in the summer, plan to handle cattle early in the morning (beginning at daylight) with the goal of being done around 10 am. While this may be easier said than done, processing cattle increases the animal’s core body temperature, and puts them at risk for experiencing heat stress earlier in the day. This translates to more time spent in the shade after working. Work cattle in smaller groups so that they are not standing in the holding area for more than 30 to 45 minutes. Use low-stress handling techniques to minimize stress during handling.

2) Establish a grazing management plan

• Rotate cattle to new pastures at night rather than in the morning. While this may seem to counter the above statement about when to work cattle, in general, moving cattle to a new pasture is less stressful to the animal. Moving cattle in the early evening also coincides with the time of day when air temperatures are coming down. Cattle often graze in the early morning or late evening, so moving to a new pasture area in the early evening may encourage animals to actively graze as the daytime temperature begins to decrease. Beef cattle produce heat during the digestion process, and grazing at night allows them time in the evening to dissipate excess heat when the weather is somewhat cooler. Establish a rotational grazing plan to move cattle to new pastures periodically during the summer based on forage availability – both the cattle and the forage will thank you.

3) Ample Supply of a Quality Water Source

• Providing access to an adequate supply of clean, cool water is important to help maintain the internal temperature of cattle within the normal range. In the warmest months of the year, mature cattle can consume up to 2 gallons of water per 100 pounds of body weight! That is between 25 to 30 gallons of water per head per day needed for the majority of our beef cattle in the Southeast. A yearling calf can drink as much as 15 gallons of water per day to avoid a loss in productivity. Careful monitoring of water sources is critical during these months to ensure an ample, algae-free supply for the beef cattle herd.

4) Transport, timing, and other tips

• Transport – If you are hauling calves to be sold during this time of the year, avoid overcrowding on stock trailers since you will likely have to wait before they can be unloaded (see Table 1 for recommended trailer density and footnote). The more crowded calves are on the trailer can accelerate shrink loss in calves under increased heat conditions. Shrink losses can be significant for stressed calves. Calves lose an estimated 1% of their body weight per hour in the first three to four hours after a major stressor has been induced such as weaning, hauling, a sudden change in environment, and marketing. This first phase of shrink is largely due to the loss of urine and decrease in body fill. A shrink rate of 0.25% per hour is usually seen in the following eight to ten hours, and calves begin to lose fluid from body tissue, making them more prone to illness.

Table 1. Maximum recommended number of cattle† to load by trailer dimensions.

†Reduce trailer stocking density by 5% for cattle with horns; reduce load by 5 to 10% during hot conditions. Adapted from NCBA, 2007. Stock Trailer Transportation of Cattle.

• Timing – When receiving a new set of calves during the warmest months of the year, move the feeding schedule to the evening if daytime temperatures are elevated. Supplemented calves dissipate more heat from the digestion process, and feeding at night will allow calves to cool down overnight after feeding.
• Other Tips – Managing against other environmental stressors can also help cattle deal with heat stress. For example, a good fly control program is important because it will help prevent cattle from bunching up, which can add more heat stress because the air doesn’t circulate among cattle as well in this situation. Additionally, cattle with mineral imbalances run the risk of heat stress. In the hot summer months, cattle crave salt. Providing a free-choice loose mineral with salt, calcium, phosphorous, copper, selenium, and zinc can help maintain the nutritional status of cows during the warmer months. The addition of microminerals such as copper may also help cattle shed excess hair, further reducing the potential for heat stress.

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