Originally posted to the Ohio BEEF Cattle Letter. To view the original post, click HERE.
By:John Yost, OSU Extension Educator, AgNR, Wayne County
We are accustomed to hearing the weatherman talk about the “actual” air temperature versus the “feels like” temperature. While we each have a “feels like” temperature where we are most comfortable, we can’t translate our comfort to the physiologic and welfare comfort of our ruminant livestock. Heat generated by the fermentation process in the rumen allows cattle to tolerate much colder temperatures than humans. Conversely, they can begin to experience heat stress at temperatures we would consider mild.
The Thermal Heat Index (THI) considers the air temperature and relative humidity to identify combinations where livestock can begin to experience heat stress (Figure 1). Critical THI values will vary depending on the type of livestock and how they are housed. Generally, cattle can begin to experience mild heat stress at temperatures as low as 76 degrees if the relative humidity is 85% or higher. While heat stress events are inevitable during the summer months, they are manageable if temperatures cool off each evening. It is when the temperature remains high through the evening and early morning hours that cattle are unable to dissipate excessive heat, compromising their health and welfare. A THI of 84 is the threshold where we need to implement management changes to help our cattle manage heat stress.
Much of our knowledge about the adverse effects of heat stress in cattle have been studied in dairy cows. Heat stress contributes to increased concentrations of the stress hormone cortisol, which has a significant impact on reproductive performance. Elevated cortisol suppresses estradiol production which reduces the duration and intensity of standing heats. There will also be subsequent reduction in follicular development females and sperm production in males. Early embryonic death is also more likely in heat stressed cattle. The efficacy of vaccinations and antibiotic treatments will also be reduced, as will overall animal performance s depressed due to due to depressed feed intake.
It is important to be able to recognize the signs of heat stress. The USDA Agriculture Research Service created a heat stress scale ranging from Stage 1 to Stage 6 utilizing animal behavioral cues. Under mild conditions (Stage 1) you can begin to cattle with elevated breathing rates, restlessness, and they will spend more time standing. As conditions worsen (Stages 2 to 6), breathing will become more labored, they will open mouth breath with their tongue protruding, they will become lethargic and have their heads down, and they will progress from grouping up to the most affected individuals isolating themselves from the herd.
Pastured cattle will instinctively manage mild heat stress conditions. They will voluntarily seek out shaded areas of pastures, only coming out to consume water, and may avoid grazing until late evening or after dark. As managers, the most important thing you can do is ensure that your cattle have liberal access to cool, clean, water and shade. Cattle prefer water that is between 40 and 65 degrees. When water temperature reaches 80 degrees, reduced water intakes are observed. You can help this by keeping your waterers and water troughs shaded. It is also helpful if the waterlines are buried or shaded by taller grasses in fence rows. If possible, allow animals to graze in pastures that have ample shade from trees or bushes and/or allow access to open barns.
As THI approaches the 84 threshold, we also need to consider avoiding animal handling and transport. Cattle body temperature will peak about 2 hours after the air temperature peaks, and it can take 4 to 6 hours for their body temperature to return to normal. If you need to work or transport your cattle, consider the following suggestions.
- Work cattle during the early morning, before 8:00 AM and never after 10:00 AM, utilizing low-stress handling practices.
- Work cattle in small groups to limit time in the handling facility to 30 minutes or less.
- Provide access to water while in the handling facility.
- Heat production from rumen fermentation peaks 4 to 6 hours after feeding. It is recommended that feedlot cattle received at least 70% of their daily ration 2 to 4 hours after peak daytime temperatures.
- Transport only those animals that are fit for transport. Avoid hauling animals that are sick, lame, or those that have low body condition score or are over conditioned.
- Reduce number of animals hauled at one time.
- If possible, transport in the early morning and arrive at your destination by 8:00 AM
- Open all the baffles of your trailer to maximize air flow.
- Don’t stop until you reach your destination, and if you do have to stop, do so later in the evening or park in an area that shades the trailer.