How do you know if your livestock needs shade? The answer has to do with heat stress. But heat stress can get complicated.
If animals are in the thermoneutral zone (TNZ), then shade isn’t needed. Outside of that zone, an animal must expend energy keeping cool. Physiologically, animals will regulate body temperature by dilating the peripheral blood vessels, expending little energy. Feed intake isn’t impacted. Outside of the TNZ range, however, animals must work to cool their body temperatures.
Kevin Ogles, Natural Resources Conservation Center Grazing Specialist and Matt Poore, Ph.D, Professor and Ruminant Nutrition Extension Specialist, North Carolina State University, presented a webinar to assist producers in understanding when livestock need shade to cool down, and how to provide it in pasture-based operations in the Eastern United States.
There is no one measure of heat stress, but the THI, Temperature Humidity Index, is commonly used for dairy cattle. As humidity increases, lower temperatures can cause heat stress events. The Noble Livestock Weather Hazard Guide is often used for beef cattle. The range of temperatures causing heat stress is a bit higher than the THI index.
Other environmental factors, however, such as wind speed, ground cover, nighttime temperatures and degree of solar radiation are not considered in these indexes, but do have an impact. Animal factors, too, can impact tolerance to heat stress. Hair color, hair type, long-term and short-term acclimation, health, diet and species all affect an animal’s ability to regulate body temperature during any given environmental situation.
When mild heat stress is indicated “there needs to be fresh, clean water” available in unlimited amounts, Ogles said. “We definitely need shade, as well as that fresh water,” during moderate heat stress events. For severe heat stress, “all the measures that we can” need to be implemented, such as fans and misters.
Latitude and location
Ogles gathered data from the National Oceanic and Atmospheric Administration (NOAA), using weather stations in Eastern regions. He found that the 40-degree latitude mark offers some general delineation, with key differences, in the duration, intensity and frequency of heat stress events. But there is no region free from heat stress concerns.
“Unfortunately, it’s not that easy,” and heat stress conditions can occur everywhere in the Eastern United States, Ogles said. “There’s a lot of things to consider.”
Southern latitudes do tend to have higher maximum summer temperatures and very high humidity levels. But in the higher mountainous elevations along the Appalachian Mountain range, this generalization doesn’t hold. Elevation, vegetation coverage, and winds, tend to keep things cooler here.
“Air can give some relief to those hot temperatures,” Ogles said. “We know that wind is a factor. There’s a reason that we have fans on our confinement dairy cattle.”
There has been another pattern, that of increasing frequency of 3-hour periods without cloudy coverage, known as CL-3. Clouds reduce solar radiation, keeping things cooler. There has been significant increases in CL-3 periods, particularly in the South, based on 2016 data.
There is “a good chance of heat stress” when the temperature is in the 90 degree range and skies are clear, Ogles said.
The warmest days of the year vary not only from state to state, but within states and within regions. For example, in Southern Florida and some of New England, as well as portions of Central and Southern New York, the hottest day normally occurs in late July or early August. In western New York, western Pennsylvania and in the Virginias, it is early to mid-July.
Nighttime temperatures “play a really big role,” Ogles said. If the temperature doesn’t drop below 70 degrees, there is “a big difference in the effect on livestock.”
When animals are grazing pastures during hot days, water availability is essential. Their daily water intake needs increase substantially as the temperatures rise above 90 degrees.
“There’s nothing wrong with having a small watering facility, but it’s got to have a quick recharge” to keep up with the animals’ demands for water, Ogles said.
It isn’t all about temperature and humidity. If livestock are grazing toxic fescue, the impact of heat stress is going to happen sooner, and be more acute. The endophyte in toxic fescue causes vasoconstriction, and hinders circulation. Studies done at NC State with toxic versus non-toxic fescue show that there are differences in the animals’ core body temperatures, under the same climatic conditions, depending on whether or not they are grazing this fescue.
“It really does cause a lot of heat stress in animals at a very low temperature,” Poore said of toxic fescue.
Genetics also influences heat stress tolerance. In tropical breed cattle, such as Brahman, Brangus, and Senepol, higher temperatures and humidity levels can occur without triggering heat stress. These breeds are adapted via long-term genetic changes. But they aren’t cold-tolerant, so selling these animals to northern markets is a concern.
Animals which have the “slick hair trait” are more tolerant of heat stress conditions. This genetic mutation offers “an opportunity for us to focus on how we might breed heat tolerance in some of our animals,” Poore said.
Another trait which improves heat tolerance is hair color. In Florida, Ona White Angus were monitored for heat stress, along with black-haired Angus cows. The core body temperatures of the animals were measured throughout the day. During the hottest periods, the black animals showed core body temperatures elevated two degrees above that of the white-haired animals in the same conditions. During morning and evening hours, core body temperatures did not differ in the animals.
Short-term acclimation occurs, too. If animals are out on pasture in spring, and the weather becomes extremely hot, heat stress may occur as they are not yet acclimated to the hot temperatures, and will still have winter hair. If the animals are also grazing toxic fescue, the impact can be severe.
If animals are given shade in one pasture, it has to be provided in all pastures. If not, any heat event that occurs in a pasture without shade will have a significantly detrimental impact as the animals have not acclimated to the heat due to the use of the shade where provided. For producers not providing shade in pastures, an emergency pasture that can be used as needed is imperative when heat stress threat is elevated.
When designing pastures, the use of natural shade is a best practice wherever possible. A shade structure often only provides shade when the sun is overhead, and as the sun lowers to the horizon, the shaded area decreases. But daytime temperatures peak in the mid-afternoon, and the animals need more shade during higher temperatures. In a tree line, the shade is extended as the sun sets. Cedar trees are particularly resilient to cattle pressures, and are good shade providers in pasture, Poore said.
Poore advocates for the use of the heat index, which is readily accessible in the normal daily forecast, as a monitor of when to provide shade. If the heat index is 100, shade is needed for beef cattle, he said.
“Shade is an important consideration, both for animal performance and for animal welfare,” Poore said.
According to Poore, animals prefer access to surface water over shade during heat stress events and would chose to submerge themselves, rather than move to a shaded area. To limit animal intrusion in riparian zones, shade should be provided away from water bodies, he said.
“If that heat index is really getting up there, then we need to do something,” Ogles said. “The more hours those animals are under stress, the worse it’s going to be for them.”