The heat is on

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A hot sunny day with high humidity and a cessation of wind. Suddenly cattle are struggling to survive. This scenario has played out many times in the Midwest, especially, which can be known for its summers with high heat, high humidity and lack of a breeze. Over the years thousands of feedlot cattle have died from episodes of heat stress which can come on within hours.

Other feedlot areas such as the Texas Panhandle and southwest Kansas where high heat in the summer can be a factor often don’t suffer the same consequences because of their lower humidities and generally higher wind speeds. As the saying goes, “it’s not the heat, it’s the humidity,” and in cattle heat stress, it’s the humidity that’s the killer.

“Humidity is a killer without airflow,” says Dee Griffin, DVM, MS, Great Plains Veterinary Educational Center, Clay Center, Neb. “The temperature-humidity index (THI) is decreased by one unit for approximately every one mile per hour wind speed once the threshold is reached (five to 10 MPH).” Griffin adds that the average wind speed in southwest Kansas and the Texas Panhandle in the summer exceeds 10 MPH, which, when added to low humidity, makes heat stress seldom a killer.”

Griffin explains that the thermo-comfort zone varies greatly for beef cattle. Young animals have a narrow comfort zone between 45° and 80°F. The comfort zone of feedlot cattle and mature cows will range from subzero temperatures in the winter to around 75°F in the summer, depending on body condition, hair coat length and plane of nutrition. “This wide comfort zone allows cattle to thrive under diverse climatic conditions with little or no need for shelter or protection,” Griffin says. But feedlot cattle have difficulty coping with temperatures above 90°F. This is particularly true when humidity is high or wind-speed is low, especially when cattle have had little or no chance to adapt to excessive heat loads.

Observing cattle will tell you when they are becoming uncomfortable from heat. Griffin says cattle will start to move and walk around the pen looking for an area that is more comfortable. They will start to slobber and their respiratory rate will increase above 75 breaths per minute. They will begin to elevate their head to make it easier to breathe and position their body to minimize their exposure to the sun, generally facing the sun.

It’s no surprise that the most susceptible animals on the feedlot are heavy black cattle. Griffin says research by Hungerford found no difference in gender or days on feed. “Hide color and feed additives were the biggest differences,” he explains. “The all-natural fed cattle died at more than four to one compared to the standard feeding program cattle.”

Water is a lifesaver

Water on the feedlot can alleviate stress in two ways — as an exterior coolant on the cattle and to drink and bring down internal body temperatures. The dairy industry has known for years how to cool cattle in hot climates with a combination of shade, water sprayers, misters and fans. For that to work, however, humidity must be lower and there must be enough of a breeze for evaporative cooling to take place. In more humid areas of the country such as the Southeast, some dairy producers use cooling ponds, but the water and the mud can lead to foot and mastitis problems, and would be quite impractical on a feedlot. 

Griffin says the principle of cooling the cattle externally needs to involve cool water that has enough volume to it so it doesn’t just increase the humidity around the animal such as a low-volume mist might do. “Air flow and water to drink are the two most important life saving measures,” he says.

In times of heat stress, it’s important that cattle have enough water to drink and enough space to do it in. “I would suspect if 18 to 24 inches per animal were available and the water temperature was below 60°F, very few animals would die even if there were not breeze,” Griffin says.

Veterinarians can play a role in helping the feedlot prepare for increased water needs prior to a heat stress incident by encouraging them to evaluate their water handling systems. Griffin says a ½-inch water line at 40 PSI will deliver 8 gallons per minute. A ¾-inch water line at 40 PSI will deliver 32 gallons per minute. Every 90-degree angle decreases the water pressure 25%.

Cattle need 20 gallons (160 lbs water per 1000-lb. animal unit) when the temperature reaches their upper critical temp (>82 degrees ). The water systems must be able to deliver that amount of water when the system is under pressure, i.e., every animal trying to drink at the same time. “Every yard in heat stress death loss country should evaluate every water tank’s ability to deliver water.” Griffin says 10 a.m. is when the system will be under pressure when it is not in heat stress period, and shortly after every animal eats its morning feed. “If the heat is occurring and you don’t have empty pens to distribute cattle to get more water to them, move in water tanks or convert part of the feed bunk into a water tank by sand bagging part of the bunk and filling it with water.” Griffin notes that this idea came from GPVEC’s Terry Mader, PhD.

Shades can also be used in times of heat stress, and if they are available in areas of the feedlot, Griffin says they will do the most good for heavy black cattle. He also says shades should be tall to promote air flow underneath.

Windbreaks can be another impediment during heat stress periods as they block any air flow (see sidebar). “If the cattle are in this dead air space created by windbreaks they become dead cattle if the situation is right.” He adds that in general that situation is three days of THI above 85 (heat index >100) with night cooling not falling to below a 70 THI. If possible, move cattle to pens without windbreaks during periods of heat stress.

WHEN HEAT STRESS HAPPENS

Griffin outlines several things that can be done in the event of feedlot heat stress conditions:

Have ample water available. At temperatures above 80 degrees, cattle may need in excess of two gallons of water per hour per 100 lbs of body weight. Consuming water is the quickest and most efficient method to reduce body temperature. Water will prevent dehydration and allows heat dissipated through evaporative cooling and urination. Put out extra water tanks if needed, in advance of anticipated need, so that cattle have access to at least five gallons per hour. Provide 5½ in. of linear space per animal and ensure that all cattle can get water when needed. Having ample linear space for cattle to drink and stay cool also can be important in maintaining cattle performance during the summer. Keep waterers clean to encourage drinking

Avoid cattle handling. Processing cattle can elevate body temperature 0.5°F to 3.5° F. During heat stress periods, if cattle must be handled, work them in the early morning (prior to 8 a.m. and not after 10 a.m.) and in a shaded facility if possible. Wait until the cattle have had at least six hours of night cooling before working them. Dissipation of body heat is needed at night and allows cattle to deal more effectively with heat stress during the day. Schedule cattle shipments at night or early morning. Start loading early so that all cattle can arrive before 7 a.m.

Cattle handled on hot days should spend no more than 30 minutes in the processing or hospital facilities. Avoiding cattle bunching is equally important. Most cattle working facilities have very poor wind movement causing cattle to gain body heat while they are in these areas. A 30-minute time limit minimizes the heat gain and allows the body core temperature to return to normal quicker. Arrange to have shade and sprinklers in those areas. Tubing (½- ¾-in.) equipped with spray nozzles (one nozzle per five animals) placed overhead will improve the cooling in handling and holding areas.

Change feeding patterns. Shifting the feeding schedule toward evening may help hold cattle on feed and even out consumption patterns. Deliver 70% percent or more of the daily scheduled feed two to four hours after the peak ambient temperature of the day has been reached to decrease fluctuating intake patterns. A late-day feeding may also minimize subclinical acidosis thought to contribute to heat stress problems. Research indicates that lowering the energy content of the diet or using a storm ration may lower the heat load on the cattle.

Assess water supply and delivery. Cattle should get half their daily need within an hour. Shoulder-to-shoulder feeder cattle can require up to 32 in. of liner space. The water must also be available when needed: the system needs to deliver a minimum of 1.1% of body weight per hour; for a 1,000 lb animal, this means 11 lb/hr, or about 11⁄3 gal/hr. Ideally, a water system should be capable of delivering within a four-hour period, the amount of water required for an entire day’s needs. Check flow rates on automatic water tanks. Alternatively, spread cattle out to more pens so that the existing water supply can better serve critical needs.

Contact the local fire department or cooperative to access equipment that can deliver emergency water. Make sure it is safe and palatable. Large volume sprinklers for evaporative cooling and lowering ground temperature can be installed if water supply is adequate. Coverage of 10–15 sq. ft. per head should be adequate.

Move cattle away from wind breaks. Windbreaks can be beneficial in the winter, but a detriment in the summer. Consider abandoning pens with windbreaks during critical heat stress, or avoid feeding finishing cattle in these pens.

Improve airflow in pens. Cut tall vegetation 150 ft. back from the perimeter of the pens. If possible, move finishing or high-risk cattle to shaded pens or pens with better wind flow. Build earth mounds, the taller the better, to help prevent cattle from bunching and enhance cattle exposure to air movement.

Provide shade. Shade reduces exposure to solar radiation and reduces heat load on cattle, but they do not affect air temperature. The most effective shade orientation is east–west to keep ground under shade cool; however, a north–south orientation will minimize mud build-up under the shade. With east–west orientation, a higher percentage of shadow lies under the shade structure than when a north–south orientation is used. The shade structure should provide approximately 20–40 sq. ft. of floor space per feedlot animal. For emergency situations, 15–25 ft./head can be beneficial. Shade height should be in the range of 7–14 ft. The higher the shade, the greater the air movement under it. The selected site should have minimal trees, buildings, or obstructions within at least 50 feet of all sides. The most effective material is a reflective roof such as white-painted galvanized or aluminum metal. Slats, plastic or other shade materials with less than total shading capabilities are less effective.

Control biting flies. Stable flies cause cattle to bunch and disrupt animal cooling. Removing weeds and brush within 150 feet of pens and spraying the shaded areas of building with a residual insecticide will help control stable flies.

Cattle feeders risk their economic welfare if they are not prepared for heat stress. “This is a costly mistake, even if nothing dies,” Griffin says. “Heat decreases intake which decreases gain and feed efficiency and increases the cost of gain. Failure to provide enough water and air flow is costly as well, and as far as providing enough water, most feedyards aren’t even close.” 


Human safety is imperative

Heat stress can also negatively affect humans working on the feedlot. To reduce the risk of heat-stress related illness, follow these precautions:

  • Minimize strenuous work during hotter times of the day.
  • During hard work, employees should take a break each hour by spending 10–20 minutes of each hour doing less strenuous work, preferably in the shade.
  • Drink one to two quarts of water per hour.
  • Use a buddy system to make sure adequate water is consumed, workload alternates between strenuous and light work, and early signs of heat exhaustion are detected. Signs of heat exhaustion include mood changes, emotional responses and confusion.
  • If a person gets overheated, he/she should not return to strenuous work that day. Inside work or taking the rest of the day off is advisable. Failure to do this may result in the person developing heat stroke.


Evaluate the potential for heat stress

Dee Griffin, DVM, MS, suggests evaluating the potential for a heat stress emergency to develop by paying attention to these factors:

  • The normal annual rainfall in your area. High rainfall areas are more susceptible to having high humidity.
  • Precipitation above normal, particularly if wet weather continues into the summer months.
  • Long-term weather forecast of hotter than normal conditions, which should signal early activation of a heat stress management plan.
  • Obstruction to airflow in cattle pens. Wind breaks and other airflow obstructions will create calm airflow up to 10 feet downwind for every one foot in height. A windbreak 10-ft. high will obstruct airflow 100 feet downwind. Wind is your friend: a one mile per hour wind will decrease the Temperature Humidity Index (THI) about one unit.
  • Availability of water for watering cattle, and wetting down cattle or pen, so animals on average can easily consume one to two gallons of water per hour, under normal environmental conditions. Watering space and water flow to watering troughs should also be evaluated to ensure cattle are protected from dehydration.
  • Special protection, water supply and airflow maybe need to help black-hided cattle and cattle on all natural feeding (no implants or feed medications) programs keep cool.


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