By Steve Boyles
Feedlot cattle consuming large amounts of feed and gaining rapidly generate significant amounts of metabolic heat begin to challenge and animals ability to handle heat stress. An animal can endure high ambient temperatures if heat gain during the daytime hours is balanced with heat loss during the nighttime hours. If nighttime ambient temperatures remain high, especially if the relative humidity is also high, there is no time for recovery.
Assessing Heat Stress in Feedlot Cattle
The ability to predict a heat stress event allows for preparation and mitigation of the effects on animal well-being and animal performance. Temperature-Humidity Indexes have been used for more than 40 years to assess heat stress in cattle. There are also Heat Load Indexes for cattle. Such indexes exits in the literature for cattle. Respiratory rate and panting score are useful indicators of heat stress in cattle because they are the first visual changes seen during hot conditions.
- Body alignment with solar radiation
- Shade seeking
- Increased time spent standing
- Reduced dry matter intake
- Crowding over water troughs
- Body splashing
- Agitation and restlessness
- Reduced or stopped rumination
- Bunching to seek shade from other cattle
Cattle Failing to Cope
- Open-mouth and labored breathing
- Excessive salivation
- Ataxia/inability to move
- Collapse, convulsions, coma
- Physiologic failure and death
Cattle can usually cope up to symptom 9. A sign that animals are failing to cope with the hot conditions occurs once cattle display behavioral symptom 10. An early indicator of heat stress in fed cattle before clinical signs are noted is a dry matter feed intake decrease of more than 10% from the previous day, when cattle are monitored early in the morning.
In a crisis the opportunity to treat individual animals is limited. If the opportunity arises the use of ice cold enemas, electrolytes, and intravenous fluids using either normal saline or 5% dextrose and supportive therapy is indicated. Provision of cool drinking water is essential and together with shade and air movement is of considerable assistance when animals are exposed to high ambient air temperatures.
Managing A Heat Stress Event
In most heat stress events there have been several days of adverse environmental conditions of high ambient temperatures, high overnight minimum temperatures, high humidity, and lack of wind speed following a rain event. During these days, the focus is on risk reduction and preparation for a possible crisis. The following presents a list of actions that should be considered to be implemented when a heat stress event is occurring.
1. Cease/Minimize All Cattle Movements
2. Pay attention to Water Availability and Supply: To encourage water intake water troughs should be clean and recharged reliably. It is also important to ensure sufficient access. Three times the normal waterer space may be needed to allow for sufficient room for all animals to have access (additional water tubs). Supplementary water troughs should be filled predawn so the water is as cool as possible.
3. Pen Surface Wetting and Sprinkling Systems: Terry Mader and Dee Griffin (University of Nebraska) recommend a 2- to 5-minute application every 30 to 45 minutes or up to a 20-minute application every 1 to 1.5 hours. Mud build-up and up to a three-fold increased feedlot water usage is associated with a sprinkler regimen. Direct wetting or sprinkling of cattle can have adverse effects, particularly when the cattle get acclimated to being wet and failed or incomplete sprinkling occurs during subsequent hot days.
4. Nutrition and Feeding Management: Heat production increases with digestion and metabolism. This is known as heat increment. Fibrous feedstuffs have greater heat increments than feedstuffs like grain. In theory, it is possible to formulate diets according to heat increment but evidence on whether this practice is effective in alleviating heat stress in feedlot cattle is inconclusive. The addition of dietary fat seems to be the best alternative for reducing heat increment, because fats have a low heat increment. In beef cattle studies, mixed results have been found for steers exposed to high heat load and fed grain diets high in fat. Neither has the addition of salts found to prevent or mitigate heat stress. Nevertheless, electrolyte demand is increased with sweating. With extended heat events, additional potassium and sodium may be needed.
5. Pen Management: Proper feedlot pen layout and design are crucial for minimizing effects of adverse climates. In wet, humid summer environments (sometimes in Ohio) particular attention should be made should promote drainage in the pen and reduce the amount of water stored in the manure pad after a rain event allowing the pad to dry more quickly and reduce the potential impact of humidity in an adverse weather event.
6. Shade: In general, the response to shade is greatest at the onset of heat stress even though shade use increases as cattle get closer to finished weight. Shade improves performance in the summer particularly when cattle are fed in facilities that restrict airflow and for cattle that have not become, or had the opportunity to become, acclimated to hot conditions.
7. People: If feedlot personnel are required to work in conditions likely to cause heat stress in cattle then strenuous work and light work should be alternated. It is essential that hydration is maintained. Wear suitable clothing and a hat and where possible work in the shade.