Heat Effects on Cattle

The thermo comfort zone for cattle varies depending on a large group of factors. These will include body condition, hair coat length, plane of nutrition, health, breed, age and acclimation. In general, cattle do not handle heat as well as humans. In the midst of a typical summer, cattle are generally less comfortable than humans at the same environmental temperature. What that means is that producers need to consider the fact that their cattle are probably hot even when they themselves are not.

In the initial or early stages, when cattle start to suffer from heat, the early signs are not always apparent. Feed and roughage intake may drop a little but the animal may be fairly uncomfortable way before that. As cattle heat up and forage and/or feed intake drops, cattle begin using additional energy in order to help keep themselves cool, therefore, heat stress reduces production and efficiency. Once this performance level drops it becomes very difficult to get it back. This is especially true in growing and feedlot cattle. Some of this loss is carried all the way through to the packinghouse. In many cases with growing and feedyard cattle the losses can equal 10 percent or more.

In breeding cattle, a similar response in terms of nutrient or feed intake and energy metabolism in an effort to stay cool. Often, this results in reduced breeding activity, reduced cycling and lower conception rates. A confounding factor in this scenario is that at a time when cows are hot and not grazing as heavily, the forage quality has also deteriorated so that the roughage or pasture that is consumed is lower in nutrients as well as less digestible. This makes a pretty strong case for summer supplementation programs.

Another area of concern is immune response. This is especially critical in newly weaned cattle that are to be preconditioned and backgrounded. It becomes even more of a concern if these cattle are transported, especially if they have been run through a sale facility of some type and co-mingled with other cattle. Although the exact relationship between heat stress and immune function is unclear, we do know that since much of the animal's maintenance energy is being used to cool itself, the requirements of the immune system may go unmet. Additionally, we also know that under different types of stress, the body tends to deplete itself of critical nutrients such as zinc and copper that are vital to immune response. Also, it has been determined that under stress, the adrenal gland will secrete steroid mimicking hormones that will essentially turn off the immune system. Yet another factor is that respiration rate (panting) is accelerated which increases the susceptibility to respiratory disease, especially if conditions are somewhat dusty. Regardless, in many cases heat stress is only one stress component in the overall physiological challenges these cattle encounter.

It has been determined that three elements are critical in hot weather situations: intensity of heat, duration and the opportunity to cool down at night. The heat intensity means that the combination of heat and humidity create a seriously debilitating or killing situation. If you add an inch or more of rain just prior to an intense heat the humidity level is increased significantly. This elevates heat losses substantially. To help monitor this problem it helps to keep handy or even post a Temperature-Humidity Index (THI) Chart and monitor it regularly in the summer months. Such a chart is found in Figure 1. When the THI reaches 84 or more for two to three days in a row, the producer needs to start taking action to help alleviate the heat.

Figure 1. Temperature-Humidity Index (THI) Chart.

Signs of Heat Stress

Producers need to watch the cattle as well as the environment and be familiar with the signs of heat stress. Signs of heat stress can include:

� Restlessness and crowding under shade or at water tanks.

� Open-mouthed breathing (panting), and increased salivating.

� Increased respiration rates (Moderate heat stress: 80 to 120 breaths per minute, Strong heat stress: 120 to 160 breaths per minute, Severe heat stress: over 160).

� Gasping and lethargic.

Often, symptoms of heat stress may look like respiratory disease. Remember that since cattle do not perspire (sweat) well, they have to use the respiratory system to remove excess heat from their system.

Planning is the Key

Having a solid plan can make the difference between weathering hot, humid periods with a minimum of production losses and finding yourself in an emergency situation. Considering what to do about heat stress periods must start long before it gets warm. Some key factors include:

Identifying high risk cattle. High risk cattle may include the following:

a. Newly arrived cattle that have experienced a fair amount of weaning, processing or transportation stress.

b. Finished or nearly finished cattle, especially heifers.

c. Cattle that are or have been grazing infested fescue pastures.

d. Cattle that have been sick in the past and may have some preexisting lung damage.

e. Black or very dark-hided cattle.

f. Heavy bred cows that will calve sometime during the summer months.

g. Older cows.

h. Cattle which may be somewhat thin due to inadequate nutrition.

High risk cattle should be placed in pastures or pens which will help them reduce their heat loads as best possible. These pastures or pens should contain adequate shading and availability of adequate water. Pens facing east or southeast have been shown to have the radiant heat load and showed the lowest death losses. Pens or pastures with features that obstruct air flow should be avoided. Also areas next to irrigated crops should be avoided since this also increases the humidity in the air. Finally, producers need to control flies since flies cause cattle to bunch which also restricts air flow.

2) Water Availability. One would think this is a �no-brainer� but surprisingly enough, every year cattle are lost because they are placed in areas with insufficient water. Drinking water is the most efficient and fastest way for cattle to reduce body temperature. In higher temperature situations, demand is increased as well. At temperatures above 80o F, they may need in excess of two gallons per hour for each 100 lbs. of body weight. Heavily lactating cows may require even more if milk production is to be maintained. For a herd of 100 average sized cows, that would require 48,000 gallons of water per day. In a 10,000 head feedyard where the average animal size might be 800 lbs. this daily requirement would equal almost four million gallons! Obviously you'd better have a good water supply. Additionally, cattle in a confined feeding situation need at least three inches of linear space at the waterers. Water has long been known as the single most important nutrient in an animal's diet. This is especially true during the heat of the summer. Finally, thought needs to be given to a contingency plan in the event that something might interrupt the water supply.

3) Shade. Although shading does not decrease air temperature, reducing radiant energy (sun exposure) to cattle is critical. In pastures with substantial numbers of trees, we often take shades for granted. In feedyards or growing operations research has shown that adequate shading can cut death losses in half. Shade also increases feed or nutrient intake. If cattle are able to stay cooler they will be inclined to eat better. Trials in Kansas have shown a three percent increase in feed efficiency and a six percent improvement in gain during hot weather periods in cattle with access to shade in the pens.

Shades should be from 7 to 14 feet high and provide from 40 to 50 square feet per head. In pastures with trees for shade, this number should be increased in order to prevent excessed grouping which can and does tend to kill the trees over an extended period of time. It may be necessary to construct some additional shades in order to increase the shade availability in pastures even when trees are available. Constructed shades can be made of many different types of materials but in northern climates they must be able to handle the weight of snow in winter. Some new shade designs have been produced over the last few years which incorporates a high strength mesh material that will reduce as much as 95 percent of radiant energy, handle environmental challenges (wind, some ice and snow) and are quite cost effective.

4) Handling and processing. Do not handle cattle in hot weather if at all possible. Research has shown that movement or handling of cattle during hot weather can change (increase) their body temperature from .5 to 3.5 degrees. Some breeds or breed carcasses are affected greater than this. If it is absolutely necessary to handle cattle during hot weather periods make sure it is done between midnight and 8 a.m. and never after 10 a.m. Even in the evenings after the sun has set, it takes a minimum of six hours to dissipate body heat and this is contingent on what the air temperature is at night. Holding and processing areas should have shade and sprinklers available.

5) Sprinklers. In confined cattle feeding situations, sprinklers can have a multifold benefit to cattle. Initially they are quite useful in keeping down the dust. On very hot days they can be considered an insurance against death losses. Sprinkling cattle helps reduce body temperatures by increasing evaporative cooling. It can also help reduce ground temperature as well. Cattle need to be really wetted down though, not just misted. Data is available which shows cattle under sprinklers gain faster and with increased feed efficiency. Sprinkling should take place intermittently over the course of the day to prevent a high humidity situation. Sprinkling of two to three minutes followed by a break of 20 to 30 minutes seems to work the best.

6) Feeding. As discussed earlier, hot weather often dictates a need for supplementation of pasture cattle. With intakes down, nutrient intake can only be maintained by feeding a more concentrated supplement to offset the reduction. For growing and finishing cattle it also makes sense to reduce energy levels. Ration energy level reduction will reduce fermentation and the associated heat production. This is typically done by reducing the grain concentration in the ration. During the hot months, however, it is often effective to reduce the grain and add back a pound or so of fat. Fat contains 2.25 times more energy than grain and can effectively replace the energy from grain. It is important that the fat level remain less than seven percent of the ration dry matter content to prevent interfering with fiber digestion in the rumen. It may also be useful to alter feeding times. This may mean feeding earlier in the morning or away from warmer times of the day.

Conclusions

We have to recognize that during the heat of the summer it will be impossible to keep cattle perfectly comfortable and performing as normal. By doing some homework and identifying critical issues a producer can effectively reduce heat stress in breeding, growing and finishing cattle. This will help maintain profits and productivity even when the environment is less than cooperative.