Rains and normal temperatures into June have taken some of the sting out of the memories of last summer's drought. For cattle kept during the summer of 1998, many of the effects of heat stress were magnified due to the extreme nature and duration of the drought. Cows had lowered conception rates and reduced milk production; therefore, they weaned lighter calves last fall. A silent problem that probably surfaced last summer was early embryonic death in cows being bred during late June, July, and August. Research done at Oklahoma State University in 1986 showed that severe heat stress may increase early embryonic death. It shows up at pregnancy check in the fall as an open cow. Stocker cattle on forage and/or feed gained at a lower rate. It was a tough summer that was hard on cattle. In reality, however, heat stress and its effects are probable during "normal" summers in this part of the country.
Heat is generated inside cattle as energy and is released during digestion due to "burning" at the cellular level through exertion and normal bodily functions. Cattle use this heat to regulate body temperature to 101 degrees Fahrenheit (+ or - one degree). In cold weather, energy needs increase, to generate more heat to keep the animal warm. When it's hot, however, cattle do everything possible to dissipate this heat. Their ability to get rid of heat depends on air temperature, relative humidity, wind, thermal radiation, and the energy level in the ration. Since cattle sweat very little, the main ways they cool themselves are through breathing, radiating heat from their bodies, and reducing feed/forage intake. As intake declines, energy needed for performance also declines, whether for milk production in cows or weight gain in growing cattle.
Anytime an animal's heat load exceeds its ability to dissipate it, heat stress occurs. The comfort zone for beef cattle is between 41 and 77 degrees Fahrenheit. Increasing relative humidity adds to the potential for heat stress. Look at the Livestock Weather Hazard Guide on page two. The vertical column on the left is the air temperature; across the top is the relative humidity. Each number in the body of the guide is a Temperature-Humidity Index (THI) that combines the effects of a given temperature and relative humidity. The THI is divided into levels of hazard potential: alert, danger and emergency.
The alert level is just that. Be aware of the possibility of moving into a higher hazard level and be prepared to take additional precautions. In the danger level, conditions can be severely stressful on livestock and losses can occur. Working, sorting, shipping, and even rotating cattle should be done early or late in the day. When conditions are in the emergency category, plans for handling cattle should be postponed if at all possible.
It's obvious that we have many days in Oklahoma and north Texas which fall into these categories. We should be aware of the potential for heat stress and manage accordingly. Regardless of the THI index, our summers demand attention to livestock water, shade, and handling.
Ideally, cool, clean, fresh water is available to cattle at all times. Water points should provide adequate drinking space for livestock, with frequent turnover of the water in whatever reservoir is used. Remember that mature cows nursing calves need 20-30 gallons of water per day during the summer; stocker calves, 10-20 gallons. A rule of thumb in the summer is a gallon of water per pound of daily dry matter intake.
Direct sunlight raises body temperature and decreases an animal's ability to dissipate its own heat. Shade can reduce radiant heat up to 40 percent. Ideally, treed areas used for shade are thick enough to block direct sunlight but sparse enough to allow natural ventilation.
Plan major workings, pasture rotations, shipping, etc., in relation to the potential for heat stress. A little common sense during our summers can pay large dividends in percent calf crop, weaning weights, and gains.