Vet Column 12-13-10
Fort Collins, Colo.
For most Bos taurus cattle (e.g., British and continental breeds), the impact of climatic heat load can be stressful. Combinations of increased temperature, humidity, solar load, and low air movement can exceed the ability of an animal to cope, resulting in a loss of productivity and even death. Research studies have documented that shade reduces the heat load of cattle and can reduce mortality in extreme weather events. Performance measures when evaluating the effects of shade have been, however, inconsistent.
In one published study, shaded cattle had greater average daily gain, more dry matter intake, and heavier final weights than cattle without access to shade in the feedlot. In contrast, another study reported that providing shade did not improve feed intake, growth, feed efficiency, or carcass quality of feedlot cattle. Another study reported that body temperature, respiration rate, and open-mouthed breathing were reduced with providing shade in the feedlot.
One important criterion for measuring heat stress is body temperature. Several studies measured body temperature, but over a short period of time. Investigators from the University of Queensland, Australia, and the University of Nebraska-Lincoln recently published a study of heat stress in feedlot cattle. The cattle were fed for 120 days in the subtropical environment of central Queensland where the temperature ranged from 88-93 degrees Fahrenheit. to 64-70 degrees Fahrenheit. with temperatures often exceeding 102 degrees Fahrenheit. The average rainfall during the feeding period was 16.6 inches.
For this study, 164 Angus steers, averaging 12 to 15 months-of-age and weighing 837 pounds, were used in a 120 day feeding period to determine the effect of feedlot shade on body temperature and performance. All cattle were obtained from a temperature region and were not adapted to the subtropical climate for this study. Shade was provided by use of an 80 percent solar block shade cloth. Prior to beginning the study, 63 animals were implanted with a body temperature transmitter providing a body temperature reading every 30 minutes. Each pen involved in the study had three implanted steers. Individual panting scores were obtained daily at 6 a.m., noon, and at 4 p.m.
Results from this study indicate that over the 120 day period of the trial, average body temperature was not different between those animals receiving shade and those that did not. For the 21-day period that averaged more than 86 degrees Fahrenheit for at least eight hours each day, shaded steers had lower body temperatures than non-shaded cattle, 104.7 degrees Fahrenheit versus 106 degrees Fahrenheit, respectively. For this same 21-day period, the average panting score for each pen was greater at all observation periods for the cattle without shade. During this same heat wave, feed intake was 51 percent less for nonshaded steers and 39 percent less for shaded steers when compared to the pre-heat wave period. Final body weight for the shaded steers was greater when compared with the non-shaded steers, averaging 38.5 pounds more body weight.
In this study, access to shade improved growth rate, feed efficiency, and hot carcass weight. It also decreased average panting scores. Shade did not, however, completely eliminate the impact of high heat load. Based upon this study, when sustained temperatures exceed the high 80s Fahrenheit, providing shade is one alternative to reducing heat stress in feedlot cattle.