By Steve Sharrow, Oregon State University
One visible effect of including trees in pastures is the shady haven which they provide for livestock on hot summer days. The benefits of providing protection from the hot rays of the summer sun are obvious. After all, that is why we wear a hat! It easily follows that animals, which are unable to shelter from the direct sun during the heat of the day, will have to expend energy to deal with their discomfort and/or reduce their feeding activity. Their productivity should decline in proportion to the time spent under these unfavorable conditions. Increased livestock production during hot weather is often promoted by agroforesters as one of the benefits of having trees. However, there is very little published research available to either verify or disprove this widely held belief.
A recent article by Cook et al. (1998) discussed a similar situation for thermal cover for elk. The need for tree canopy cover to shelter deer and elk is so widely accepted that forest management prescriptions often include thermal cover guidelines. However, most evidence of benefits from thermal cover for animals is circumstantial. We know that deer and elk prefer shady habitat during hot weather (Ockenfels and Brooks 1994). However, shady areas may differ from unshaded areas in terms of topographic position, understory vegetation, and proximity to other habitat factors such as water. Benefits from any of these factors could be attributed to shade. Animals will also sometimes prefer things which they do not actually need. So do animals really benefit from the shade or just like it? Surprisingly, the only available direct comparison of shade trees vs. unshaded habitat (Cook et al. 1998) shows no effect of shade on elk summer grazing habits or weight gains. While elk prefer shade, they no not need it in Oregon's Blue Mountains.
Similar to elk, livestock are attracted to shade on hot sunny days.Cattle may orient their pasture use to be near sources of shade. But how much does livestock performance really improve when summer shade is provided? This is a harder question to answer than you might think. Millions of cattle are successfully raised in the hot humid summers of the Midwestern and Southeastern states without shade. So, would shade increase their performance? Shade has generally been useful in increasing milk yields of dairy cattle and liveweight gains of feed lot cattle in hot climates. This is especially true for European breeds of cattle in areas that are both hot and humid (Blackshaw and Blackshaw 1994). However, high concentrations of high producing animals consuming relatively high-energy rations are somewhat different than the conditions typical of forage-based cattle production in pastures and rangelands.
Surprisingly few reliable studies of cattle productivity with and without shade have been done under realistic pasture conditions. I could only find two controlled comparisons of cattle grazing in pastures with vs. without shade in the regularly published literature. The greatest effect of shade trees on summer weight change in free ranging cattle was reported for European breeds of cattle in Louisiana (McDaniel and Roark 1956). Averaged over 4 years, cows grazing under scattered pines spent about half an hour more time grazing each day and gained 1.29 lb./head/day compared to cows grazing open pastures, who lost weight during the summer. Unfortunately, no pasture data are provided.
It is not possible, therefore, to completely separate shade effects upon forage quantity and quality from its effect on animal thermoregulation. Much more modest increases in summer weight gains were reported for cattle with access to artificial shade structures in Oklahoma (McIlvan and Shoop 1970). During 4 years, weight gains of Hereford cattle were increased 6, 9, 1, and 11% by artificial shades, with the greatest impact of shade seen during hot and humid weather.
Only Cook et al.'s (1998) elk data are available to represent the western states. Humidity is probably as important as temperature in summer heat stress of grazing livestock. We often think of the daily high temperatures as the main issue in heat stress. However, night conditions are probably equally important. As those who have lived without air conditioning in the southern U.S. will agree, it is not just the hot humid days that wear you down, it's also the hot humid nights. Some small daily variation in body temperature is normal for mammals. A slight increase in body temperature during the heat of the day, followed by a similar reduction in body core temperature during the cool of night allows large mammals to use their body mass as a buffer. The issue becomes serious when animals are not able to recover from the previous day's heat stress before the next day begins. Mercifully, heat and humidity are an uncommon combination in the more arid, higher elevation areas of western rangelands. This probably explains the lack of shade effects on elk reported by Cook et al. (1998)
Blackshaw and Blackshaw (1994) summarized the literature relating shade to heat stress in cattle. As they explain the situation, cattle receive heat directly from the sun when standing in the open. Dull, dark colored hair absorbs most sunlight, which is converted to heat. White, shiny coats reflect much of the energy and less is converted to heat. Shade can be very helpful in reducing heat loading from the sun's rays, especially for dark colored animals. However, quite a bit of heat can be received after being reflected from bare soil or nearby objects. Shade is not very effective in protecting cattle from reflected radiation. For example, a California study found that 33% of the heat load received by a shaded animal was reflected from the ground and 28% came from the overhead shade material. Total heat load was only reduced by 30% under shade compared to out in the sun.
Metabolic heat generated from the digestion and use of energy contained in food can also be a significant source of heat. Approximately 35-70% of the energy which cattle extract from food is converted to heat. Active animals and those consuming large amounts offeed can generate significant amounts of internal heat. Shade has no direct effect upon this heat source. So, although shade may reduce heat loading of cattle, it probably only directly affects less than a quarter of the total heat energy cattle must deal with on a sunny day. This reduction may be important under severe heat conditions or when livestock are under stress because of other factors. It is less likely to be crucial for acclimated cattle that are otherwise well fed, well watered, and healthy.
Cattle lose heat primarily by transferring it to cooler air, and by evaporation of water from sweat and from moist tissues in the respiratory system. The cooler air temperatures under trees are very helpful in increasing heat transfer from animals. This is probably as important in the general cooling effect of shade as is protection from direct sun light. It is less effective, however, for naturally well insulated animals, such as some of the northern European breeds of cattle, which have thick coats of hair. Shade trees do not reduce air temperatures under windy conditions and still air under trees may be more humid as well as cooler than that out in open pasture. Because of the major role that evaporation of water plays in heat transfer, heat stress is much greater under humid conditions than under dry heat.
The issue of heat stress is further complicated by both physiological and behavior adaptations which cattle make to reduce the effects of heat. Heat stress may reduce animal performance in two ways. First, it may depress grazing and food intake. And second, animals may resort to panting or other actions that consume energy. Cattle breeds differ markedly in their ability to tolerate heat. In general, Brahman type cattle are more heat tolerant than are northern European breeds. However, much heat tolerance is behavioral. Cattle may successfully deal with lack of shade by restricting their grazing and traveling to the cooler hours of the day or night and by standing together in areas of good airflow.
So, where does all this leave agroforesters? There are many sound esthetic, ecological, and economic reasons for including trees in pastures. It makes intuitive sense to most of us that shade should increase comfort and improve animal performance. However, the factual basis for improved livestock production due to shade is very limited. We need to be careful about advocating shade as a means of reducing heat stress. Protection from sunlight only reduces one of several sources of heat which contribute to thermal stress in animals. Livestock have both physiological and behavioral mechanisms they can use to counter environmental stresses. Livestock performance is unlikely to be greatly improved by shade unless heat stress is sufficiently great and of adequate duration to overcome these coping mechanisms. Such conditions are probably less common under traditional range and pasture production systems than we think.
Blackshaw, J.K., and A W. Blackshaw. 1994. Heat stress in cattle and effect of shade on production and behaviour: a review. Aust. J. Exp. Agr. 34:285-295.
Cook, J.G., L.L. Irwin, L.D. Bryant, R.A. Riggs, and J.W. Thomas. 1998. Relations of forest cover and condition of elk: a test of the thermal cover hypothesis in summer and winter. Wildl. Mono. 141:1-61.
McDaniel, A.H., and C.B. Roark. 1956. Performance and grazing habits of Hereford and Aberdeen-angus cows and calves on improved pastures as related to type of shade. J. Anim. Sci. 15:59-63.
McIlvain, E.H., and M.C. Shoop. 1970. Shade for improving cattle gains and rangeland use. J. Range Manage. 24:181-184.
Ockenfels, R.A., and D.E. Brooks. 1994. Summer diurnal bed sites of coues white-tailed deer. J. Wildlife Manage. 58:70-75.
*This article originally appeared in the July 2000 issue of the Temperate Agroforester. Dr. Sharrow is Professor or Range Management and Agroforestry, Department of Rangeland Resources, Oregon State University, Corvallis, OR. 97331