RUMINANT HEAT STRESS: EFFECT ON PRODUCTION AND MEANS OF ALLEVIATION 1

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1 RUMINANT HEAT STRESS: EFFECT ON PRODUCTION AND MEANS OF ALLEVIATION 1 S. R. Morrison 2 University of California, Davis Summary A review of the literature indicates heat stress generally causes lower milk production, decreased growth rate for cattle and lambs, but little effect on wool production. Breed and diet affects the degree of adverse response. Heat stress is caused primarily by high air temperature, but can be intensified by high humidity, thermal radiation and low air movement. Improving performance of animals under warm conditions involves breeding and management and modifying the environment. The former includes selection for heat tolerance, use of crossbred animals, diets with low heat increment in relation to energy for production and control of diseases and parasites. Environmental modifications may include provision of shades, use of water for evaporative cooling and increased air movement. (Key Words: Ruminant, Heat, Temperature, Cooling, Shades, Evaporation.) I ntroduction Livestock performance is affected by heat stress because an animal having difficulty in losing heat will decrease its heat production by lowering feed intake. This act results in lower production or growth and probably less production per unit of feed. Heat stress also affects the maintenance energy because the body may be at higher temperature resulting in greater metabolic action, and also, energy is used to increase heat dissipation. However, digestibility of feed may be increased either through direct means or due to the fact decreased feed intake Invitational paper presented at the Symposium on "The Effect of Environmental Extremes on Ruminant Requirements," held on August 11, 1982, during the joint Annu. Meet of the Can.-Amer. Soc. Anita. Sci., Univ. of Guelph, Guelph, Ontario, Canada. 2Professor of Agr. Eng., Dept. of Agr. Engineering. Received August 25, Accepted February 18, may result in longer retention time. There also may be indirect effects on productivity because the factors causing heat stress may affect insects and disease-causing organisms. Heat stress is caused by those factors that decrease heat transfer from an animal to its environment, which would include high air temperature, high air humidity, low air movement and thermal radiation load. Air temperature is usually the primary cause of heat stress, although other factors may intensify the stress. Thus, in studies of the effect of temperature on production values of humidity, air velocity and thermal radiation conditions should be noted. Although tests using constant temperatures are common, caution must be used in applying them to natural conditions where diurnal variation of temperature is usual. Possible acclimation to high temperature and compensatory production after relief of stress must also be considered. Research is ordinarily conducted with healthy animals, but it seems reasonable to expect that animals suffering from disease or parasites would be more adversely affected by heat stress than healthy ones. The references cited in this paper represent only a fraction of the voluminous literature on the subject and often were selected to illustrate certain points. References are restricted to those dealing with cattle and sheep. Reports on the effect of heat stress on fertility and reproduction are not included. Effects on Production Cattle. Much research has been conducted on dairy cattle at the University of Missouri Climatic Laboratory. Ragsdale et al. (1949, 1951) reported the effect of high temperature on milk production of Jersey, Holstein and Brown Swiss cows. These breeds under the conditions of the test, showed a rather marked drop in feed intake and milk production at temperatures above about 25 C, although JOURNAL OF ANIMAL SCIENCE, Vol. 57, No. 6, 1983

2 butterfat content tended to increase especially at their highest temperature when production was very low. In studies of diurnal temperature, Brody et al. (1955) found for Jersey and Holstein cattle about the same depressing effect of 21 to 38 C diurnal rhythm on milk production as for the mean of the two extreme temperatures. However, Folman et al. (1979) found only about an 8% decline in milk yield for summer conditions when the mean afternoon temperature was 39.8 C compared with winter conditions. They attributed this result to possibly the higher concentrate diet (65 to 75%) than in the Missouri studies or the adaptation of the Israeli-Friesian cattle. Kamal et al. (1962) reported cattle raised at 27 C showed significantly milder heat stress reaction when exposed to high temperatures compared with those reared at 10 C. Thus, heifers raised at warmer temperatures may later have better lactation performance. Johnson et al. (1967), in a study of the time necessary for cattle exposed to 18 C temperature to reach a "stabilized state" at 29 C, found the lower milk productior~ paralleled the expected production (at 18 C) after 3 or 4 wk. Interaction between roughage level and temperature on the fat content was found by Stanley et al, (1975) in that temperature had no effect on fat percentage of milk f, rom cattle fed a high roughage diet, whereas a high grain diet resulted in lower fat content at the cool temperature and a further depressed level at the high temperature. Maintenance energy for dairy cattle was shown to be considerably higher under thermal stress conditions by McDowell et at. (1969). Hales and Findlay (1968) indicated most of the additional energy is due to the effect of increased body temperature on metabolic rate (van't Hoff-Arrhenius effect) and that panting requires relatively little energy. Ragsdale et al. (1957) reported the growth responses of three breeds, Brahman, Santa RUMINANT HEAT STRESS 1595 Gertrudis and Shorthorn calves, at constant temperature of 10 and 27 C and found only the Shorthorn growth rate adversely affected by the higher temperature. Morrison and Lofgreen (1979) found a significant reduction in feed intake and rate of gain for Hereford and Hereford-Angus steers at 29 compared with 20 C. The data (table 1) from Colditz and Kellaway (1972) show the value of FI Brahman Friesian crossbred cattle under high temperature conditions as compared with either pure breed. Holmes et al. (1980) also reported the improved performance of Brahman-Friesian crossbred cattle over Friesians with a highly digestible diet. They also reported improved digestibility of diets at 37 compared with 17 C. Warren et al. (1974) likewise found evidence of an increase in digestibility of roughage with increase in temperature. Hahn et al. (1974), in a study of Hereford cattle on a high concentrate diet, found that after 5 wk of "moderate" heat stress and then a return to thermoneutral conditions, the cattle exhibited compensatory growth and within 1 or 2 wk were back at the weight of a control group. However, cattle with more severe heat stress had only limited recovery of growth. Sbeep. Because sheep are seldom raised under confinement conditions, there have been fewer reports of tests under controlled conditions. Ames and Brink (1977) have reported the detrimental effect of high temperature on the rate of growth of shorn iambs. Bhattacharya and Hassain (1974) reported that the effect of heat stress on depressing feed intake was more pronounced with sheep fed a high roughage compared with a low roughage diet. Bhattacharya and Uwayjan (1975) found that feed intake of Awasi sheep was not affected until temperatures rose to 32 C. Singh et al. (1980) reported on the difference in genetic groups of sheep exposed to high temperature. Entwistle (1975) investigated the relationship TABLE 1. PERFORMANCE OF CATTLE: INTERACTION OF BREED AND AIR TEMPERATURE Item Friesian Brahman X Friesian Brahman Air temp., C Average daily feed, kg/100 kg wt Average daily gain, kg O

3 1596 MORRISON ~,oo v 0.J - - I z O 0 o V--_ 150 '.S I00 ~ 5o >- I'-'T _z o 0 _ Convection 0 o nr I-- tic btj lad I _ Recto?/7-- - / \ 20-- fo I I I I III I III I0 I IIIIII~LIL dune, 1971 PDT Figure 1. Temperature and heat transfer for a 500-kg steer outside on a clear summer day. PDT -- Pacific daylight time. between plane of nutrition and air temperature on wool growth of tropical Merino sheep. He compared a simulated summer circadian temperature with temperatures in excess of 38 C for 7 h with a natural winter temperature and found no effect on feed intake and gain. Wool growth rate was unaffected by high temperature on a high quality diet, but was lower on a low quality diet compared with that at the winter temperature. Ames et al. (1971), studying the energy balance during heat stress in shorn Suffolk ewes, recorded about a 1 C rise in rectal temperature with increase in air temperature from 25 to 45 C. They found about the same values and same increase in respiratory and surface heat loss with this increase in tempera- ture. They further reported both panting and the Ql0 effect increased maintenance energy. Alleviation From consideration of the previously cited papers, one means of reducing the effects of heat stress is in choice of breeds, selective crossbreeding and selection of heat-tolerant strains within breeds. Several papers have shown the interaction between diet and resistance to heat stress. One approach is to select feed ingredients with lower heat increment per unit of net energy for production as was shown to be successful for beef cattle by Lofgreen (1974).

4 RUMINANT HEAT STRESS 1597 Shades. Solar radiation loads on animals can be very large as indicated in figure 1 (Morrison, 1972). That animal, from which the data were collected, in addition to being in the sun, was on a concrete floor on the west side of a barn. Thus, it was receiving reflected solar radiation and emitted radiation from surroundings. Bond et al. (1967, 1969) have indicated the surroundings can contribute substantially to the radiant heat load of an animal. Shades, while not affecting air temperature, are effective in reducing radiant heat load. They have been shown to be beneficial for beef feedlot cattle in the Imperial Valley of California (Itmer et al., 1954), but not for cattle on pasture or drylot in Southern Georgia (McCormick et al., 1963). Roman-Ponce et al. (1977) reported a 10% increase in milk production for cattle under shades in the subtropical environment of Florida. Ingraham et al. (1979) reported an average 4.0-kg increase in daily milk production with shades under rather mild heat stress conditions in Hawaii. No doubt variation in breed, lactation level and diet as well as temperature, humidity and radiation load of the various locations account for the differences. Moose and Ross (1964) reported 12% higher gain and 15% greater efficiency in feed conversion for lambs provided with shades in Missouri. Evaporative Cooling. Because evaporative cooling is probably the most practical means of cooling livestock, it will be given considerable attention. Because it involves using the latent heat of moisture added to air to cool the air, the effect of humidity on animal heat loss is relevant. The absolute humidity or vapor pressure of water in air affects the rate of evaporation from a water surface so it would be expected to affect respiratory heat loss. Sheep and cattle do not usually have water on their skin, so the effect of air humidity might be expected to have less effect on surface evaporation. Yet the skin is an important avenue of evaporative heat loss and as Kibler and Brody (1950) indicated; the surface loss from dairy cattle is two to three times that of the respiratory tract it high temperature. Hopkins et at. (1978) found tropical Merinos lose 90% of total evaporative heat loss by nonrespiratory means, whereas Hales and Brown (1974) report 40% athe equation for this index, in Fahrenheit units, is.55 times dry bulb temperature plus.2 times dew point temperature plus for this characteristic for sheep. Kibler and Brody (1950) state that, "in sparsely-sweating cattle, evaporative cooling is mainly limited by the moisture secretion rate of the body surface." Morrison et al. (1967) found that swine, another slightly sweating species, under a constant 30 C, but with relative humidity increasing from 30 to 90%, were able to maintain surface evaporation rate. These results indicate that the decline in evaporative heat loss with increase in humidity is not as large as from a free water surface and the decrease in temperature with evaporative cooling should more than compensate for the increase in humidity. Of course, with constant high temperature, increasing humidity should result in decreased performance as Ragsdale et al. (1953) found for milk production. The temperature-humidity relationships can be visualized by the use of a psychrometric chart, such as shown in figure 2, where air temperature is plotted along the horizontal axis and absolute humidity (humidity or vapor pressure; scale not shown) along the vertical axis. Several lines indicating conditions of equal comfort or production are shown on the chart. They were chosen for one particular value of each index with a common point at 30 C air temperature at saturation. Line "A" represents the "wet bulb" temperature or the process of evaporative cooling and would be the line of constant productivity if effect of increase in humidity was just offset by decrease in air temperature. Lines with steeper slopes indicate a lesser effect of humidity and that evaporative cooling would be beneficial. Line "B" represents conditions of equal comfort for humans determined by allowing them brief exposure to various environmental conditions (Houghton et al., 1926). Line "E" is also for humans, but these resulted from tests where people were allowed to remain at various conditions for some time (Jennings and Givoni, 1959). Evidently, acclimation to high humidity occurred. Line "C" represents a value of the Temperature- Humidity Index (THI) 3 which is used in some weather reports and with which milk production has been correlated (Johnson et al., 1962). One means of assigning the appropriate significance to humidity is to multiply the wet bulb temperature by a fraction and the dry bulb temperature by one minus that fraction. Roller and Goldman (1969) determined.25 to be the value of this fraction for swine physiological response to acute stress. Morrison et al. (1969), in a study of the effect of humidity on

5 1598 MORRISON I00 % 90 % 80 % 70 % 60 % 50 % 40% 30 % 20 % 10% / / / /. / / / _ f J / t J I l,// / / /A) / I/ 20 Z DEGREES C Figure 2. Temperature-humidity indices on a psychrometric chart. See text for details. rate of gain of swine, found lines of equal production also were equivalent to assigning the fraction a value of.25, and such a line is labeled "D" in figure 2. It is not clear that the THI gives the best correlation with milk production, but Hahn (1981) used it to determine losses in milk production for various United States locations and then determined the expected production benefits of evaporative cooling. Experiments on the effect of evaporative cooling on milk production include research conducted by Brown et al (1974) in Mississippi, who found it was effective in reducing high temperatures, but effect on milk production was mixed. A significant improvement was recorded one year, positive but nonsignificant effect another year and negative nonsignificant effect a third year. Stott and Wiersma (1974) and Wiersma and Stott (1966), studying evaporative cooling under shades in Arizona, found improvement not only during the hot weather but for the entire lactation season, with an / / annual average increase of 550 kg of milk over that from cattle under shade with no cooling. Higher air temperature and lower relative humidity in Arizona probably account for the difference. However, in a similar Arizona study with beef cattle, comparing evaporative cooled air, sprinkling shade roof and ground surfaces, and high pressure fogging with conventional shades, Wiersma et al. (1973) found no significant improvements in gain or feed efficiency. Other Means of Cooling. Direct application of water to beef cattle under shades in the Imperial Valley of California has been studied by Morrison et al. (1973, 1974, 1981) with mixed results. Sprinkling at 30-min intervals when the temperature was above about 27 C usually resulted in about a.24 kg/d higher gain for British breeds weighing about 300 to 350 kg at start of tests; however, the performance of heavier British or British-Brahman crossbred cattle was not improved. As long as air temperature is lower than surface temperature, increasing air velocity should increase convective heat loss. The overall effect on heat loss is complicated, however, as the resulting lowering of surface temperature will decrease radiant and perhaps evaporative heat loss. Early research in the Imperial Valley of California (Bond et al., 1957) showed improved performance of beef cattle under shades by using fans. However, subsequent research (Garrett et al., 1960) failed to substantiate the results. Variation in climate, diets or animals could account for the difference. Pontif et al. (1974) in Louisiana showed fans or shade improved performance of feedlot cattle, but fans with shade resulted in no further improvement. Zone cooling, in which cooled air is applied only to the head, has been tried in Louisiana (Gomila et al., 1977) with resulting slight increase in milk production. It would seem that in most circumstances, if there is reasonable natural air movement, shades will provide sufficient protection from heat stress and the cost of fans, evaporative coolers or even sprinkers may not be justified for beef cattle. Another means of cooling that has been suggested is the use of cooled panels for radiation cooling (Shanklin and Stewart, 1958). Lofgreen et al. (1975) studied the effect of cooled drinking water (18 vs. 32 C) on beef cattle in the Imperial Valley of California. They found the increased feed intake for British breeds was that expected from the increased

6 RUMINANT HEAT STRESS heat production possible due to the cooling effect of the water, but they found no increase with British Brahman crossbred cattle. The use of the earth's temperature at shallow depth is a possibility and is being investigated for both heating and cooling of swine barns (Goetsch et al., 1981). L iterature Cited Ames, D. R. and D. R. Brink Effect of temperature on lamb performance and protein efficiency ratio. J. Anita. Sci. 44:136. Ames, D. R., J. E. Nellor and T. Adams Energy balance during heat stress in sheep. J. Anim. Sci. 32: 784. Bhattacharya, A. N. and F. Hassain Intake and utilization of nutrients in sheep fed different levels of roughage under heat stress. J. Anita. Sci. 38:877. Bhattacharya, A. N. and M. Uwayjan Effect of high ambient temperature and low humidity on nutrief'.t utilization and on some physiological responses in Awasi sheep fed different levels of roughage. J. Anita. Sci. 40:320. Bond, T. E., C. F. Kelly and N. R. lttner Cooling beef cattle with fans. Agr. Eng. 38:308. Bond, T. E., C. F. Kelly, S. R. Morrison and N. Pereira Solar, atmospheric and terrestrial radiation received by shaded and unshaded animals. Trans. Amer. Soc. Agr. Eng. 10:622. Bond, T. E., S. R. Morrison and R. L. Givens Influence of surroundings on radiant heat load of animals. Trans. Amer. Soc. Agr. Eng. 12:246. Brody, S., A. C. Ragsdale, R. G. Yeck and D. Worstell Milk production, feed and water consumption, and body weight of Jersey and Holstein cows in relation to several diurnal temperature rhythm~ Missouri Agr. Exp. Sta. Bull Brown, W. H., J. W. Fuquay, W. H. McGee and S. S. Iyengar Evaporative cooling for Mississippi diary cows. Trans. Amer. Soc. Agr. Eng. 17:513. Colditz, P. J. and R. C. Kellaway The effect of diet and heat stress on feed intake, growth, and nitrogen metabolism in Friesian, F~ Brahman Friesian, and Brahman heifers. Australian J. Agr. Res. 23:717. Entwistle, K. W The influence of high ambient temperature and plane of nutrition on wool growth rates of tropical sheep. Australian J. Exp. Agr. Anita. Husb. 15:753. Folman, Y., A. Berman, Z. Herz, M. Kaim, M. Rosenberg, M. Mamen and S. Gordin Milk yield and fertility of high-yielding dairy cows in a subtropical climate during summer and winter. J. Dairy Res. 46:411. Garrett, W. N., T. E. Bond and C. F. Kelly Effect of air velocity on gains and physiological adjustments of Hereford steers in a high temperature environment. J. Anita. Sci. 19:60. Goetsch, W. D., W. H. Peterson and A. J. Muehling Field study of earth tempered swine ventilation systems. Amer. Soc. Agr. Eng., paper No St. Joseph, MI. Gomila, L. F., J. D. Roussel and J. F. Beatty Effect of zone cooling on milk yield, thyroid activity and stress indicators. J. Dairy Sci. 60:129. Hahn, G. L Housing and management to reduce climatic impacts on livestock. J. Anita. Sci. 52:175. Hahn, L., N. F. Meador, G. B. Thompson and M. D. Shanklin Compensatory growth of beef cattle in hot weather and its role in management decisions. Proc. Int. Livestock Environ. Syrnp., Amer. Soc, Agr. Eng., St. Joseph, MI. p 288. Hales, J.R.S. and G. D. Brown Net energetic and thermoregularity efficiency during panting in the sheep. Comp. Bioehem. Physiol. 49A:413. Hales, J.R.S. and J. D. Findlay The oxygen cost of thermally-induced and COl-induced hyperdilation in the ox. Resp. Physiol. 4:353. Holmes, C. W., C. T. King and P.E.L. Sauwa Effects of exposure to a hot environment of Friesian and Brahman X Friesian cattle, wfth some measurements of the effects of exposure to radiant heat. Anita. Prod. 30:1. Hopkins, P. S., G. I. Knights and A. Leuvre Studies of the environmental physiology of tropical Merinos. Australian J. Agr. Res. 29:161. Houghton, F. D., W. W. Teague and W. E. Miller Effective temperature for persons lightly clothed and working in still air. Trans. Amer. Soc. Heat. Vent. Eng. 32:315. Ingraham, R. H., R. W. Stanley and W. C. Wagner Seasonal effects of tropical climate on shaded and nonshaded cows as measured by rectal temperature, adrenal ~ortex hormones, thyroid hormone and milk production. Amer. J. Vet. Res. 40:1792. Ittner, N. R., T. E. Bond and C. F. Kelly Increasing summer gains of livestock. J. Anim. Sci. 13:866. Jennings, B. H. and B. Givoni Environment reactions in the 80 to 105~ zone. Trans. Amer. Soc. Heat. Ref. Air Cond. Eng. 65:115. Johnson, H. D., L. Hahn, H. H. Kibler, M. K. Shanklin and J. E. Edmondson Heat and acclimation influences on lactation of Holstein-cattle. Missouri Agr. Exp. Sta. Bull Johnson, H. D., A. C. Ragsdale, I. L. Berry and M. D. Shanktin Effect of various temperaturehumidity combinations on milk production of Holstein cattle. Missouri Agr. Exp. Res. Sta. Bull. 791, Kamal, T. H., H. D. Johnson and A. C. Ragsdale Metabolic reactions during thermal stress (35 ~ to 95~ in dairy animals acclimated at 50 ~ and 80~ Missouri Agr. Exp. Sta. Bull Kibler, H. H. and S. Brody Influence of tem- perature, 5 ~ to 95~ on evaporative cooling from the respiratory and exterior body surfaces in Jersey and Holstein cows. Missouri Agr. Exp. Sta. Res. Bull Lofgreen, G. P Ration formulation for relief from heat stress. 13th California Feeder's Day, Univ. of California, Davis. p 81. Lofgreen, G. P., R. L. Givens and S. R. Morrison Effect of drinking water temperature on beef cattle performance. J. Anita. Sci. 40:223. McCormick, W. C., R. L. Givens and B. L. Southwell.

7 1600 MORRISON Effect of shade on rate of growth and fattening of beef steers. Georgia Agr. Exp. Sta. Tech. Bull. N. S. 27. McDowelL R. E., E. G. Moody, P. J. Van Soest, R. P. Lehman and G. L, Ford Effect of heat stress on energy and water utilization. J. Dairy Sci. 52:188. Moose, M. G. and C. V. Ross Effects of shades and concentrate levels on lambs. J. Anita. Sci. 23:1201 (Abstr.). Morrison, S. R Physical principles of energy exchange. J. Anim. Sci. 35:624. Morrison, S. R., T. E. Bond and H. Heitman, Jr Skin and lung moisture loss from swine. Trans. Amer. Sor Agr. Eng. 10:691. Morrison, S. R., R. L. Givens and G. P. Lofgreen Sprinkling cattle for relief from heat stress. J. Anita. Sci. 36:428. Morrison, S. R., H. Heitman, Jr. and T. E. Bond Effect of humidity on swine at temperatures above optimum. Int. J. Biometeor. 13:135. Morrison, S. R. and G. P. Lofgreen Beef cattle response to air temperature. Trans. Amer. Soc. Agr. Eng. 22:861. Morrison, S. R., G. P. Lofgreen and R. L. Givens Sprinkling cattle for heat stress relief: Breed differences and sprinkling interval. Proc. Int. Livestock Environ. Syrup., Amer. Soc. Agr, Eng., St. Joseph, MI. p 310. Morrison, S. R., M. Prokop and G. P. Lofgreen Sprinkling cattle for heat stress relief: Activation temperature, duration of sprinkling and pen area sprinkled. Trans. Amer. Soc. Agr. Eng. 24:1299. Pontif, J. E., W. A. Nipper, A. F. Loyacano and H. J. Braud Effects of windbreaks and roofs in winter and shades and fans in summer on feedlot performance of cattle in the South. Proc. Int. Livestock Environ. Syrup., Amer. Soc. Agr. Eng., St. Joseph, MI. p 305. Ragsdale, A. C., C. S. Cheng and H. D. Johnson Effects of constant environmental temperatures of 50~ and 80~ on the growth responses of Brahman, Santa Gertrudis, and Shorthorn calves. Missouri Agr. Exp. Sta. Res. Bull Ragsdale, A. C., H. J. Thompson, D. M. Worstell and S. Brody Influence of increasing of temperature, 40 ~ to 105~ on milk production in Brown Swiss cows and on feed and water consumption and body weight in Brown Swiss and Brahman cows and heifer~ Missouri Agr. Exp. Sta. Res. Bull Ragsdale, A. C., H. J. Thompson, D. M. Worstell and S. Brody. 1953, The effect of humidity on milk production and composition, feed and water consumption and body weight in cattle. Missouri Agr. Exp. Sta. Res. Bull Ragsdale, A. C., D. M. Worstell, H. J. Thompson and S. Brody Influence of temperature, 50 ~ to O~ and 50 ~ and 95~ on milk production, feed and water consumption and body weight in Jersey and Holstein cows. Missouri Agr. Exp. Sta. Res, Bull. 449; Roller, W. L. and R. F. Goldman Responses of domestic swine to acute heat exposure. Trans. Amer. Soc. Agr. Eng. 12:164. Roman-Ponce, H., W. W. Thatcher, D. E. Buffington, C. J. Wilcox and H. H. Van Horn Physiological and production responses of dairy cattle to a shade structure in a subtropical environment. J. Dairy Sci. 60:424. Shanklin, M. D. and R. E. Stewart Relief of thermally-induced stress in dairy cattle by radiation cooling. Missouri Agr. Exp. Sta. Bull Singh, M., T. More and A. K. Fau Heat tolerance of different genetic groups of sheep exposed to elevated temperature conditions. J. Agr. Sci. (Camb.) 94:63. Stanley, R. W., S. E. Olbrich, F. A. Martz, H. D. Johnson and E. S. Hilderbranc Effect of roughage level and ambient temperature on milk production, milk composition and ruminal volatile fatty acids. Trop. Agr. 52:213. Stott, G. H. and F. Wiersma Response of dairy cattle to an evaporative cooled environment. Proc. Int. Livestock Environ. Sym., Amer. Soc. Agr. Eng., St. Joseph, MI. p 88. Warren, W. P., F. A. Martz, K. H. Asay, E. S. Hilderbrand, C. G. Payne and J. R. Vogt Digestibility and rate of passage by steers fed tall rescue, alfalfa and orchardgrass in 18 and 32 C ambient temperature. J. Anirn. Sci. 39:93. Wiersma, F., D. Ray and C. Roubicek Modified environment for beef in hot climates. Trans. Amer. Soc. Agr. Eng. 16:348. Wiersma, F. and G. H. Stott Microclimate modification for hot weather stress relief in dairy cattle. Trans. Amer. Soc. Agr. Eng. 9:309.