Overview Of Livestock Heat Stress Biology Essay


Livestock (mainly bovine cattle) maintain their normal rectal temperature between 37.5- 38.5 0C(100-101 °F), when they are within their comfort/thermoneutral zone (between -5 and 200C, or more precisely -4 to 18.50C ); in theory .But the every day environmental conditions prevent the animals from maintaining their rectal temperature at an optimal constant value; animals attempt that by homeostatic thermoregulatory mechanisms, which is successful most of the time. But the efforts that animals exert to maintain perfect rectal temperature aren't always satisfactory for profitable businesses. In warm climatic conditions, prolonged exposure to excessive heat or heat and humidity, the physiological heat-regulating mechanisms ultimately become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb exponentially. The generated heat reaches levels beyond a healthy animal's ability to cool itself, because the heat and humidity of the environment reduce the efficiency of the body's normal cooling mechanisms(When more heat accumulates than the cow can dissipate, heat stress occurs); thus forcing the animal to sacrifice some productivity in order to maintain thermal homeostasis. That's exactly when humans intervene to better manage the animal surrounding environment to motivate the animals to keep up their maximum productivity potential. When it comes to judging heat stress for livestock, the biggest miscalculation that many farmers make is that they go by the outside ambient temperature instead of the combination of temperature and humidity, explains Maurice Eastridge, an Ohio State University extension dairy specialist. Cattle can exhibit mild heat stress (a combination of temperature and humidity known as the THI index) with temperatures as low as 75 degrees Fahrenheit and a relative humidity of 65 percent. An animal's response to heat stress is to eat less. For each pound of dry matter not consumed, 2 pounds of milk can be lost. Heat stress also can increase a cow's susceptibility to diseases, specifically mastitis and digestive disorders, and create fertility problems as it is difficult to get heat-stressed cows pregnant, or keep them pregnant. In this paper we are going to concentrate on the practices/strategies that humans usually use to control heat stress that livestock animals are subjected to under hot ambient conditions. In order to minimize the effects of heat stress, those in the cattle industry usually use three management strategies. The first is improved nutritional (feed) management practices. The second is physical modification of the environment. The third is the genetic development of heat-tolerant breeds.

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Heat stress is no laughing matter; its frequent occurrence and severity usually cause mild to dangerously high economic losses in profitable animal based businesses. Different environmental factors cause an animal's body temperature starts to rise above normal , consequently leading to Heat stress .The combined effects of : air temperature, humidity, air flow around the animal, radiation from hot surfaces, direct and diffused sunlight, and ground temperature influence the ability of the animal to control its internal body temperature. In addition, the animal's breed, age, health, lactation status, hair coat color, and diet furthermore affect its ability to maintain its internal body temperature.  

Heat stress has been repeatedly observed in cattle ranging from mild to major economic losses. The economic costs of heat stress are very difficult to measure in a production environment, because there are so many different factors involved. But one of the best studies that have been conducted in a manner that has isolated a single factor was reviewed in the Journal of Dairy Science in 2003 ("Effects of Heat-Stress on Production of Dairy Cattle", by J.W. West, vol. 86, pgs 2131-2144). This article reported on Holstein, Brown Swiss, and Jersey heifers raised for 13 months at a constant temperature of 50degrees F (100C) or 80degrees F (26.670C) in environmentally controlled chambers. The weight difference was 18 pounds (8.18kilograms) at 3 months and 67 pounds (30.45kilograms) at 11 months. Heifers raised at the higher temperature took 1.5 months longer to reach 700 pounds (318.18 kilograms). This was an important result because it reveals that heifers don't need to be subjected to severe heat stress to suffer decreased growth in warm climates. It also confirms that heat stress can occur without changes in temperature, meaning that farm heifers may be heat stressed even in the presence of very little temperature change for several days or weeks. In addition, in July 1995, 3,750 cattle in one feedlot in Iowa died in 24 hours, due to heat stress. Also in the summer of 2006, more than 30,000 dairy cows died in California after several days of 100 degrees F (37.780C). Estimated loss of milk production in U.S. due to heat stress in 2003 was calculated to be $900 million. In other literature reviews, its was observed that 70 % of cows who had body temperature over 103 degrees F (39.40C) experienced a 10% reduction in Dry matter intake (DMI), also any healthy cow with body temperatures over 104 degrees F (400C) went into a coma, finally death usually occurs when the cow's rectal temperature reaches temperatures ranging between 106 degrees F to 108 degrees F (41-420C).

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The first heat stress management strategy involves improves nutritional practices, these include modification in the amount of: water, feed nutrient density (fiber, fat, protein, minerals, feed additives), in addition to the time and frequency of feeding.

Water Intake and Its Quality

Water is probably the most important nutrient for animals, and the most under estimated in importance to cattle productivity, as well as all living creatures. A loss of only 20% of body water is fatal in cattle bodies. Cow bodies normally contain 55 - 65% of water (by weight). Lactating dairy cows require more water than other livestock, since 87% of milk is water. Drinking water is the main source of water, and provides 80 - 90% of dairy cows' total water needs. Water consumption is inconsistent, and contingent on many factors, such as: ambient temperature, DMI, milk yield (MY), sodium intake (NaI), physiological stage, and other factors. Murphy et al. (1983) proposed an equation to predict water consumption:

Water intake, lbs/day = 35.25 + 1.58 * DMI (lbs/day) + 0.90 * MY (lbs/day) + 0.11 * NaI (grams/day) + 2.65 * average minimum temperature (oF/1.8 _ 17.778).

It is observed that cattle water intake would significantly increase by 120 - 200% when ever they encounter heat stress. This increased water intake assists in dissipating heat through the lungs (respiration) and by sweating. On average, cows drink about 3 kg of water / kg DMI when the temperature is below 5oC, however they drink 7 kg water / kg DMI at high temperatures. High-producing dairy cows are capable of consuming 190 liters of water each day (Beede 1992).

Cows follow an interesting drinking behavior. They spend about six hours a day eating, while allocate only 5 to 10 minutes for drinking. They mainly drink after being milked and when fresh feed is offered. To fit this special drinking pattern, in high end dairy farms, water systems are usually designed to deliver water to each station at the proper rate and keep up with peak demand.

In arid areas, it is recommended that for groups consisting of 200 cows or less, water stations should accommodate 15% of the herd at the same time, allowing 60 cm of accessible perimeter per cow (McFarland 2000). Since it is challenging to define exactly how much water is adequate under different conditions, it is crucial to supply abundant, clean and easily accessible drinking water to cows all the time.

Water quality may affect the water consumption and performance of cows. Total dissolved solids (TDS) refer to the level of salts dissolved in water. These include sodium chloride, sulfate, potassium, calcium, magnesium, etc. Of these salts, sulfate and chloride are more likely to have a negative effect than sodium. During warm weather, Challis et al. (1987) found that desalinated (reverse osmosis) drinking water increased milk yield by 28% (35 vs. 27.3 kg/day). It also increased water intake by 20% and grain intake by 32%, compared to highly saline water.

Nutrient Density and Adjustment

The DMI (Dry matter intake) generally plummets during hot weather. This means that cows are possibly eating inadequate amounts of nutrients (energy, protein, ADF, NDF, and effective NDF). Feed digestion and metabolism usually creates heat, and this heat production should be reduced as much as possible. Heat increment (HI) is defined as energy expenditure associated with the digestion and assimilation of food. Each kind of feed has its own HI value. A diet with a higher nutrient density and low HI (higher energy conversion efficiency) for lactating cows under heat stress is desirable. Conversion efficiencies of intermediate products, such as acetate and glucose, to end products, such as fatty acids, are 68 - 72% and 82 - 85%, respectively. Partial efficiencies for the conversion of acetate and dietary fat to milk fat are in the ranges of 70 - 75% and 94 - 97%, respectively (Baldwin et al. 1985).


Low-fiber rations should be fed during hot weather since there acetate metabolism is associated with greater heat production compared with propionate. Feeding more concentrates at the expense of fibrous ingredients boosts the energy density of rations, and should diminish total HI (West 1999). Although increasing the level of grain feed is widely practiced in summer, any drop in fiber levels should be approached cautiously. Corn and other concentrates are sometimes called "hot" feeds. This is in reference to their higher energy content compared to hay or straw (cool feeds). However, corn and other concentrates contribute less to the heat of fermentation or digestion than hay. Therefore, cattle actually produce less heat when consuming corn than when consuming hay. Experiments have indicated that the highest percentage of grain diets (61.8, 51.8 and 31.8% in diet DM for early, mid and late lactation) gave lower milk yield, depressed milk fat, inferior DMI and feed efficiency for multiparous cows (Tessmann et al. 1991). Giving cows more grain in their feed leads to a lower rumen pH, especially in hot summer (Mishra et al. 1970), and sorting the feed to get rid of fiber could make the rumen pH even lower. The maximum benefit from grain appears to be when it is approximately 60 - 65% of the diet (Coppock 1985).

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Feeding very high-quality forage to lactating cows in hot summer is advocated, because it reduces heat build-up and supplies necessary long fibers. Another option is high-fiber, easily fermented feed by-products. Soybean hull, brewers' grain and beet pulp pellets are all swiftly digested in the rumen. One and a half kilograms per day per cow of beet pulp pellets were a good substitute for corn and Pangola grass. They maintained DMI, the same milk yield, milk fat and chewing activity, but cost less, thus giving a higher net income of US$ 0.93 per day per cow (Lee et al. 1999). The level of effective dietary fiber should be sufficient to avoid rumen acidosis and metabolic difficulties. The ADF level should be maintained at a minimum of 18 - 19%, or otherwise the NDF should be at least 25 - 28% of diet DM.


It's a common practice to add fat into lactating dairy cow diets. During hot weather, it may be particularly beneficial to offer cattle high-fat diets due to their higher energy density and high energy conversion efficiency. However, research on the effects of dietary fat during hot weather gives inconsistent results (Huber et al. 1994). It has been reported that cows fed a diet supplemented with fat could improve their fat-corrected milk yield by 22% compared with the control group (33.2 kg vs. 40.5 kg) during warm weather but not during cool weather (Skaar et al. 1989). However in one experiment, diets containing 5% added fat fed to cows in either thermoneutral or hot environment conditions didn't improve milk yield (Knapp and Grummer 1991). In a recent study, diets supplemented with 3% prilled fatty acids were offered to cows, kept either in the shade or the shade plus evaporative cooling. Again, milk yield was improved only by the cooling not the fat treatment, supplementation (Chan et al. 1997).

Although the results seem conflicting, biological principles argue in favor of fat supplementation under conditions of heat stress. Extension nutritionists still suggest fat supplements to give a final fat content of 6 - 7 % of diet DM, especially for high-producing cows. Sources of fat supplements include whole oilseed, tallow and protected fat products.


Heat stress usually reduces cattle feed intake which often results in cattle have a negative nitrogen (N) balance. Both the quantity and quality of protein in the diet need to be considered when feed is being provided for heat-stressed cows. Simply increasing the level of crude protein (CP) may increase energy requirements and cause problems of environmental pollution. Excess dietary protein is converted into urea and excreted. It is estimated that for each gram of urea synthesis, 7.3 kcal of energy is expended.

Researchers in Arizona USA conducted 2 x 2 dietary protein experiments with high (18.5%) or low (16.1%) CP and high (65% of CP) or low protein degradability (59% of CP) in both hot and moderate environments. Results showed that cows under heat stress fed a diet with high CP and high degradability (18.5% and 65%) showed a decrease in milk yield of 11% (3.1 kg/day) compared with the other three groups. In hot environments, a high CP (18.5%) diet caused DMI to fall by 1.5 kg per day, compared to the 16.1% group (Higginbotham et al. 1989b). Cows in a temperate environment did not response to dietary protein treatments. Milk yields were similar for all treatments (Higginbotham et al. 1989a). Based on these experimental results, it was recommended that during heat stress, the level of crude protein (CP) in the diet should not exceed 18%, while the level of rumen-degradable protein should not exceed 61% of CP or 100 grams of N/day (Huber et al. 1994).

Protein quality was also studied. High-quality protein sources (soybean, fish meal, and blood meal) with a 1% lysine content, were compared with a low-quality protein source (corn gluten meal), containing 0.6% lysine. Cows fed the 1.0% lysine diet increased their milk yield by 3.2 kg/day (Huber et al. 1994).

Studies in Taiwan of dietary protein level and quality gave results consistent with those from Arizona. In the hot summer months, cows fed diets with a high CP level (16.5% vs. 15.0%) and high degradability (63% vs. 58% of CP) reduced their milk yield by 11.3% (2.2 kg/day). Cows fed diets with a low level of degradable protein had a higher percentage of milk fat and milk lactose, and a lower level of urea nitrogen in their blood. Fish meal and blood meal were fed as supplements to study protein quality. Cows fed blood meal increased their milk yield 1.6 kg and 1.0 kg/day, compared to the control. It may be that the first limiting amino acid in a diet of Pangola grass is lysine rather than methionine (Lee et al. 1998).


Electrolyte minerals, sodium and potassium are important in the maintenance of water balance, ion balance and the acid-base status of heat-stressed cows. Some scientists recommend increasing the level of potassium to 1.3-1.5 percent of total ration dry matter; sodium to 0.35-0.40 percent; magnesium to 0.35 percent. But more importantly, the mineral requirements recommended by the National Research Council (NRC), United States, in 1989 do not seem high enough for cows suffering from heat stress. When heat-stressed cows sweat, they lose a considerable amount of K. Increasing the concentration of dietary K to 1.2% or more result in a 3 - 9% increase in milk yield, and also an increased DMI. Increasing the concentration of sodium in the diet from the NRC recommended level of 0.18% to 0.45% or more improved milk yield by 7 - 18% (Sanchez et al. 1994). If magnesium oxide (MgO) was added, thus increasing the Mg concentration from 0.25% to 0.44%, the milk yield of heat-stressed cows increased by 9.8% (Teh et al. 1985).

The dietary cation-anion balance (DCAB, Na + K _ Cl or Na + K _ Cl _ S) contributes to maintain the acid-base status of cows in hot weather. For heat-stressed cows, alkaline diets are preferable. DMI increased linearly with increasing cation content from 120 to 464 mEq/kg DM, while Na or K was equally effective as a cation source (West et al. 1992). In hot weather, the level of milk fat is usually lower. Buffers such as sodium bicarbonate (NaHCO3) and magnesium oxide (MgO) are commonly supplemented. Adding sodium bicarbonate to the diet may help to maintain the pH of the rumen, and also contribute to a more cationic DCAB value to increase DMI and milk yield.

A diet with high chloride content, with a DCAB value of -144 mEq/kg DM, depressed DMI and was associated with low blood pH and reduced blood buffering (Escobosa et al. 1984). Diets high in chloride also resulted in a lower milk protein percentage (Sanchez et al. 1997). [Thus, it is recommended that the level of dietary chloride does not exceed 0.35% of DM (Sanchez et al. 1994)]. A mixture of potassium sulfate (K2SO4) and magnesium oxide (MgO) were blended to increase dietary concentrations of K, S, and Mg from 1.02, 0.23 and 0.21% to 1.46, 0.41, and 0.45%, respectively. There was no improvement in milk yield or DMI of the cows under heat stress which received this supplement. In fact, the percentage of total solids in the milk fell, and the milk fat percentage was also lower (Lee et al. 2000). The reasons for this adverse effect are not yet clear. The DCAB values for both diets were similar, 288 and 281 mEq/kg DM. A simultaneous increase in the anionic S concentration may partially explain this result.

Feed Additives

As well as the nutrients fed in large quantities, some minor nutrients were studied in terms of their ability to relieve cows suffering from heat stress. Generally speaking, the results were inconsistent. An improvement in performance was more likely to occur in high-producing cows fed a high-energy diet.

Niacin can prevent ketosis, and is involved with lipid metabolism. Cattle fed with 6 g niacin per day in summer increased their milk yield by only 0.9 kg/day. However, with cows yielding more than 34 kg a day, there is a clear improvement in milk yield (2.4 kg/day) (Muller et al. 1986). Under moderate to severe heat stress conditions, niacin at a rate of 12 - 36 grams a day did not improve milk yield, but lowered skin temperature by about 0.3oC (Di Costanzo et al. 1997). Similarly, there was no response in milk yield from cows receiving 10 - 20 grams of niacin per day. A group given 10 grams of niacin had a slower respiration rate in the afternoon. Both groups given niacin supplements had a higher molar percentage of the butyrate in the rumen (Lee et al. 2001).

Most research with lactating cows concerned with microbial or "probiotic" products deals with either Aspergillus oryzae (a mold classified as a fungus) or Saccharomyces cervisiae (a yeast). The effect of the mold A. oryzae on cows under heat stress was evaluated. Results indicated that 3 grams of A. oryzae addition to the feed had a slight effect on rectal temperature, respiration rate, or milk composition, but gave a 4% increase in milk yield (1 kg/day) (Huber et al. 1994). Both A. oryzae and S. cervisiae may influence the fermentation pattern and microbial population in the rumen (Yoon and Stern 1996).

Nutritional strategies used during heat stress

During the hot seasons, we observe a real change in animal feeding behavior. Animals would consume more feed during cooler evening hours (West 1999). The quantity of feed and the feeding schedule must be fine-tuned to accommodate this behavior. In order to encourage DMI, fresh feed is offered to lactating cows after milking. When the weather is very hot, at least 70% of the daily feed should be given fresh at night. This will move more of the heat of fermentation to the cooler, nighttime temperatures. During the early morning hours, the air temperature and the temperature of cow's body reach their daily lows. This coincides with the morning milking time on many farms. If cows are cooled with fans and sprinklers during the milking process, cow temperatures will fall even further. To take full advantage of this, it is critical that fresh feed be present for all cows when they leave the milking parlor. This is the best opportunity during the day to increase the cow's feed intake.

Steers ate more frequent but smaller meals in a hot environment, with the result that less feed was consumed overall (7.39 vs. 8.12 kg/day), than in cool conditions (Hahn 1999). More frequent feeding could keep feed fresher, and encourage cows to eat more frequently, thus stimulating DMI. Theoretically, more frequent feeding might decrease the diurnal fluctuations in metabolites and increase feed utilization efficiency in the rumen (Robinson 1989). A fixed amount of high grain offered 12 times a day did not increase milk yield, but raised the milk fat percentage from 2.21 to 2.60%. It also increased the mean rumen pH value from 5.7 to 6.2, and shortened the period during which the rumen pH was less than 6.0 from 14 hrs a day to 4 hrs, compared to cows fed twice a day (French and Kennelly 1990).

Frequent feeding is believed by dairy farmers to be crucial in achieving and maintaining high productivity. This practice might even more important during hot weather, because feed is fermented faster after preparation when air temperatures are high. However, research studies conclude that the effect of more frequent feeding on milk yield is moderate, and of little economic importance to producers (Gibson 1984).

It's said that feed bunk management actually ahs an impact on cattle dry matter intake.

First ,feed intake by cattle can be stimulated by simply voiding secondary fermentation in the feed bunk (indicated by higher feed temperature and undesirable odors).Second, higher levels of organic acid and lower feed pH can actually extend feed bunk life fermented forage (such as corn silage)present in the feed. Third, by simply adding water (2 to 5 liters), farmers can increase total dry matter intake, reduce sorting, and avoid feed fines and /or dust that can reduce palatability; furthermore adding water may increase the undesirable feed secondary fermentation risk, so caution id advised. Fourth, both feed bunk life and feed palatability can be extended by applying a propionic acid based feed stabilizer to the TMR at mixing time. Fifth, dry matter intake might be improved by

removing stale and/or hot feed (weigh back), which is critical before adding fresh feed; offering ration dry matter during the night period (cooler time), multiple feedings, and fresh feed after each milking .Sixth, While avoid getting the feed wet, farmers should

soak the cows and place fans over the feed bunk as an effective ways to improve cow comfort. Seventh, in order to encourage cows to remain at the feed bunk longer periods of time, rubber mats or belting should be placed next to the feed bunk manger under the shade.

As already mentioned before, water is essential to control heat stress. Cool water adlibiltum is desirable (not cold), this is especially important when cattle are experiencing heat stress, in that situation, cattle water requirements increase by 1.2 to 2 times normal levels. Second, the water troughs should be place under shaded areas and ten centimeters (four inches) of space per cow (or 1 linear foot for every 20 cows) should be provided. Finally, water should be offered as cows exit the milking parlor (as in lactating dairy cows).

During heat stress, cattle may face mineral loss due to increased water consumption that increases excretion of urine. This loss happens in certain minerals, such as sodium (a part of salt), potassium and magnesium. Free-choice trace mineral salt should be provided in a location that the animals will consume it. Loose salt will be more readily consumed than block salt.

 Cattle producers using management-intensive grazing might consider several grazing options. The first option is to rotate through fields at a more rapid rate. Taller grass tends to be a cooler surface to maintain cattle on versus pastures with shorter grass stands. A second option is to rotate cattle in the evening rather than the morning. The assumption is that the grass will be consumed in the evening and the "heat of fermentation" or digestion is mostly dissipated by mid-morning, thereby reducing the heat load produced by the animal. A third possible option is to graze paddocks that allow access to temporary shade or trees during the heat of the day. This will reduce equal distribution of manure throughout the paddock but might be a suitable compromise during excessively hot weather. Limit feeding is a fourth viable option. Similarly, high quality forage produces less heat of fermentation than low quality forage. This might be another argument for moving cattle to higher quality pasture or moving them more frequently through the paddocks.

Furthermore, cattle should be handled early in the morning under heat stress conditions. Bulls are particularly vulnerable to handling during the heat of the day. This is mainly due to the fact that they have less surface area per pound of body weight exposed to dissipate heat. Cattle should be handled quietly because once they get excited it will take 20 to 30 minutes for their heart rates to return to normal. When hauling cattle, farmers must load early in the morning and shouldn't stop during the heat of the day.

Physical modification of the cattle's surrounding environment

The holding pen is often an area of elevated HS conditions. Cows are crowded into a confined area for several minutes to hours. Cows must not spend more than 60 to 90 minutes in the holding area. In addition, provide shade, fans, and sprinklers in the holding pen. An Arizona study showed a 3.5°F drop in body temperature and a 1.76 lb increase in milk/cow/day when cows were cooled in the holding pen with fans and sprinklers (Wiersma and Armstrong, 1983). Cattle handling such as sorting, adding cattle to the herd, vet checks, and lock-up times should be completed in the early morning. The cow's warmest body temperature occurs between 6 p.m. and midnight. Reducing lock-up times can also reduce HS, especially in facilities with little or no cooling above head locks.

Providing enough shade is vital for proper cow comfort due to its cooling effect. Shade height should be 8-14 feet tall and should be large enough to provide 20 to 50 square feet of shade per mature dairy cow to reduce solar radiation. A shade tree is a source of relief for cattle as it is to humans on a hot summer day. Trees serve as the best shades, but can be killed when cow density is high. If trees are to be used, they must be protected from cattle by fences Shade can also be constructed. The most effective shade is a solid reflective roof constructed of white colored, galvanized, or aluminum materials. Shading with wooden slats, plastic fencing, or other materials that allow flecks of sunlight to hit the animals are less effective. Improved efficiency of metal roofs has been noted by painting tops white, underside black, by placing about 2.5 cm of insulation directly beneath the underside, and by sprinkling the roof with water. Polypropolene fabric for shade cloth has become popular, especially the fabric providing 80% shade. It is recommended that cattle in arid climates have 3.5 to 4.5 m2 per lactating cow (38-45 sq ft). Shades should be at least 4 m (12 ft), preferably 14 to 16 ft, high to minimize radiation reflection from the shade roof back to the cow and maximize air velocity flowing over the roof, thereby reducing roof temperature. Ridge vents of barns for warm climates should be at least 1 ft wide with an additional 2 inches for each 10 ft of structure width over 20 ft 2. A good rule of thumb for ridge vent width is 12 per 20, then 2 per 10 (in inches and feet, respectively) 7. Roof slopes should be at least 4:12 (33%). Temporary shade structures should be oriented with the long axis north and south in order for the sun to assist in drying the ground under the shade during morning and afternoon. Permanent shade structures and those in hot, arid areas should be oriented in an east-west manner as to keep the area under the shade cooler. Temporary shades cost approximately $1.75 to $2.25 per square foot, about $25 to $50 per cow, and last approximately 5 years. Permanent shades cost about $150 to $300 per cow, and last approximately 3 years, making the benefit to cost ratio about 2.2:1. Therefore shaded cattle need to produce about 700 pounds of additional milk for shades to be cost effective. If possible two shaded areas are recommended, one over the feed area to increase feeding time, and another away from the feed area to encourage the cattle to rest. Water should be made available under both shaded areas, to increase the water consumption during heat stress period. If the structure is left up year-round, construct a frame adequate for snow load. Shade is insurance against mortality loss. Any performance benefits are a bonus.

Stable flies are pest who coincidentally make a huge appearance under hot conditions. They usually cause cattle to bunch and disrupt cooling. As result, farmers have to monitor the situation and control the flies as needed (either by physical or chemical means). Another approach to the problems is eliminating any shallow pools or muddy areas nearby, since they are common breeding areas for flies.

In order to diminish the effects of heat stress, cattle housing facilities should be designed via economically justified inputs. Areas that prove beneficial for cooling systems are feed lines, free stalls, holding pens, milking parlors, and exit lanes. Feed line cooling costs approximately $4 per cow, but this cost is justified by the increased DMI and milk production. Cattle feeding under cooling systems will spend more time eating in the feed bunk than cows lacking a cooling system. The longer duration of standing and eating also allows sufficient time for the teat sphincter to close and thus may reduce the opportunity for pathogenic microbes to more easily enter the teat. Strategic cooling barns, such as for fresh cows, could be thought of as analogous to maintaining a sick barn for unhealthy cattle.

Fans are considered a practical method for increasing cooling, especially at night, by increasing heat loss at the animal surface through evaporative and convective means. Fans, that increase heat loss by convection, do not cool the air but work best if the ambient temperature is less than the cow's body temperature. Fans reduce dependence upon panting; allowing cows to more easily feed or ruminate, consequently increasing DMI. Sprinklers should wet the hair coat of the cow, not just form a fine layer of mist over it. Sprinkling, as opposed to misting, allows direct evaporative cooling. Whereas misting relies upon convection cooling with evaporatively cooled air. Misting is also thought to be less effective in humid climates than arid climates because it forms an insulating barrier between the body surface and the ambient air, thereby decreasing effective heat exchange.

A popular example of a fan cooling system is tunnel ventilation. This system is more widely adopted in the eastern US than in parts west at the present time, tunnel ventilation helps keep cows cool by providing a fan-driven breeze at rates as high as 600 ft/min (6.8 mph). Fan banks in one endwall operate to draw fresh air in large air inlets located on the opposing endwall; thus large volumes of high-velocity air moves through the barn in a linear or "tunnel" fashion. Evaporative systems (sprinklers, foggers, or pads) may be incorporated into the design to provide even greater cooling benefit. Furthermore, an improvement to the usual ventilation cooling system is the TIV (Time-Integrated Variable Control). Considerable time is needed for cows to dissipate body heat accumulated over the course of a warm day--hours longer than the time it takes for the surrounding air temperature to drop off in the late afternoon and evening. Traditional ventilation controllers tend to reduce ventilation rates too quickly during this time of day, which can hinder cow heat loss. The new TIV (time-integrated variable) ventilation controller tracks air temperature during the day and when heat stress occurs, keeps ventilation rates elevated and cooling systems operating longer into the evening to help cows cool off.

Although evaporative cooling works best in areas of low humidity it is the most economical way of cooling cattle in the hot, humid southeast U.S. Evaporative cooling, also known as forced-air ventilation, works by using heat energy from the air to evaporate water, lowering the temperature of the air and raising relative humidity . Fog and mist systems spray water into the air cooling the air as the droplets evaporate. Evaporative cooling pad systems and fans are effective in hot, arid climates and lower air temperature by 8-12 F, but raise relative humidity. An effective evaporative cooling system is to run the misters 1-3 minutes every 1 minutes and the overhead fans the remaining time of each cycle. Another option is to run fans and misters during the day and just fans at night. In addition, water misters provide a spray of water that helps in cool down the animals. However it is important to place misters over a clean, concrete area. Also, running misters over dirt creates mud and increases the potential for mastitis or other bacterial diseases. Furthermore, a timer should be used to run the mister long enough to cool, but not wet the cattle. Farmers ought to prevent the mist from wetting nearby feed. Wet feed spoils rapidly, especially under hot climate.

Fog or mist systems are the most widely used evaporative cooling methods in the air southwest United States. Such systems cool the air, which in turn cools the cows-unlike sprinkler systems (generally used in more humid climates) that directly wet and cool the cows' hides. Also, according to a recent study done by Ohio State University study, it's suggested that, based on net return, a combination of fan and sprinkler set-up is the best cooling system for producers across most of the United States. In addition, several companies offer stir fans that oscillate in order to increase the effective cooling area. One of note uses a special controller to program the movement of a bank of stir fans to track along with the movement of shaded areas as their location changes with sun position over the course of a day. In this way, cows are always befitting from the combined cooling effect of the shade and the air movement .Additionally, its has been unanimously agreed that combining fog or mist systems with stirring fans boosts the evaporative process efficiency. Finally, the mist system has a potentially dangerous defect, when using high-pressure misting systems, a fine spray or mist is produced that increase humidity,which may do as much harm as good, as causing animals to inhale water droplets contaminated with dust from dried manure.

Genetic strategies to decrease heat stress

Particular genes that supposedly control traits related to thermotolerance are already being carefully selected by cattle geneticist, making it possible to produce novel cattle breeds that are thermally resistant to heat stress without inadvertently selecting against milk yield (Hansen and Arechiga, 1999). Traits that could possibly be selected for include coat color, genes controlling hair length, and genes controlling heat shock resistance in cells (see review by Hansen and Arechiga, 1999).This was evident in the superior thermoregulatory ability of zebu cattle compared to European breeds; this difference is due to reduced heat production, and increased capacity for loss of heat to the environment, or some combination of both. Clearly, a major contributing factor to thermotolerance trait in many zebu breeds is due to the low metabolic rates resulting from reduced growth rates and milk yields. There is also evidence that the basal metabolic rate of B. indicus is lower than for B. taurus. In addition, genetic modification or altering biochemical properties of the embryo before embryo transfer may be possible to improve thermal resistance and increase summer fertility.

Optimal cattle productivity require facilities that will avert excessive environmental heat stress, while at the same time assisting cattle to dissipate surplus body heat, in addition to feeding and management practices that help minimize internal heat production . The combined use of improved nutritional (feed) management practices, environmental physical modification, and the genetic development of heat-tolerant breeds will surely alleviate much of the HS properly designed cattle facilities. All the reasons to reduce HS mentioned earlier lead to increased cow comfort and productivity ultimately will most probably increase profits for cattle operations.