- Introduction to Heat Stress
- Measuring Heat Stress
- Reducing Heat Stress and Mastitis During Warm, Humid Months
Two of the greatest challenges for dairy operations in the Southeast are heat and humidity. When combined, heat and humidity contribute to the development of heat stress in dairy animals, which negatively impacts feed intake, immune function, and cow comfort, inevitably resulting in decreased milk production and animal well-being. More specifically, heat stress is associated with increased risk for mammary gland (intramammary) infections—commonly known as mastitis—and reduced milk quality.
A primary indicator of milk quality and mammary health is somatic cell count (SCC); a high SCC, in combination with the presence of pathogens in milk, impacts cow health and an operation’s economic sustainability. Estimates differ, but some reports claim that economic losses in the Southeast because of heat stress could be close to $700 per cow per year. Operators must implement management strategies that reduce the impact of heat stress, prevent mastitis, and ultimately improve milk production and quality in dairy animals.
Modern dairy breeds, such as the Holstein, are of northern European origin and are thermoneutral—in balance with the environmental temperature—within a temperature range of approximately 30 to 70 °F. Holsteins tend to be more sensitive to heat stress than Brown Swiss, Guernsey, and Jersey cattle, although these others are not resistant to heat stress. Additionally, older, heavier, high-producing cows are more susceptible to heat stress than smaller, younger animals. The incidence of mastitis in the United States is greatest during July, August, and September, accompanying an annual elevation in SCC (Figure 1). This is followed by a decrease in milk production during August, September, and October.
Heat stress exerts several adverse physiological and other harmful effects (see section below) on dairy cattle that contribute to a decline in milk yield and an increase in SCC. Early-lactation cows and high producers are most affected, but all ages and stages experience heat stress at some level. Dry-matter intake is depressed, and milk production may decrease by up to 50%. Above the thermoneutral zone, the cow’s immune system becomes compromised, leading to an increase in the incidence of intramammary infection (IMI). The overall losses to the U.S. livestock industry because of heat stress are approximately $2.4 billion per year, with greater than 50% of losses attributed to the dairy industry.
Heat Stress Causes Changes in Animal Physiology and Other Parameters
Heat stress in dairy cows causes decreases in:
- Dry matter intake
- Rate of feed passage
- Blood flow to organs
- Rumen buffering capacity
- Milk yield and quality
- Reproductive efficiency
- Body condition score
- Heifer growth
- Immune function
Heat stress in dairy cows causes increases in:
- Weight loss
- Clinical mastitis
- Respiration rates
- Rectal temperature
- Water intake
- Health care costs
Measuring Heat Stress
The cow’s thermoneutral zone ranges between 30 and 70 °F, in which she maintains both a normal body temperature and basal metabolic rate. Above this zone, she experiences heat stress and her body alters its nutrient and energy partitioning. This alteration is in an attempt to transfer energy into heat-dissipating activities such as increased respiration (panting), sweating, and increased blood flow to the periphery of her body for cooling. Since these cooling mechanisms are heavily impacted by heat (temperature) and humidity, the method of evaluating heat stress encompasses both factors.
The temperature-humidity index (THI) evaluates heat stress in a cow’s surrounding environment and accounts for the combined effects of temperature and relative humidity (Figure 2). For example, at a temperature of 72 °F and 30% relative humidity, the milk production of a lactating cow is not affected by the thermal environment. However, at 87 °F and 30% humidity, the THI value is 76, which represents mild to moderate stress on the cow.
Cows adjust to mild stress by seeking shade and increasing their respiration rates slightly. Mild stress slightly decreases milk production. Feed consumption decreases while a cow seeks ways to cool down, and her energy is repartitioned to adjust to these changes. During moderate stress, the body temperature increases along with respiration, salivation, and water consumption, while there are decreases in feed intake, milk yield, and reproductive efficiency.
If heat stress becomes severe, body temperature continues to rise along with excessive panting, salivation, and water consumption. The animal will be visibly uncomfortable. Feed intake and milk production are severely depressed and, if environmental conditions continue, convulsions may occur, followed by the death of the animal.
Physiological Changes as a Result of Heat Stress
As ambient temperature and relative humidity rise, the ability of the cow to dissipate heat decreases, resulting in excessive heat accumulating in the cow’s body and a subsequent increase in her body temperature. As a result, several varied physiological and behavioral mechanisms of heat loss take over to maintain thermoneutrality.
When heat-stressed, an animal will seek air currents (wind). Through the process of convection (hot air rising away from a source of heat), body heat is removed from the immediate surface of the cow by air currents. Cows also will lie in moist, cool earth (mud) to conduct heat from their bodies into the ground. Unfortunately, laying on cool ground may lead to environmental mastitis, which increases in incidence during the summer season, especially in animals housed outside or on pasture.
Additionally, the cow’s superficial blood vessels will dilate, which helps to dissipate heat from the surface of the cow’s body into the atmosphere by the process of radiation. This process is referred to as vasodilation. Finally, the cow will increase her respiration rate, expelling hot, moist air from her lungs, which causes heat loss via evaporation. Likewise, the sweat on her skin absorbs the heat from her body and evaporates, making her cooler.
One of the best practices to reduce heat stress is to provide adequate drinking water. Water must be readily available, fresh, clean, and cool to encourage consumption. Cows will drink 50% more water when the ambient temperature is 80 °F than when it’s 40 °F. Instead of consuming an average of 30 gallons per day, their intake may increase to 45 gallons or more. As evidenced by decreased respiration and rectal temperature, chilling cows’ drinking water to 50 °F alleviates heat stress and results in increased feed intake, rumen motility, and milk yield. To maximize water access in confinement barns, it is important to provide at least 2 in. of trough space per animal.
Other methods of cooling include shade, commercial coolers, tunnel ventilation, shower or fanning stations, fans, cooling ponds, and center pivots. Shade is the easiest and least expensive option to cool cows. For small operations, shade trees in the pasture work well. However, a high-cow density will kill trees in a matter of months because of the toxic effects of urine pH and excess nitrogen on root systems. Permanent shade structures in pastures work well but must be mounded periodically—otherwise, cows will dig holes under the structure and create wet, muddy areas that are conducive to environmental mastitis. Permanent dry-lot shades provide relief from the sun, but it is important to keep shade structures away from feed bunks because cows tend to defecate and urinate where they eat. Portable shades on skids are better because they can bring shade to the cows and can be relocated to other areas of pasture as mud and manure accumulate. Shade alone will reduce a cow’s respiration rate by 30%, and adding sprinklers will reduce the respiration rate by 67%. Both methods of cooling will also lower rectal temperatures.
The use of shade along with fans and sprinklers has an additive effect. The use of fans is important, especially in confined structures. Approximately 20% of a cow’s gross energy goes into producing body heat, which is released to the surrounding air, contributing to even greater heat stress in a confined space. Fans remove this body heat via convection, thereby cooling down the surface of the animal. Sprinklers are used to moisten the cow’s hair coat to the skin with water, allowing the loss of body heat via conduction (i.e., heat is conducted through the skin to the water). Fans and sprinklers together allow for conduction and evaporative cooling, as the fans help to vaporize the water that has been warmed by the release of body heat.
Figure 3 illustrates the additional cooling effect on cows by using sprinklers and fans. Under the conditions of this study, heat-stressed animals were respiring at approximately 100 breaths per minute. The addition of fans resulted in some relief by decreasing respiration to almost 90 breaths per minute within 95 min. However, marked relief was observed by the use of fans and sprinklers, which reduced respiration by 50% (50 breaths per minute) within 95 min.
In some situations, showers are also implemented in exit lanes to provide moisture as cows are heading back to pens or pastures. However, there is a balance that must be considered—too little water and the cow will not be appropriately capable of cooling, but too much and the water may run down to the udder, increasing the risk of mastitis.
Tunnel ventilation in barns and structures provides air movement and air exchange through a series of fans placed in one gable end wall of a closed barn. Fans create a negative pressure in the barn, causing air to be drawn into the opposite end-wall opening. Fresh, cool air flows longitudinally at a speed of 400–600 cubic feet per minute over the cows from the intake wall opening through the barn and is exhausted by the tunnel fans. The intake wall opening commonly contains a series of cooling cells, which remove heat from the incoming outside air by water evaporation. The temperature may be up to 8 °C cooler inside compared to outside the barn, which can lower cows’ body temperature up to 1.2 °C, improving cow comfort and possibly resulting in a 5–6 lb increase in milk production per day.
Commercial coolers combine air turbulence and high-pressure water injectors to lower the ambient temperature under shades. One study showed increased milk production (about 10%), an increase in the body weight of cooled cows (+49 lb) versus uncooled cows (-54 lb), and a lower culling rate among cooled cows. However, these systems are expensive to operate. Mechanical refrigeration with evaporatively cooled shades is another option, but they also are expensive and limited to areas with low relative humidity.
Cooling ponds have been used successfully in Florida to cool cows between milkings and just before entering the parlor. These cooling ponds are constructed either with a continuous flow or a circulating water supply system in which fresh water is provided. Importantly, these are not stagnant ponds. Prototheca zopfii, a colorless algae, often grows in stagnant ponds and results in mastitis that cannot be cured with antibiotic therapy. Cows should not have access to stagnant ponds. Cows with access to properly designed cooling ponds exhibit less lying down in mud and manure, and less clinical mastitis.
Before milking, cows can be cooled in the holding pen with fans and sprinklers. The holding pen tends to be an unfavorable environment for cows during the summer because of the combined stresses of crowding, body heat, and elevated ambient temperature. Cows must be allowed a drip-dry time of 10–15 min before entering the parlor. Otherwise, water contaminated with bacteria runs down flanks and udders during milking. From there it can enter the teat cups and get collected into the bulk tank, increasing milk bacteria count. In addition, milking-machine unit liner slips, vacuum fluctuations, and faulty pulsation cycles in the presence of excess water on teats all can contaminate the claw and lead to machine-induced infections from environmental pathogens.
After milking, cows can be cooled down one more time using a shower/fanning station in the milking parlor exit lane. Spraying should cover only the top and sides of the cow so that the postmilking germicidal teat disinfectant is not washed off. This way, the cows are temporarily relieved from the sun, and instead of returning immediately to the shade they are more inclined to eat and drink after milking. This keeps them on their feet and allows time for the teat ducts to close before contact with soil, manure, and bacteria that cause environmental mastitis.
In grazing operations typical of New Zealand-style dairying, center pivots and traveling irrigators are used to cool cows during the summer, but these require a close and reliable water source. The pivots both irrigate grass pastures to maintain plant growth and cool the milking herd, as shade is usually not provided to these animals. The grazing cows learn that if they stand under the pivot’s spray to graze or go off to graze elsewhere in the pasture after being cooled, the evaporative cooling effect helps to lower their body temperatures.
Dry cows typically exhibit far less dry-matter intake than lactating cows and can cope with heat stress more effectively. However, controlling heat stress when milk-producing tissues are developing can dramatically impact the cows’ transition into subsequent lactation. It’s important to manage heat stress during the entire dry period because cows cooled for only the final portion of the dry period may exhibit lower production than cows cooled for the entire dry period.
Most studies that investigated the benefits of alleviating heat stress have placed dry cows in free stalls that provided shade, fans, and sprinklers. Animals cooled with fans and sprinklers had lower rectal temperatures and respiration rates, longer dry periods, higher body condition scores, gave birth to calves with heavier body weights, and produced more milk (7–10 lb more) than those afforded only shaded free stalls. In addition, heat-stressed cows had compromised immune function, and their white blood cells exhibited a decreased capacity to kill disease-causing bacteria, such as those that cause mastitis. Reduced immune function also could negatively impact vaccine effectiveness.
Similar to dry cows and pastured lactating cows, dairy heifers also need relief from hot and humid environmental conditions. These young stock represent the future milking herd, so they must remain healthy as calves and heifers and follow expected body growth patterns, reproductive cycles, and mammary gland development to ensure maximum milk production in their first and subsequent lactations. Shaded areas in pastures and paddocks, and sprinklers and fans for housed animals, are necessary to counter heat stress and maximize animal comfort.
Hot and humid environmental conditions are ideal for the proliferation of biting insect pests such as the horn fly (Haematobia irritans). Horn flies irritate and cause stress to heifers by inserting their proboscis through the epidermis, mainly on the animals’ backs, and sucking blood from capillaries near the skin surface. However, these flies also will attack the hairless teat skin, causing lesions that become infected with Staphylococcus aureus. This places these mastitis-causing bacteria in an opportune position to enter the teat orifice, colonize the teat duct keratin, and subsequently cause IMI. Heifer mastitis caused by S. aureus causes chronic inflammation in the affected gland, preventing the milk-producing tissues from developing normally during the heifer’s first pregnancy, which reduces milk production when the heifer calves.
Unfortunately, even when relief from heat stress is provided (shade, fans, and sprinklers), a dense fly population will cause heifers to bunch together, which can disrupt cooling. However, stress caused by horn flies can be managed by reducing contact between flies and animals through the application of insect repellents, such as pour-ons, sprays, or ear tags, as well as by reducing fly populations through the incorporation of larvacides in feed additives. Such larvacides are consumed and pass through the animal’s body into the feces. Adult horn flies lay eggs in feces that hatch into larvae (maggots), which consume the larvacides and perish, thus reducing the adult fly population and their subsequent procreation. While fly control is necessary for all populations of dairy animals, research shows that our replacement heifers are our most vulnerable population, especially when considering their productive potential.
In most instances, the cooling of cows involves the use of water, which provides favorable growing conditions for environmental mastitis pathogens when combined with warm temperatures in the cows’ surroundings. These bacteria require only warm temperatures, nutrients, water, and a proper pH in order to thrive; hot and humid summer conditions are ideal for the growth of these organisms. Environmental bacteria, such as streptococci and coliforms like E. coli, can double their numbers every 20–30 min, greatly increasing the bacterial load on the udder and teats. Producers must improve herd-management practices, including cow hygiene, bedding management, and premilking udder-prep practices to maintain excellent milk quality during periods of environmental stress.
The environmental streps include Streptococcus uberis, S. dysgalactiae, S. parauberis, and S. equinus, while the coliforms include E. coli, K. pneumoniae, and Enterobacter, as well as Citrobacter and Serratia spp. Environmental bacteria counts in bedding materials are directly related to counts on teat ends, which can lead to IMI if bacterial numbers are excessive. In addition to providing clean bedding, soiled teats can be minimized by hair flaming or clipping of udders and frequent alley scraping. Tail trimming, but not docking, also can be useful in preventing the accumulation of manure on the udder and flanks. Also, areas where cows calve should be clean and dry. Clean pasture areas for calving are preferred.
In addition to clean housing and surroundings, strict milking hygiene is critical for reducing environmental bacteria. When a cow enters the milking parlor, any remaining sprinkler water from the holding pen and organic matter on the udder surface must be removed because they contain numerous mastitis-causing bacteria. If left on the udder surface, these skin contaminants would be removed by the flow of milk through the milking cluster and into the bulk tank, increasing the bacteria count. It should be noted that psychrophilic (cold-loving) bacteria from the environment can thrive at refrigerated bulk-tank temperatures, increasing the bacteria count even more. These bacteria also may survive pasteurization and reduce the shelf life of dairy products. The bacterial load present on teat ends when cows are being prepared for milking is best reduced by using teat germicides, a practice known as predipping. Premilking teat sanitization, whether accomplished by dipping teats in a germicidal solution or by using sanitized towels, foaming devices, or sprays, is 40% to 50% effective in preventing infections with environmental bacteria—as long as these procedures are done correctly (Figure 4).
When the cow leaves the milking parlor, producers should offer her fresh feed so that she remains standing for approximately 1 hr and does not lay down in mud and manure. During this time, her teat canals remain dilated from the machine-milking process, and environmental bacteria have easy access to the interior of the gland. After 1 hr, the teat sphincter muscle has contracted around the teat canal keratin and formed a seal against bacterial penetration.
Producers should implement a milk-monitoring system to ensure that mastitis prevention techniques are working properly and to maximize milk quality, especially during periods of environmental stress when milk quality will suffer. Monitoring may be as simple as the daily checking of milk-pipeline filter socks and cow-side screening for clinical mastitis, or as sophisticated as periodically collecting bulk tank and/or individual cow milk samples for SCC and bacteriology.
Dairy producers who perform regular bulk-tank milk-quality monitoring are better able to keep SCC low during the hot, humid summer months. For example, Figure 5 shows the 2-year monthly bulk-tank SCC averages for several upper Midwest dairies, illustrating the seasonal variation between low and high bulk-tank SCC herds. The low SCC herds experienced better overall herd management and more comprehensive bulk-tank monitoring. For years 1 and 2, the bulk tank SCC increases in months 7, 8, and 9 (July–August) were greater in the high-SCC herds than the low-SCC herds. The conclusion is that better practices for bulk-tank monitoring and udder health held the mastitis level and bulk-tank SCC in check over both years in the low-SCC herds despite the hot and humid summertime conditions.
To maximize yield, it is imperative to keep cows as comfortable as possible and optimize feed intake for conversion into milk. Heat stress negatively affects cow comfort, dry matter intake, and subsequently milk yield. Therefore, management strategies must be applied to counter hot and humid environmental conditions. Countering these risky environmental conditions can prevent seasonal increases in mastitis and SCC and protect milk quality. Control is based on the provision of fresh, cool, clean drinking water, increased energy density of rations, use of feed additives, and the use of cooling mechanisms, including shade, fans, sprinklers, tunnel ventilation, commercial coolers, cooling ponds, exit-lane sprinklers, and center pivots.
Unfortunately, most cooling systems result in excess water in the cow’s environment; along with warm temperatures, this provides ideal conditions for the growth of mastitis-causing bacteria. Thus, the cows’ surroundings must be kept as clean and dry as possible to reduce microbial growth. Additionally, recommended premilking udder prep and milking-time hygiene must be followed precisely to avoid new infections with environmental mammary-gland pathogens. Bulk-tank monitoring is critical during times of heat stress to ensure that mastitis-control practices are indeed working and that maximum milk quality is maintained. Finally, heat stress control practices should also be applied to replacement heifers, as these animals constitute the future milking herd, and their well-being, mammary health, and future productive life must be considered in an overall herd health program.
Status and Revision History
Published on Jan 31, 2014
In Review for Major Revisions on Sep 01, 2022
In Review on Jun 20, 2023
Published on Jun 20, 2023
Published with Major Revisions on Dec 08, 2023