Susan Schoenian, Sheep & Goat Specialist, University of Maryland Small Ruminant Extension Program
(Previously published on the Maryland Small Ruminant Page)
Over the past weekend, 
my family and I spent some time installing a new water line to give us access to more grazing area. As we spent most of both days in the sun, I began to work on 2019’s farmers tan. As I write this up, my arms are still feeling the heat of the weekend. With this being said, I thought that it would be timely to talk about heat stress in our favorite livestock species, sheep and goats. Any time we talk about feeding livestock, we note the importance of fresh, clean water. This is always a given regardless of the time of year. There is also discussion about wool on sheep during the summer months. Wool is actually quite beneficial when it comes to protecting against the hot summer sun. For more on these two topics and others related to heat stress, be sure to check out this weeks discussion provided by Susan Schoenian.
Extreme heat is stressful to livestock, as well as people. High temperatures are even more problematic in states like Maryland, because high temperatures are also often accompanied by high humidity. The heat index (temperature + humidity) is a more accurate measure of heat stress (hyperthermia) than temperature alone.
Heat tolerance
Some livestock (and people) tolerate heat better than others. Sheep and goats tend to be less susceptible to heat stress than swine, cattle, llamas, and alpacas. Hair sheep usually tolerate heat better than wooled sheep. This is why they are often used for training and trialing herding dogs. Fat-tailed sheep are also more heat tolerant. The European sheep breeds are usually the least heat-adaptive because they tend to have shorter bodies and legs, short, thick ears, tight skin, and dense fleeces.
Goats tend to tolerate heat better than sheep. Goats with loose skin and floppy ears may be more heat tolerant than other goats. Angora goats have a decreased ability to respond to heat stress as compared to sheep and other breeds of goats. Dark-colored animals are more susceptible to heat stress, while light-colored animals may be prone to sunburn. Females usually handle heat better than males. The heat is especially hard on fat animals.
Horned animals dissipate heat better than polled (or disbudded) animals. Young animals are more susceptible to heat stress than older animals, though the geriatric animal is also very vulnerable. In fact, any animal with a poor nutritional status or compromised immunity will be more susceptible to environmental extremes.
Wool
Wool protects sheep from extreme heat as well as extreme cold. A thick fleece is mostly immune to temperature changes due to its insulating properties. According to research, sheep with a one-inch fleece are more comfortable than sheep with less wool, as wool fibers dissipate heat more rapidly.
However, woolly animals should be sheared prior to the onset of hot weather. Spring shearing allows sheep to have adequate wool growth to keep them cool in the summer (and avoid sunburning) and a full wool coat in the winter to keep them warm. Sheep and goats should not be sheared in extreme heat. Shearing lambs will improve their growth performance (and welfare) during the summer months, if temperatures and humidity are elevated.
Water
Plenty of clean, cool, and fresh water is paramount to preventing heat stress in livestock. During periods of extended heat and humidity, it may be necessary to provide extra water and clean and change waterers more often. On-average, a sheep or goat will drink 1-2 gallons of water per day. Lactating females will drink even more water.
A study conducted with 3-year old ewes showed that consumption of water is 9% – 11% of body weight in the winter and 19% – 25% during the summer. In a 1958 study, Merino sheep drank 12 times more water in the summer than winter when it was dry and temperatures exceeded 100ºF (38ºC).
High temperatures are often accompanied by dry weather, resulting in lower moisture content in grazed forages. Dry forages increase water needs. Salt consumption increases water intake. Young animals need more water (on a percent body weight basis) than adults because a greater percentage of their body weight is water. Young animals need to drink more often because they drink less water at a time and have a more rapid metabolism.
Sheep will drink more water than they need for metabolism, perhaps as a pre-adaption to heat stress and water deprivation. Another way sheep adapt to heat stress is by producing more concentrated urine.
Shade
Access to shade is another important aspect of managing livestock during hot weather. Livestock shelters do not need to be complicated or elaborate. Mature trees provide excellent shade (and shelter) and are usually the least-cost alternative. If natural shelter is not available, many sheep and goat producers use quonset huts, plastic calf hutches, polydomes, and/or carports to provide shelter for grazing animals.
Simple shade structures can be constructed from shade cloth, mesh fabric, tarps, canvas, or sheet metal. Movable shade structures are suitable for intensive rotational grazing systems. All livestock should be able to lie down in the shade structure or area at the same time. Lying down in a cool spot provides additional relief from the heat.
While there is disagreement as to whether grazing livestock require shade, numerous studies show the benefits to shade. In addition to improving animal welfare, access to shade may improve weight gain, milk production, and reproduction. The benefits (to shade) would be greatest in humid environments.
When livestock are housed, the key is good ventilation and air movement. It may be necessary to install fans or other cooling systems in barns and similar structures. Research has shown cool water spraying to reduce heat stress and improve welfare of goats.
It goes without saying that livestock should not be handled, worked, or transported during the heat of the day. If livestock must be worked, they should be handled in the early morning or late evening hours.
More nutrient-dense diets are usually preferred during periods of high heat and/or humidity. This is because animals generate more body heat when they digest poor quality feed. Though grains (e.g. corn) are considered “hot” rations in other respects, less body heat is produced when livestock digest grain as compared to forages, especially poor quality forages. The feed supplement that produces the least amount of heat is fat.
Heat stress
Under normal circumstances, livestock are able to maintain their body temperature at a safe range, so long as they have shade and plenty of water. In extreme heat, they will decrease their grazing time and spend more time in the shade, especially during the heat of the day. They will graze mostly in the evening and early morning hours. They should be allowed to rest during the heat of the day.
While heat stress (exhaustion or stroke) is not very common in sheep and goats in temperate climates, it may occur, especially if stock are handled during the hottest part of the day. Clinical signs of heat stress include continual panting, rapid breathing, weakness, inability to stand, and an elevated rectal temperature (over 105ºF/40.6ºC). If rectal temperature exceeds 107ºF (41.7ºC), death may occur, as the animal’s cells begin to degenerate.
Animals suspected of being heat-stressed should be moved to a cool, shaded area with good air circulation. The obvious goal of treatment is to lower body temperature. Sheep should be cooled by applying rubbing alcohol to the area between their rear legs. Besides not being covered with wool, this area has a lot of vascular activity. Wooled sheep should not be sprayed with cool water as this will prevent cooling. Air will not be able to pass through the wetted fleece.
It’s okay to spray cool water over other livestock or to spray water over a sheep or goat’s woolless areas. Other cooling treatments include ice applications, submersion in ice, and cool water enemas. Always be careful to make sure the cold treatment isn’t too great a temperature shock to the animal’s vascular system.
Heat-stressed animals should be offered ample water and encouraged to drink small amounts. It may be necessary to administer fluids to animals that have become dehydrated as a result of their exposure to extreme heat and/or humidity. Woolly (or hairy animals) should be sheared as conditions allow.
Productivity
Extreme heat can have a profound effect on productivity, especially if the onset of heat is sudden, not giving livestock ample time to adapt. It goes without saying that growth rates are reduced in hot weather, as livestock forage less and have reduced appetites. This situation is often worsened by dry, poor quality forage. If temperatures subside, there is often a risk of acidosis or bloat as livestock engorge on feed.
Prolonged high temperatures (above 90ºF/32.2ºC) can impair reproduction. Overheated rams may lack libido (sexual desire). Ideally, rams should be sheared six to eight weeks before the onset of the breeding season. Woolly scrotums should be sheared. In extreme heat, rams can be housed during the day and put with the ewes at night.
After a ram or buck has been affected by heat stress, it will take six to seven weeks before he produces semen that is capable of fertilization. Fully-developed sperm are less susceptible to heat stress than sperm in the developing stages. High temperatures can also be detrimental to embryo survival and fetal development.
Heat stress lowers the natural immunity of animals, making them more susceptible to disease. It is not uncommon to see cases of pneumonia in extremely hot weather. In general, animals will have less tolerance for parasitic and other opportunistic diseases.
During periods of high heat and/or humidity, livestock should be checked frequently for signs of distress.