Minimizing impacts of higher temperatures on pigs raised outdoors

Producers who raise pigs outdoors can implement several low-cost adaptive measures to help minimize adverse effects of high temperatures on pig health and performance.

Pigs outside in pasture
Figure 1. Photo by Jim Isleib, MSU Extension.

U.S. National Weather Bureau long-term projections show higher summer temperatures and humidity, milder winters and increased precipitation are likely to continue in areas of the U.S. where most pork production occurs. These conditions could adversely affect the health and performance of pigs which are more sensitive to heat than other livestock. Whether animals are raised in barns where considerable control of the environment is possible or outdoors where control is more challenging as pork producers should be prepared to adapt their practices to minimize the adverse effects of these weather conditions and the increased exposure to diseases they bring. This article focuses on pigs raised outdoors.

High external temperatures adversely affect pig health and performance directly by compromising appetite, gut function, and the immune system, and indirectly by increasing exposure to numerous pathogens that infect pigs. Pigs are challenged by the combination of high heat plus humidity (quantified by the heat stress index, see Figure 2) that occurs late spring and throughout the summer due mainly to their low lung capacity and minimal ability to sweat. These factors contribute to the minimal ability of pigs to self-cool by panting or by evaporative cooling that depend on sweat production.


livestock hot weather safety index
Figure 2. Weather safety index for livestock. Photo by National Pork Board.


There is a clear relationship between body size and susceptibility to heat stress in pigs; smaller (less than 60 lb.) animals can maintain optimum growth rate at ambient temperatures 3 to 5 degrees Fahrenheit (F) warmer than large (greater than 175 lb.) animals. Key performance and health indicators for how high temperatures can affect pigs include reduced feed intake, growth rate and feed efficiency, lower conception and piglet survival rates, higher incidence and severity of scours, and higher mortality rates during transport.

Heat stress in pigs can also become an important factor in susceptibility to disease.  When blood is shunted away from internal organs (including the intestinal lining) to muscle and skin in effort to maximize evaporative cooling it reduces the amount of oxygen delivered to the gut.  Over time, this leads to damage of the intestinal lining and a condition referred to as “leaky gut characterized by loss of ability by the gut lining to prevent viruses, bacteria, and toxins from entering the bloodstream. Pigs become more vulnerable to infection by anything ingested. Left unchecked, this can damage the animal’s immune system to an extent that animals also become less able to withstand infection by respiratory and systemic pathogens.

How are warmer and wetter weather conditions expected to impact pigs raised outdoors? 

Pigs raised outdoors are generally more exposed than pigs raised inside to the adverse effects of all types of weather extremes, including higher ambient temperatures, humidity, and precipitation. They are unable to benefit from the consistent protection from direct sunlight and moving air currents generated by properly ventilated barns. In addition, higher external temperatures and moisture are projected to expand the geographical range of vector-borne diseases that affect pig health and performance. Pigs raised outdoors are likely to come in frequent contact with a broad array of parasites (including insects, roundworms, and protozoa) and other wildlife such as migratory birds and rodents known to carry those parasites along with bacteria and viruses that infect pigs. In contrast, pigs raised inside modern, slatted, and well-ventilated facilities where strict biosecurity measures are often practiced (including barn disinfection, all in/out movement patterns and pest control) are typically exposed to fewer and lower levels of many of these pathogens. Pigs raised outdoors are also more likely to be exposed to weather conditions that lead to greater frequency and severity of drainage or runoff-related contamination of fields, pastures and surface water by manure containing pathogens that can infect pigs, especially parasitic roundworms, coccidia, enteric bacteria (e.g., E.coli, Salmonella) and Erysipelas.

These same weather conditions are projected to expand the range of several fungi, including tar spot and mycotoxins which adversely affect corn production/storage and pork nutrition in different ways. Tar spot infestation increases with higher precipitation and causes production losses in corn while it is growing; this may increase feed costs when corn is used to supplement pasture/forage diets. Mycotoxins grow in corn during and following harvest can compromise pig health when contaminated corn is fed to the animals.

Adaptive measures to increase farm resilience to weather-sensitive conditions that affect pigs raised outdoors

Minimizing risk of heat stress and damage it causes to the barrier function of the gut is essential to preserving pig health and performance. Adaptive measures for pigs raised outdoors to minimize direct effects of high external temperatures include creating shaded spaces and wallows. Useful measures to minimize the indirect effects excessive heat and heat stress on pigs include implementing stricter farm biosecurity measures, updating vaccination plans, rotating pens/pastures and other measures described below.

Provide shade to minimize direct risk of excessive heat and heat stress

It is important to provide ample space where pigs can avoid direct exposure to sunlight. Radiant energy from sunlight can raise an animal’s surface temperature and work against the natural physiological response of the animal to shunt blood to the skin during periods of excessive heat. It can also cause sunburn and blistering, adding to the discomfort of animals and increasing their susceptibility to skin infections.

Make sure that pigs can find ample space in shaded areas during all hours of the day. Small pigs should have at least 4 square feet and pigs weighing over 100 lbs should have at least 6 sq ft of shaded space to sit or lie down which was explained the USDA Space Requirements for Swine, Volumes 3-10. Strategic placement and orientation of sheds or portable sunbreaks to maximize airflow during hot weather is usually a low-cost adaptive measure. Additionally, if these facilities are built with a floor, it can be beneficial to raise the structure 6 to 8 inches from the ground using blocks to allow airflow. While half-barrels or sided-sheds can provide reliable barriers to direct sun exposure, their fixed walls allow less flexibility than natural tree-shaded areas for capturing the benefits of air movement.

Maintain a wallow 

Pigs enjoy rolling in mud, especially in hot weather. Since pigs are unable to generate sufficient sweat on their own to allow for evaporative cooling, consider constructing a wallow to achieve the same end result as part of your adaptive strategy. Mud dries slowly on their skin allowing moisture to be retained for longer periods than water alone, helping the pig remain cooler for a longer period. Mud from the wallow can also provide protection from biting insects and from sunburn, as it is an effective block to ultraviolet (UV) radiation.

Wallows can be constructed by simply tilling soil in a low area of pasture or allowing pigs to work up an area of the pasture (adding water is usually necessary), ideally not far from a shaded space. A wallow should be large enough to accommodate all pigs housed in a pen at one time, otherwise they are likely to fight for space, especially on the hottest days. Pigs should be able to submerse their legs and at least one side of their body in mud. A well- constructed wallow typically requires little management, but it is important to keep the mud moist (muddy) all summer; an external source of water may be required.  Wallows should be monitored regularly to prevent build-up of stagnant water and the contaminating pathogens it might contain and should be located in a way that minimizes contact with manure from pigs or other livestock. It is helpful to locate wallows away from the feeding site, and to move it to new locations periodically to minimize exposure to parasite eggs and oocysts which can build up to high levels in and around wallows. If maintaining a wallow for your pigs is not an option, then it is essential to provide some form of water misting/spraying periodically during summer months.

Make sure fresh drinking water is always available

High temperatures and heat stress can quickly lead to dehydration. Most pigs will want to drink a lot more water in hot weather. Making sure plenty of fresh, clean water is available to your pigs is the most cost-effective way to prevent dehydration.

Adjust feed and consider use of supplements

High temperatures and heat stress can cause pigs to eat up to 50% less than they would normally. The risk of heat stress in pigs raised outdoors on pasture may be higher because they typically consume a lot of fiber, which generates a lot of heat as it is metabolized. In the case of pork producers who supplement their pigs’ diet, increasing fat while reducing protein and fiber can help reduce heat produced during digestion while encouraging feed intake. Supplementing the diet during extended hot periods can also help ensure that antioxidant, trace-mineral, and vitamin needs are met. Your veterinarian or feed supplier are good sources of information on diet and use of feed supplements during hot weather.

Implement and maintain solid biosecurity measures

Many pathogens enter by the way of new pigs introduced to the farm and by wildlife, especially rodents (but also deer, fox, skunks, raccoons, and birds). Taking actions to minimize introduction of pathogens to your barns, outdoor pens and stored feedstuffs provides a form of low-cost insurance. It is challenging to accomplish this where pigs are maintained outdoors. Some useful tactics are described below:  

  • Quarantine new animals for 3 to 4 weeks before co-mingling them with your herd. This buys time to assess the health of new animals more closely, and for treatment and recovery from some infections. It also provides time to treat animals for parasites, especially worms (Ascaris, whipworm, lungworm) that can otherwise contaminate outdoor pens or pasture with their eggs following placement.
  • Rotate pastures periodically to allow time for parasite eggs and larvae close to the surface to desiccate. Tilling or deep raking soil around high traffic areas (e.g., feeder, water source, wallow) can be helpful, as it exposes more eggs to the surface drying. Eggs from some important parasites, including Ascaris and Trichuris (whipworm) are resistant to desiccation and some eggs may persist for up to 10 years in soil; pasture rotation will be helpful but will not ensure protection from reinfection by eggs from these species. A management plan that includes parasite control is a necessity when raising pigs outdoors. 
  • Use bait traps for rodents and install fencing to limit specific feeding/watering areas of pasture to larger wildlife.
  • Keep feeds off the ground where rodents and other wildlife that serve as vectors for many pig diseases, especially parasites, can gain easy access. If feed spills occur, clean these up immediately to limit attracting more rodents or wildlife to the area.

Keep pens and pastures as clean as possible, minimize contaminated water run-off

Work with a farm environmental specialist to develop a manure and water management strategy that minimizes risks associated with re-entry of manure into outdoor areas where pigs spend most of their time or surface water that animals may encounter. A farm environmental specialist or your veterinarian can also help develop a pen/pasture rotation strategy to reduce exposure to diseases such as parasitic worms and protozoa that pigs already at risk for heat stress may be especially sensitive to.

Conduct periodic disease surveillance

Implement a disease surveillance program that allows early detection of diseases in your herd. Oral fluids sampling is an ideal tool for early detection of most of the key pathogenic bacteria and viruses along with some key worm species that infect pigs. Oral fluid collection is easy and inexpensive, and a few samples can provide a good window to the health status of your herd. When collected over time, surveillance information can be used, in collaboration with your veterinarian, to develop an effective vaccination strategy and to evaluate the effectiveness of other preventive measures.

Periodically review and update your farm vaccination and parasite control strategies

It is useful to work with your veterinarian to update your farm’s vaccination strategy and parasite control strategy to prevent diseases that become more prevalent in your area during prolonged hot spells. For several key diseases, existing vaccines will provide full or partially effective control options. Diseases against which vaccines are available and that may become more important on farms raising pigs outdoors include Lawsonia, E.coli and Salmonella. Effective vaccines are not yet available for parasites that typically infect pigs, so strategic use of parasiticides is strongly recommended.

Use antimicrobials responsibly to treat infections that occur

On small farms the most efficient option for antimicrobial use usually involves treating individual pigs when a suspected infection is observed. Use of an injectable antimicrobial is recommended for most enteric (intestinal) or respiratory infections involving pathogenic bacteria. Antimicrobials are not effective against viruses. Your veterinarian is the best source for advice regarding product, dose regimen and safety precautions.

Minimize exposure to contaminated grain supplements

On farms that grow their own corn or soybean to supplement to pasture feeding, be sure to dry stored grains greater than 15% moisture soon after harvest and test stored feedstuffs periodically for mycotoxins frequently (as a preventive measure). Low-cost methods of detecting mycotoxins or fungi that produce them include commercial immunoassay kits that are easy to perform.

Follow NPB-recommended guidelines while transporting pigs in hot weather

Regardless of the type of farm pigs are raised on, transportation events can be stressful for the animals, but they can be especially hard on pigs during hot, humid weather. It is important to follow recommendations provided by the National Pork Board (NPB) whenever transporting pigs. Key recommendations include: 

  • Schedule transportation early morning or evenings when temperatures are lowest 
  • Make sure trailer vents are open 
  • Sprinkle pigs with water (large droplets, not mist) before transporting to increase evaporative cooling 
  • Handle pigs gently while loading and unloading; use sorting boards instead of electric prods and do not yell
  • Use minimal bedding, 1 bag per small trailer is sufficient in hot weather; wood chips are better in hot weather because straw holds more heat 
  • Do not exceed recommended load density; a 300 lb. pig should have at least 5 sq ft 
  • Keep loading and unloading times to a minimum 
  • Make sure the destination is prepared for and expecting your load of pigs 
  • Have a back-up plan in case unexpected events (re-routing, truck breakdown) occur 

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