In mid-July, the Midwest experienced one of the most brutal heat waves in recent history. As the heat index climbed into the triple digits, most people were looking for the nearest air-conditioned shelter to stay cool. Unfortunately, there just isn’t enough space to take the animals inside with us.

As a result, everyone in livestock production and meat processing stand to lose as the mercury rises. Although air-conditioning is not an option, there are other heat-mitigation strategies that can help to alleviate heat stress and associated meat-quality issues from the farm to the plant.

You may not have thought of the issues that other sectors of livestock agriculture face during hot weather, so let’s take a look down (or up) the supply chain. We’ll use a single pig as the running example for this article.

Depending on the time of year in which our pig’s mother became pregnant, our pig may have a decreased chance of even existing. This is due to multiple factors, but it appears that the length of the day and the seasonal temperatures play a major role in decreased pregnancy rates over the summer months. A French study1 of 601,117 sows and 22,773 litters of pigs between 2003 and 2007 called this difference in pregnancy rate ‘seasonal infertility.’ The authors of the study reported that pregnancy rates over the five years of their study were 2.8% lower in the summer than the winter. They concluded that this phenomenon was partly due to the longer days of summer, which cause some animals, such as sheep and deer, to refrain from breeding. They also concluded that heat stress likely played a role, as hotter summers resulted in larger reductions in pregnancy rate.

So let’s assume that our pig was born — otherwise our example is over. The next step in our pig’s life is the growing and finishing phase. This period lasts from the time the pig is weaned, which typically varies from three to six weeks of age, until the pig is slaughtered at five to six months of age. During this time, the management goal is typically focused on rapid growth and lean muscle development.

Many swine producers and animal scientists have identified a large difference between our pig’s response to hot temperatures and his predecessors’ average response. A study2 conducted at the USDA Meat Animal Research Center at Clay Center, Neb., in 2000, reported that our modern pig generates about 18.5% more body heat than a typical pig in the late 1950s. At the time that the study was published, the standards for heat removal from swine facilities were largely based on equations that were developed in 19592. As a result, our pig may be exposed to summer temperatures that are more difficult for him to handle than his predecessors because his body is creating more heat and he might be living in a building that was not designed to remove as much heat as he and his modern-day penmates create.

It would be likely that if our pig were in an older growing or finishing building during a heat wave similar to the one we saw in mid-July, he would become heat stressed. Simply put, this means that his body would not be able to easily maintain its normal operating temperature of about 102.5°F. As a result, he would start to display changes in body function and behavior that could impact the characteristics of his body composition. This may impact the characteristics of raw materials that arrive at your plant if you’re in the further-processing business.

Some of the most common physical signs of heat stress that we see in growing and finishing pigs is panting and reduced feed intake2. In our pig, we would likely start to see increased panting and reduced feed intake as temperatures climb above 72°F.3 As it got hotter, he would spend more time lying down and less time eating. This is because muscle contractions and the digestion of food both generate heat. So, our pig is less likely to get up and walk to the feeder unless he is motivated by hunger. The impact of reduced feed intake can lead to changes in average daily weight gain of 8% to 30%.4

Even though our pig is not eating as much, his body still needs energy to live. As a result, his body will start to burn stored fat to support itself. Since heat waves don’t typically last for long periods of time, the impact of heat stress on the percentage of muscle, fat and bone in the carcass of pigs doesn’t tend to change significantly during bursts of hot weather.5

Let’s say that the day our example pig is shipped for slaughter just happens to be the hottest day of the year. During the processes of transport and movement to slaughter, there are some important factors that need to be considered to assure he survives the trip and the quality of his carcass does not deteriorate. On this hot day, it is more difficult than normal for our pig to get rid of excess body heat, so we need to do things to prevent him from heating up and help him to remove built-up body heat.

During the loading process, our pig’s internal temperature will increase due to exercise. Since it is the hottest day of the year, we should load him and his penmates during the early morning hours. Once the truck is loaded, it should be moving as soon as possible to prevent heat build-up in the trailer. Pigs also need adequate time to recover from the stress of loading. Some researchers5 have found that maintaining highway speeds for greater than three hours allows pigs more time to calm down and cool off from unloading.

Although a load of pigs may have had ample time to recover from loading during transit, improper handling of logistics can ruin the positive effects of a longer haul. It is vitally important that the truck has a scheduled time to unload upon arrival at the plant. The longer a truckload of livestock is stopped in the hot summer sun, the hotter it will get inside. Once the truck has been unloaded, the pigs should be held in pens with stocking densities that are lighter than a plant would typically use in the winter. If sprinklers can be used in the holding pens, they should be.

From a meat-quality standpoint, it is more likely to observe lighter-colored pork, which is indicative of the pale, soft and exudative (PSE) condition, during the summer months.6 The color of pork is partially controlled by the amount of heat that is generated in the muscles immediately before slaughter and during rigor mortis. To help prevent the onset of PSE in the carcass of our example pig, he should be given time to rest and cool down between the time he is unloaded and the time he is moved to stunning. When he is moved to slaughter, he must be handled in a manner that will not cause him to become overly excited. This means that electric prod use should be minimized in hot weather, and more time may be necessary to move pigs to stunning.

Once the slaughter process has begun, it is important to cool the carcass as quickly as possible to prevent the onset of muscle quality issues.

Throughout the entire pork production chain, it is important to focus on finding ways to keep our pigs cool during those dog days of summer.


Kurt D. Vogel, Ph.D. is an assistant professor of animal science with an emphasis in livestock behavior and welfare at the University of Wisconsin - River Falls. He is also the president of Vogel Livestock Solutions, an animal-welfare consulting agency. For more information, contact him at or (715) 425-3704.


1—Auvigne, V., P. Leneveu, C. Jehannin, O. Peltoniemi. And E. Sallé. 2010. Seasonal infertility in sows: A five year field study to analyze the relative roles of heat stress and photoperiod. Theriogenology. 74:60-66.

2— Brown-Brandl, T. M., R. A. Eigenberg, J. A. Neinaber, and S. D. Kachman. 2001. Thermoregulatory profile of a newer genetic line of pigs. Livest. Prod. Sci. 71:253-260.

3— Huynh, T. T. T., A. J. A. Aarnink, M. W. A. Verstegen, W. J. J. Gerrits, M. J. W. Heetkamp, B. Kemp, and T. T. Canh. 2005. Effects of increasing temperatures on physiological changes in pigs at different relative humidities. J. Anim. Sci. 83: 1385-1396.

4— Christon, R. 1988. The effect of tropical ambient temperature on growth and metabolism in pigs. J. Anim. Sci. 66: 3112-3123.

5— Haley, C., C. E. Dewey, T. Widowski, and R. Friendship. 2008. Association between in-transit loss, internal trailer temperature, and distance travelled by Ontario market hogs. Can. J. Vet. Res. 72:385-389.

6— dalla Costa, O. A., L. Faucitano, A. Coldebella, J. V. Ludke, J. V. Peloso, D. dalla Roza, and M. J. R. Paranhos da Costa. 2007. Effects of the season of the year, truck type and location on truck on skin bruises and meat quality in pigs. Livest. Sci. 107:29-36.

Hot summer temperatures affect everyone in the muscle-to-meat transition.