When winter in Missouri went straight to summer, practically skipping spring, Brent Sandidge worried about his pigs.

“Then May felt like June and June felt like July,” says Sandidge, a pig farmer whose family owns a 3,000-sow farm in Marshall, Mo. Hotter-than-normal weather means farmers like him will have to sell lighter pigs, because heat slows the animals’ muscle growth. Although high temperatures reduce meat production in a number of livestock animals, pigs are especially vulnerable because their sweat glands are inefficient.

Farmers have long observed that hot weather makes their hogs lethargic and thin, but the underlying science to support this has only started to emerge in the last decade. A growing body of research shows temperature increases above 25 degrees Celsius cause pigs to begin storing less protein. High temperatures also reduce the animals’ fertility. And the impact is not limited to one generation—if a pregnant sow suffers heat stress, she will birth fewer babies, which will grow slower, store less protein and have fewer—and lower-quality—eggs and sperm compared with pigs born to mothers raised at cooler temperatures.

Agricultural scientists say pork could get more expensive in a warmer and more humid world because raising pigs will require more food, energy, water and labor to meet the protein requirements of a growing human population. The impact of warming on the pork industry is primarily an economic issue in the U.S, which could adapt by investing in high-end cooling technology and raising more pigs. But poor, tropical countries such as Vietnam and Philippines, where pork is a main source of meat, lack those resources. Lower pork yield in these countries could threaten food security, says Lance Baumgard, an animal science professor at Iowa State University. Studies have already shown a warming climate is making crops less nutritious, so a reduction in animal protein is a reason to worry.

As the climate warms, hot days will only become more common. A study published in Nature last year estimates that a 1 degree Celsius increase in temperature could add 30 more days of heat wave per season over large parts of Africa, Central and South America, and Southeast Asia, and 15 more such days in the U.S. The Midwest, where 75 percent of the U.S.’s pig farms are located—will be especially hard hit (pdf). Scientists have not yet quantified how that 1-degree increase in temperature would affect global pork production. “It is pretty hard to do those studies because there are too many variables which depend on breeds, life cycle stage, intensity and length of heat stress, and the facilities in which pigs are kept,” says Jay Johnson, a U.S. Department of Agriculture scientist.

Nevertheless, worries about the impact of heat on pigs drove the National Pork Board to call a meeting of pork producers, USDA representatives and scientists in Iowa in 2013 to address some startling industry trends. “Regardless of which data set we looked at, there was about a 10 percent decrease in overall productivity [pork yield] caused by heat stress every year,” says Chris Hostetler, director of animal science on the Pork Board. That could threaten the U.S. pork industry, which is currently worth about $20 billion. The USDA had been funding this research for several years, but following the 2013 meeting the Pork Board poured in additional funding.

 Roasted Pigs

Until about 2010 the only explanation scientists offered pig farmers for why their animals gained less weight in the summer was that the heat-stressed pigs eat less to reduce their metabolic heat production. But that was only a part of the reason, according to Baumgard and other scientists. Their research—first in cows and then in pigs—over the last couple of years has revealed the immune system also plays a role. Heat stress fires-up pigs’ immune systems, which sends the hormone insulin into overdrive. The insulin acquires substantial amount of amino acids (protein building blocks) that would otherwise go into making muscle and uses them to fuel the immune system. “Heat-stressed pigs prioritize [the] immune system over muscle building,” Baumgard says.

Adding to the problem, the pork industry has over the years selected for pigs that grow leaner (produce less fat and more protein) and more quickly. But those pigs produce about 50 percent more heat compared with those raised in the 1980s. Modern pigs are like body builders—except that pigs barely sweat, so they cannot cool themselves effectively, Johnson says.

In a 2015 study Johnson and his colleagues found heat-stressed pigs stored 23 percent less protein in comparison with those raised at a comfortable temperature. Heat-stressed pigs also ate less. But when scientists in other studies fed both groups the same amount of calories to control for the effect of food intake, they still found the heat-stressed pigs converted less energy into protein for every pound of body weight they gained in comparison with the non-stressed animals. His experiments also showed babies born to heat-stressed mothers stored 16 percent less protein as they grew up and reached market, compared with those born to mothers raised under normal temperature conditions.

In the last few years scientists have additionally begun to notice excess heat makes pigs and their offspring less fertile. Sandidge has observed that during the summer his pigs’ fertility drops by 5 to 10 percent. Heat-stressed pregnant pigs have on average one piglet fewer than non-stressed sows, which typically have about 13 piglets, according to a study led by Tim Safranski, an animal science professor at the University of Missouri–Columbia. “Those numbers at the commercial farm level add up to a lot of pigs not reaching market,” he says.

Kara Stewart, an assistant professor of animal sciences at Purdue University, analyzed the semen of boars born to heat-stressed mothers, and found each ejaculate had about seven billion fewer sperm than that of boars whose mothers were not stressed. That translates to three fewer doses of semen a week that can be used to fertilize a sow via artificial insemination. (One dose contains about 2.5 billion sperm.) And the sperm the boars did produce had tails that were abnormally coiled and twisted. Likewise, researchers have found the female offspring of heat-stressed mothers produce fewer eggs. This fertility decrease also likely contributes to the lower pork yield in hot weather.

Scientists are now trying to figure out exactly how heat stress affects a fetus. Part of what’s going on, they think, is blood gets shunted away from a pregnant pig’s uterus and to its skin to dissipate heat. To dig deeper, Johnson is currently testing the hypothesis that heat stress impacts the development of the fetus’s brain—especially the hypothalamus, a region that acts as the body’s thermostat. His hypothesis rests on the consistent observation that the heads of babies born to heat-stressed mothers are about 5 percent shorter than those of babies whose mothers did not experience extreme heat.

Problem of Plenty

Every summer—and more so during hotter ones—farmers like Sandidge are forced to sell pigs that are about 10 percent lighter than the preferred weight of about 270 pounds because they need to vacate the barn to make way for the next batch of pigs. This schedule is fixed in advance. Sandidge does not have a choice. This should theoretically reduce the amount of pork in the market, yet annual production continues to rise, and is expected to jump by 5 percent in 2018 from 11.6 million tons in 2017. That is because the industry is raising more pigs, which offsets the reduction in pork as a result of heat stress, says Lee Schulz, a livestock economist at Iowa State. But the question, Johnson asks, is how many more pigs you would need to raise to make up for this reduction, and whether that is sustainable. “In a hotter world, it could get expensive to raise pork,” Missouri’s Safranski adds.

Although the scientists and economists agree the market is dynamic and difficult to predict, they say a pig-farming model based primarily on raising more pigs will not be a sustainable way to grow protein for a booming population. Scientists say the end goal of their research is to figure out ways to efficiently grow more animal protein with fewer resources. “Heat stress is a drag on efficient production,” Johnson says.

Cooling Technology and Better Breeds

Techniques to keep pigs cool have evolved over the years, from shade structures in 1940s and ‘50s, when pigs were raised outside, to the use of fans, ventilated buildings, misters, water drippers and wet pads to cool the air inside today’s barns. Although these techniques have helped pigs better withstand heat, their efficiency is reduced during heat waves. There have not been any major advances in the technology to cool pigs in the last decade or so, says Brett Ramirez, an engineer who specializes in cooling infrastructure at Iowa State. “We need more efficient and economic cooling technology,” he says.

Scientists have also been investigating whether supplementing pig food with minerals and chemicals—such as zinc and betaine—could protect the animals’ guts, which are damaged by heat stress. Zinc strengthens the intestinal barrier, and betaine is known to protect cells from environmental stress.

Some farmers in the U.S. and many in tropical countries may not have the money to invest in cooling technology, however. “If you cannot afford to pamper pigs, then you choose an animal that is resilient,” Safranski says. Scientists say traditionally farmed pig breeds tend to withstand environmental changes better than today’s industrially bred ones do, but are less efficient in converting feed into body mass.

Safranski believes it is essential to save and study such indigenous breeds. “Genetic resources are disappearing because of modernization, if you will,” he says. “It is scary because once [these breeds] are gone, they are gone.” Researchers say that if they could discover a heat-resistant gene in one of these breeds, they could splice it into commercial pig lines to make them even more efficient. “We talk about feeding [more than] nine billion people,” he notes. “We’ve got to be more innovative.”

For his part, Sandidge is hopeful science will have some answers for him in future summers. He is open to trying new techniques that could help his pigs weather the heat better. “I love my pigs,” he says.