Improving heat stress resilience should be priority for swine industry

Management, nutrition and genetic selection may be key to mitigating heat stress-related health issues, production losses.

Ann Hess, Content Director

July 28, 2022

10 Min Read
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National Pork Board

Mean global temperatures have risen in the past century and this means that animals are increasingly being raised in regions where heat stress is prevalent. However, Jay Johnson said the question remains, how do we define heat stress when it comes to our livestock?

"When we think about heat stress, we think about the balance between heat gain and heat loss by the animal. When heat gain and heat loss are balanced, both by heat gain through body temperature, the environment and metabolic processes, and heat loss to the environment, that animal would be considered in thermal neutral conditions and would have a normal body temperature," Johnson said. "However, when we get an imbalance in that heat gain to loss ratio, this is when heat stress can occur. For instance, when environmental temperatures go above that animal's critical temperature point, they can reach a state of hyperthermia and heat stress can occur."

A research animal scientist with the USDA Agricultural Research Service, Johnson was the first to present Wednesday in a free USDA webinar series titled, "ARS Research to Mitigating the Impacts of Heat Stress on Animal Health and Well-being in the Livestock Industry." Johnson shared the production and economic impacts heat stress has had on the swine industry. When sows are lactating or gestating are, he said the effects of heat stress can be compounded.

"In an attempt to remain euthermic and maintain homeostasis, pigs will have a variety of different biological responses to heat stress, and these are performed in an attempt for that animal to maintain survival and it prioritizes survival over its productive state," Johnson said. "Due to this, we see a variety of different production losses, things like reduced lean muscle growth and meat output per pig, reduced body condition, poor animal welfare outcomes and in severe cases, mortality can occur."

In 2003, it was estimated that the swine industry loses approximately $299 million per year from heat stress. Considering the effects of both climate change, as well as inflation, Johnson said when estimating in today's dollars, this could be up upwards of $481 million in losses. Another 2010 estimate showed $55 in losses per sow per year due to reproductive issues related to heat stress.

"Adjusting for that inflation factor, we see this number increase to around $88.55 per sow today. However, it's important to note that these values do not account for the postnatal effects of in utero heat stress on swine production," Johnson said.

In the past decade, Johnson's research team and others have been trying to define the negative effects of in utero heat stress on swine during postnatal life.

"We know that in utero heat stress, or IUHS, is associated with negative economically relevant postnatal phenotypes in pigs. The effects of IUHS can be observed throughout a pig's lifetime and we've seen that the first half of gestation generally elicits the most negative responses," said Johnson. "A primary risk factor for IUHS is elevated maternal body temperature, so either a chronic and low increase, or an acute and high increase in body temperature. We see a direct piglet impact of IUHS, things like reduced birth weights, greater pre farrowing embryonic, or fetal losses, reduced weaning weight and lower survival rate of these piglets."

Johnson's research team is interested in finding out the future impact on offspring after they're born. IUHS has a wide variety of post negative postnatal impacts in pigs that occur during all phases of the swine life cycle, Johnson said. These can elicit negative responses such as a greater stress response in animals, reduce growth rate, decreased feed efficiency, increased heat stress sensitivity, impaired reproductive performance, altered immune function and decreased meat quality and quantity.

"These all occur independent of the postnatal environment that these pigs are raised under, and so the question remains, how do we fix this?" Johnson said. "What strategies can we take to mitigate both the negative effects of postnatal and prenatal heat stress in our pigs?"

Johnson's group has been focused on improving heat stress resilience — the ability to cope with and recover from a heat stress challenge — through a three-pronged approach examining management, nutrition and genetics.

Through research funded by the USDA National Institute of Food and Agriculture, the team has been looking at pigs’ behavioral thermal preference, as well as their physiological thermal stress response. To do this, they've developed "thermoclines."

"They are two chambers that have a gradually increasing thermal gradient, and it allows the animal to actually choose what temperatures they prefer to be in and which temperatures they prefer not to be in," Johnson said. "Taking this data, we can model it and we can determine what is the peak temperature preference of our sows. In this case, we were looking again at nonpregnant, mid gestation and late gestation and what we found is that our late gestation sows prefer to be at a temperature of approximately 14 degrees Celsius or 57 degrees Fahrenheit. Whereas our mid gestation and nonpregnant animals prefer to be at about 14.8 degrees Celsius or around 59 degrees Fahrenheit."

In addition, the research team performed physiological heat stress testing, looking at the body temperature and physiological responses of the sows at three different gestation stages to increasing temperatures. With this information, they've been able to develop some heat stress thresholds, defined as either:

  • Mild heat stress: the point at which active heat loss attempts by that animal start.

  • Moderate heat stress: the point at which active heat loss attempts fail and body temperature starts to rise.

  • Severe heat stress: the point at which body temperature can rise uncontrollably, possibly leading to death.

With this information, Johnson's team has developed physiologically based heat stress threshold equations that can predict when both respiration rate and body temperatures will go above thresholds based upon both the temperature and dewpoint. These are differentiated by gestation stage to reflect the physiological changes that are occurring as gestation advances, Johnson said.

The research team has now integrated both their behavioral thermal preference and physiological stress response into a decision support tool in collaboration with both Purdue University and the University of Illinois.

"With this tool, producers can put three different inputs into the thermal index — the reproductive stage of their animal, the ambient temperature and the dewpoint — and they can get six different categories," Johnson said. "That animal is either cool, comfortable, warm, mild, moderate or in severe thermal stress."

The decision support tool has now been integrated into a smartphone app, called HotHog, which is being beta tested. Johnson said the app is going to be targeted towards swine producers, but researchers performing heat stress studies may also find it useful, as it can provide real-time alerts of heat stress risk levels, as well as management recommendations for producers.

In addition to management practices, the research team has also been examining nutrition, specifically in lactating sows.

"We know that reduced feed intake is often cited as a primary reason for decreased sow milk production and litter growth performance during heat stress and this is generally attributed to decreased energy available for lactogenesis or milk production," Johnson said. "Because of this, there have been several nutrition and management-based mitigation strategies — proposed things like changes in feeding patterns, dietary supplements or diet formulations. However, we were curious as to whether or not feed intake alone really explained that reduced milk production and decreased litter growth."

In collaboration with Purdue University professors Allan P. Schinckel and Robert M. Stwalley, and with support from Pork Checkoff, Johnson examined the use of electronically controlled sow cooling pads developed at Purdue University. Knowing that they're effective in allowing sows to maintain normal body temperatures under heat stress conditions, they decided to combine the pads with indirect calorimeters that were developed at the USDA-ARS Livestock Behavior Research Unit in West Lafayette, Indiana, in order to look at individual sow metabolic heat production.

Johnson said this was an indirect indicator of lactation output or milk production by the sows.

"What we find is that our animals that are cooled under heat stress conditions do have a greater metabolic heat production, and as we've seen in previous literature, heat production is closely associated with milk production," Johnson said. "In addition, we observed improvements in piglet growth rate, as well as weaning weights for our cool animals. But despite the fact that we see these improvements in both milk output and little growth performance, we did not really see any differences in feed intake."

What does this mean? Johnson said the researchers concluded that increasing feed intake during heat stress may not completely rescue lost sow lactation performance, and that there's likely a direct biological effect of heat stress occurring on lactation performance, which is similar to work in dairy cattle. Feeding management should be coupled with effective barn cooling strategies to get the most benefit, Johnson said.

The last area Johnson has been studying is genetic selection for heat stress resilience, in collaboration with fellow researchers Luiz Brito, Christian Maltecca and Francesco Tiezzi, and with support from both Smithfield and USDA-NIFA. Knowing that heat tolerance is heritable, and selection for heat tolerance must consider both production and welfare relevant traits, the team aims to develop thermotolerance breeding value estimates and integrate these within a selection index for sows.

In order to do this, they've been taking direct physiological measures of swine heat stress under commercially relevant conditions.

"During the summer months, we've recorded things like skin temperatures, body temperatures, respiration rate, body condition score, panting scores, cortisol, and in total, we currently have 1,645 animals with phenotypes and a little over 1,600 animals with genotypes," Johnson said.

With this information, the team has been able to develop heritability estimates and have found that automatically recorded vaginal temperature is moderately to highly heritable across sows. However, Johnson questions is this a feasible measure for the industry?

"Can we expect geneticists to go out and really measure 1,600; 2,000; 3,000 sows at a time? Probably not, and so looking at this a bit closer, we looked at time specific vaginal temperature predictions," Johnson said. "When we compare individual hourly measures of these sows against our 24-hour automatically collected data, we do find that our vaginal temperature heritability is relatively similar and that the genetic correlations between individual hourly measurements are actually very highly correlated with our 24-hour data, indicating that you may be able to go out and simply take one measure at a certain time of day and get a similar heritability estimate."

In addition, the researchers wanted to know whether or not some anatomical characteristics would be correlated with the vaginal temperature response. They found that both ear area and ear length were significantly genetically correlated with the body temperature response observed as well as hair density.

"With this information, we've been able to develop genomic breeding values for heat tolerance, differentiating between both heat sensitive and heat tolerant animals," Johnson said. "And we do find that the genetic merit of heat tolerance and sensitivity do differ, and so this means that selection is possible."

At the Livestock Behavior Research Unit, the team has selected 20 of the most tolerant and 20 of the most sensitive sows, based upon their genomic breeding values for heat, stress, tolerance and sensitivity. They are currently doing some intensive testing and phenotyping under these controlled heat stress conditions with the goal of identifying genes associated with both positive and negative traits of the sensitive and tolerant sows.

"Again, to develop these thermotolerance breeding value estimates for integration into a larger genetic selection index," Johnson said.

The team is also conducting some follow-up work, looking at mitigating the effects of IUHS through the use of genetic selection and currently in year two of the project with some of the live animal testing upcoming.

Johnson said the reason heat stress has so many negative effects on swine health, performance and welfare, is ultimately because pigs are prioritizing survival over productivity.

"We know that the economic costs of IUHS may actually rival those of postnatal heat stress," Johnson said. "They haven't yet been determined, but we know that the negative production and health effects of IUHS occur throughout a pig's lifetime, regardless of season and are not simply dependent on environmental conditions. Improving heat stress resilience through management, nutrition and genetic selection should be a priority for the swine industry."

About the Author(s)

Ann Hess

Content Director, National Hog Farmer

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