By Jack Dekkers and Fazhir Kayondo, Iowa State University; Hayder Al-Shanoon, Yolande Seddon, David Janz, Dylan Carette, Carmen Cole and John Harding, University of Saskatchewan; Fred Fortin, Centre de développement du porc du Québec inc.; Mike Dyck and Graham Plastow, University of Alberta; and PigGen Canada

During their lifetime, pigs encounter many stressors, including weaning, interactions with pen mates, especially after mixing, human handling, transportation, etc. How a pig deals with that stress can affect its productivity, health and meat quality.

Stress triggers the production of so-called stress hormones, the most notable one being cortisol, along with its precursor cortisone, as well as dehydroepiandrosterone and its sulfate form DHEA-S. Cortisol is known as ‘the stress hormone’ and is the most important regulator of an individual’s response to stress. It is produced by the adrenal gland in response to stress in order for the animal to stay on high alert after it has released its “fight or flight” hormones (in the case of acute stress), such as adrenaline. DHEA counters the effects of cortisol, moving the body back to a non-stressed state.

Although stress hormones can be measured in blood, their levels fluctuate, depending on the time of day and stress status. Stress hormones can, however, also be measured in hair because hormones that are in the blood that feed the hair follicle are incorporated in the hair as it grows. Thus, the hair that you grow today contains stress hormones at levels that reflect the current levels in your blood. Thus, for a person with long hair, e.g. 24 inches, you could measure stress hormone levels in each half inch of hair, which would represent that person’s stress response during the time that that half inch was grown. With human hair growing about a half inch per month, measuring hormone levels in each half inch of that person’s hair would give you insight into that person’s stress response over each of the past 48 months, like rings on a tree (Figure 1).

Note, however, that a person’s stress hormone levels are a combination of how much stress that person was under and how that person responded to that stress. In a group of pigs that were born and raised on the same farm at the same time, all pigs are exposed to similar amounts of stress and at the same time, i.e. they are weaned, mixed, handled and transported in a similar manner. Thus, measuring stress hormones in the hair of a group of pigs can provide information on how different pigs responded to the similar stressors that they were exposed to.

Figure 1. How cortisol is incorporated into hair. (adapted from Ghassemi Nejad, J., Ghaffari, M.H., Ataallahi, M., Jo, J.H. and Lee, H.G., 2022. Stress concepts and applications in various matrices with a focus on hair cortisol and analytical methods. Animals, 12(22), p.3096.)

To investigate this, we evaluated 15 groups of 60 or 75 Yorkshire x Landrace barrows, for a total of 863 pigs, with each group originating from a healthy multiplier farm from one of the breeding company members of PigGen Canada (Alliance Genetics, Alphagene, DNA Genetics, AccuFast, Genesus, Hypor, Topigs-Norsvin). Pigs were transported right after weaning to the research station the Centre de développement du porc du Québec in Quebec, Canada, where they were kept in a quarantine nursery for 19 days. At the end of those 19 days (~40 days of age), hair was shaved on an area on the rump of each pig, washed, ground and analyzed for levels stress hormones.

The hormone levels obtained reflect each pig’s responses to all the stressors that the pig was exposed to during its first 40 days of life. To better understand what these hormone levels tell us, all pigs were also subjected to a ‘back test’ at ~21 days of age, which is a standard technique to measure a pig’s innate stress response, or its coping style. The back test involves putting the pig on its back (see Figure 2) and counting the number of vocalizations and struggles that the pig performed during the first 30 seconds of being in this uncomfortable position, as well as the intensity of the vocalizations and struggles.

Figure 2. The back test.

Substantial variation in stress hormone levels in hair were found between pigs, and a substantial % of these differences were found to be the result of genetics, with heritabilities of 33% for cortisol and 30% for DHEA-S. These heritabilities show that it would be possible to select for pigs that produce lower, or higher levels of cortisol and DHEA-S, if that were desirable. Heritabilities for cortisone and DHEA in the hair of these healthy pigs were much lower (7 and 0%).

Interestingly, 9 to 17% of the differences between pigs were common to litter mates, which may reflect the impact that the environment the sow provides to her piglets has on the level of stress they are exposed to or, on how they respond to stressors. In addition, genomic analyses showed that 45% of the genetic differences in cortisol were caused by a genomic region on chromosome 2. This region harbors the glucocorticoid receptor gene, which codes for the receptor that cortisol and cortisone (they are called glucocorticoids) bind to in order to create its effect on tissues throughout the body. So, this gene likely has a mutation that is responsible for a substantial part of the differences observed in cortisol levels between pigs. The frequency of the allele that reduces cortisol levels was only 9% across all groups of pigs and pigs that carried one copy of that allele (very few were homozygous for this allele) on average had a 30% lower cortisol level in hair.

The levels to the four hormones in hair were positively correlated, with phenotypic correlations ranging from 0.26 to 0.55. However, the genetic correlations between the hormone levels were substantially larger, nearly one (0.96) between cortisol and cortisone, and 0.81 between cortisone and DHEA-S. This implies that a substantial proportion of the genes that increase cortisol also increase cortisone and DHEA-S. So, when you select to reduce cortisol levels, you also expect to decrease cortisone and DHEA-S (DHEA was not heritable, so no effect of genetic selection on this hormone at this age).

Responses to the back test were also substantially determined by genetics, with heritabilities of 57% for number of vocalizations and around 27% for the other three response traits. The four back test response traits also had fairly high phenotypic correlations, ranging from 0.53 to 0.85, and even higher genetic correlations, ranging from 0.60 to 0.99. Phenotypic correlations of the back test responses with stress hormone levels were, however, close to zero. But, genetic correlations between back test responses and stress hormone levels were moderately positive with cortisol and cortisone, as high as 0.22 and 0.58 for cortisol with vocalization number and intensity, respectively. Thus, when selecting for lower cortisol levels, you would expect to breed pigs that, behaviorally, may present a more passive coping style which may be positive, but would need to be explored to confirm the effects first.  

Ongoing research is investigating whether cortisol in hair of these healthy nursery pigs can be used to select pigs that are more disease resilient. This is based on the fact that cortisol is also associated with the immune system. In fact, an acute increase in cortisol reduces inflammation in tissues around the body and, therefore, boosts the immune system by limiting the associated damage from inflammation. However, chronically high levels of cortisol can make the body resistant to the effects of cortisol, which can lead to inflammation and a weakened immune system. We will be able to investigate this in these pigs because they were entered into a natural disease challenge right after their hair was shaved at ~40 days of age.

If the genes that affect cortisol levels in the hair of young healthy pigs also affect disease resilience, breeding companies can use this to select their lines to become more resilient to disease without ever having to expose them to disease and without requiring blood samples; a haircut is all that is needed! But more on that in a future issue. For now, we can conclude that a pig’s hair contains a lot of information on how a pig responds to stress and that a substantial portion of that is the result of genetics.

This project was funded by USDA-NIFA grant #2021-67015-34562, Genome Canada, Genome Alberta, Genome Prairie, Alberta Pork, PigGen Canada, and Results Driven Agricultural Research (RDAR). The natural disease challenge project was approved by the Animal Protection Committee of the Centre de Recherche en Sciences Animales de Deschambault (15PO283) and by the Animal Care and Use Committee at the University of Alberta (AUP00002227). Members of PigGen Canada are acknowledged for providing the pigs and input into the project, including: Canadian Centre for Swine Improvement, Fast Genetics, Genesus, Hypor, ALPHAGENE, Topigs Norsvin, DNA Genetics, the Canadian Swine Breeders Association, and Alliance Genetics Canada.