In the past few decades, the livestock industry has made several advancements through genetics, nutrition and reproductive technologies that has improved litter sizes, growth, reproductive efficiency and disease resistance in our animals. Despite those advancements, stress still remains a limiting factor to efficient livestock production.

"We know that the stress response is going to impact their health and performance outcomes. We see things like decreased growth performance, impaired reproductive success and an increase in disease susceptibility in these animals that are succumbing to stress," said Jay Johnson, Research Leader with the USDA-ARS Livestock Behavior Research Unit. "And when we think about pigs specifically, they could be exposed to a variety of potential stressors throughout their life cycle. We see things like thermal stress, particularly heat stress, transport stress, immunological stressors, as well as weaning stress."

During the recent Kemin Intestinal Health Symposium in Palm Springs, California, Johnson shared the work his team at the USDA-ARS Livestock Behavior Research Unit in West Lafayette, Indiana, are conducting in terms of gut health and the effects of weaning and transport stress in the pre-weaned and post-weaned pig.

While weaning and transporting pigs prevents disease transfer and improves overall herd health long term, the events often occur simultaneously and can negatively impact pig health and performance in the short term.

"Transport is a period of high stress on a pig's life. We know that multiple stressors are present during this process, things like handling the animal, removal from the dam, mixing of unfamiliar animals, restriction of food and water in the trailer and transport conditions, things like transporting pigs in the winter months or even in the summer months, can increase the stress response in animals," Johnson said. "As potential consequences, we can see that the pig strain response to weaning and transport can have impacts on the short-term growth and health of that animal. We can see things like depressed feed intake and growth initially, and increased prevalence in treatment for enteric diseases and greater morbidity and mortality at least in the first one to two weeks after the weaning and transport process. We also know that that strain response can be exacerbated by environmental factors and/or preexisting conditions in that pig."

Johnson's research team is focused on seasonal stress and in utero stress events. For example, when pigs are transported during summer months, it can exacerbate post-weaning and transport growth depression, the pigs can have hyperthermia-related health issues, such as greater intestinal damage and increased inflammation, and an increased stress response with greater circulating cortisol. For pigs that were transported in the summer versus pigs that were transported in the spring, the researchers found a 311% increase in circulating cortisol following transport.

In addition to seasonal heat stress, exposure to heat stress while in utero can have negative long-term impacts on the ability of the weaned and transport pigs to recover, said Johnson. This can exacerbate post-weaning and transport growth depression, increase the behavioral stress response and alter the physiological stress response, with increases in cortisol and adrenocorticotropic hormones.  

While there are various management and nutritional strategies to try to combat heat stress, the pressure to decrease antibiotic use in swine production has had repercussions on stress events.

"Unfortunately, we know that removing antibiotics or removing this technology from swine production can have a variety of negative effects on pigs," Johnson said. "We see a decrease in feed intake and growth performance of these pigs, an increase in therapeutic treatment events. We also see a variety of poor welfare outcomes in these pigs … we see things like larger tear stains, and this is a proxy for greater stress that Dr. Jeremy Marchant, a USDA-ARS research animal scientist, discovered. We've also seen an increase in lethargy in these pigs. They're less active after that weaning and transport event if you removed antibiotics from the diet, and we see an increase in lesion counts of these pigs, which is indicative of greater aggression post weaning and transport. Finally, we see an increase in morbidity and mortality in these pigs."

In the past 10 to 15 years, researchers have been examining a variety of different antibiotic alternative supplements, such as bacteriophages, probiotics, prebiotics, minerals, plant extracts, essential oils and lysozymes.

Johnson's research team has been particularly interested in the use of a conditionally essential amino acid called L-glutamine. A major energy source for enterocytes and lymphocytes, the immunomodulator can inhibit pro-inflammatory cytokines and its use by the body is greater than its synthesis during catabolism and stress. L-glutamine is commercially available for human use for muscle development, immune function and intestinal health, and the European Food Safety Authority recognizes it as "safe and efficacious" for swine.

Research conducted on L-glutamine has demonstrated it improves the health and growth performance of piglets and is more effective than glutamate in improving intestinal health and nutrient digestibility. However, most studies have been conducted to just understand its mechanisms.

Johnson's research team decided to examine the applied use of L-glutamine in weaned pigs when compared to antibiotic use, specifically chlortetracycline and tiamulin, which are commonly used to improve growth and reduce morbidity in the nursery phase. Their weaning and transport model followed a 12-hour timeline to mimic a trip from North Carolina to Indiana, since prior research has shown an 8-hour transport is the peak stress point for weaned piglets. The L-glutamine was supplemented at 0.20% as-fed.

"We do this for two reasons. One, we found that it's cost competitive with some antibiotic programs, at least based on commercial prices, and there is one research paper out there that had shown a numerically greater gain to feed at this level, at least about a 9% improvement," Johnson said.

The team examined two different weaning transport procedures. The first was a stimulated transport plus controlled conditions, with every stressor associated with transport included — social mixing, feeding, water withdrawal, space requirements and air removal. The researchers housed the pigs in a trailer model and then individually housed the pigs to get a controlled look at how L-glutamine performs.

However, Johnson said the majority of their research was done under actual transport and production conditions, where pigs were weaned and transported via a 12-hour loop on 50% two-lane and 50% four-lane roads. The pigs were then group housed.

For the simulated transport, the researchers found in the first 14 days post-weaning and transport, the L-glutamine fed animals actually consumed more feed, particularly in the second week post-weaning and transport when compared to the antibiotic or no antibiotic fed pigs.

"As a likely direct result of this, we found that our L-glutamine animals gained significantly more body weight compared to both our antibiotic and our no antibiotic fed pigs during this time period," Johnson said. "We didn't actually expect to see this, but it's something that we were very excited to see."

The research team also found a decrease in lying behavior in the pigs fed antibiotics and L-glutamine and more activity/less lethargy when compared to pigs not provided antibiotics.

For the research group with full transport and production conditions, the team saw a great improvement in the L-glutamine animals in terms of feed intake and growth. A similar response was noted in average daily gain under the production relevant conditions. They also looked at things like inflammatory biomarkers and found the antibiotic included and L-glutamine-fed animals had a similar reduction in biomarkers when compared to pigs not given any antibiotics.

In addition, the team examined the pigs for aggressive behaviors or stress indicative behaviors.

"Looking at aggressive behavior, two days post weaning and transport, what we found was that there were fewer skin lesions for our L-glutamine and our antibiotic fed pigs when compared to the pigs not fed antibiotics, indicative of potentially reduced aggression for these pigs provided with L-glutamine or antibiotics," Johnson said. "We have that social mixing; we have fighting to establish that social hierarchy. We expect to see more aggression that period, but when we supplement the L-glutamine and antibiotics, we actually see a decrease in this effect."

With larger tear stains generally associated with increased stress in pigs, the team also examined tear stain size. At day 84 and day 110 post-weaning and transport, they saw smaller tear stains for L-glutamine fed animals, a potential indicator of decreased stress, Johnson said.

"What's really interesting about this effect is that our dietary treatments only lasted for 14 days post-weaning and transport, but we're seeing a long-term effect all the way to the grow-finish period of a decrease in stress response from pigs supplemented with L-glutamine," Johnson said.

The team also wanted to look at behavioral indicators of anxiety and fearfulness in the pigs and conducted a novel object test. With dietary treatments ending on day 14, they presented the pigs with a novel object on days 16, 46, 85 and 111. The objects presented included a pool noodle, a traffic cone, a PVC pipe and a metal trash can. They then timed how long it took for those animals as a group to come and interact with that new object in the pen, with the idea that the longer it took for them to interact, the more anxiety or fearfulness the pigs must have.

"What we found was that both our L-glutamine and our antibiotic fed pigs were less anxious and fearful later in life. They spent more time exploring the object on day 85, and the latency to interact or the time period that it took for the animal to go and touch that object after we presented it, was much shorter versus the animals not given any antibiotics on day 111," Johnson said. "Again, that's a long-term effect of that short-term dietary treatment that we're seeing."

Since the research team had success in just feeding L-glutamine by itself, they decided to mix it with a combination of probiotics and prebiotics in a follow up study.  This time the stocking density was about 172% greater than industry standard, and treatments were 0.20% glutamine, symbiotics, 0.20% L-glutamine and synbiotics, antibiotics, or no antibiotics for 14 days followed by a common diet at 21 days.

Johnson said the "moral of this story was if you house pigs at 172% greater than normal stocking density, you're not going to see a lot of dietary differences." Although they did observe some slight dietary treatment benefits to immune and intestinal measures, no other positive effects of the diet treatments were observed, which potentially means that the overall reduction in stress led to reduced efficacy of the dietary treatments, even antibiotics, Johnson said.

"We know that the negative effects of weaning and transport stress can be compounded by external or pre-existing conditions. L-glutamine supplementation may be a viable solution to improve post-weaning and transport health, performance and welfare in pigs," Johnson said. "We found improvements in things like intestinal function, growth performance and physiological behavioral stress responses, and that physiological stress response may directly interact with some of our nutritional supplements, which I think we've known anecdotally for years, but we're really interested in looking at this more experimentally."