The Physiological Effects of Stressors on Performance

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Understanding physiological responses is key to explain why stressors, such as heat, cause a decline in performance during the summer months.

Stress has been defined as the sum of all biological reactions to mental, emotional and physical stimuli that disrupt an individual’s homeostasis, as it relates to bodily functions. There are a myriad of internal and external stimuli that can affect the homeostasis of an individual. Once homeostasis is disrupted, the body elicits physiological stress responses involving the immune system in an attempt to reestablish homeostasis.

An environmental stressor that will be on many swine producers’ minds as we enter into the summer months is heat stress. Heat stress will cause an animal to redistribute blood to the periphery to maximize radiant heat dissipation in an attempt to maintain safe body temperatures. The redistribution of blood causes vasoconstriction of the blood vessels supplying to the gastrointestinal tract. The vasoconstriction leads to barrier dysfunction, resulting in compromised intestinal integrity due to decreases in nutrient and blood flow.

This leads to decreased water and feed consumption in the pig, causing dehydration. Decreased water intake and osmotic imbalance intensifies the damage and sloughing of intestinal cells, exacerbating the damage. The intestinal epithelial barrier consists of tight junctions and secretory proteins that are in control of selectively absorbing the nutrients and preventing the absorption of harmful molecules or toxins from the luminal contents. The breakdown of the intestinal epithelial barrier will elicit an immune response due to the increase of harmful molecules and toxic substances, including bacterial lipopolysaccharide (endotoxin, LPS) and mycotoxins from the intestinal lumen passing through and entering into the bloodstream. The activation of the immune response will also increase energy costs.  

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Figure 1: Intestinal lining (ileum) in pigs before (left) and after (right) eight days of heat stress.

Furthermore, heat stress and the resulting damage to the intestinal barrier increase maintenance requirements due to amplified energy expenditure originating from panting, changes in body temperatures (maintaining a safe body temperature becomes the top priority) and upregulated biological processes (activation and maintenance of the immune function) in attempts to maintain homeostasis. Increased body temperature significantly affects biological systems. A 1 degree C change in body temperature alone has the potential to increase caloric need by 7 to 15%.1                                         

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Figure 2: Effect of temperature on performance of growing-finishing pigs in thermoneutral and heat stress temperature ranges.2

The swine industry experiences significant economic loss each year due to heat stress. These losses can be attributed to reduced growth and efficiency, decreased fertility, increased health care costs, decreased carcass value and increased mortality. Heat stress has the potential to alter normal metabolism by increasing lipid production and decreasing muscle mass. Paradoxically, heat stress can also increase basal concentrations of the anabolic hormone insulin and increase insulin sensitivity despite having decreases in feed intake. This leads to reduced basal plasma non-esterified fatty acids concentrations, which are a significant source of energy for animals under stress conditions. Lack of NEFA available as a fuel source causes animals to increase their production of and reliance on glucose as a fuel source by other measures, including catabolic breakdown of muscles.

Heat stress is a pertinent factor that will significantly affect the productive efficiency of pigs over the next few months; however, many of the physiological responses to heat stress are similar to other stressors. Specifically, many of the gut level insults and decreased productivity discussed here are also experienced by pigs as they experience weaning stress, abrupt dietary change or a prolonged out-of-feed event. Thus, it is imperative that the animal’s nutritional requirements are being met in order to help them through these stressors with minimal effects on productivity.

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References
1) Kluger, M. J. (1978). The evolution and adaptive value of fever: Long regarded as a harmful by-product of infection, fever may instead be an ancient ally against disease, enhancing resistance and increasing chances of survival. American Scientist, 66(1), 38-43.
2) Adapted from Coffey, R. D., Parker, G. R., & Laurent, K. M. (1995). Feeding growing-finishing pigs to maximize lean growth rate. University of Kentucky, Lexington, and Kentucky State University, Frankfort. Cooperative Extension Service. ASC-147.



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