Shedding light on the effects of heat stress in growing pigs

Tuesday, June 3, 2014

U.S. research suggests that the largest metabolic difference between heat-stressed and nutrient-restricted thermal-neutral pigs was the increase in circulating insulin levels, causing differences in response between muscle and adipose tissue

by JANICE MURPHY

Heat stress has a significant impact on the swine industry, with estimated annual losses of over $300 million in the United States and billions globally. These economic losses result from increased morbidity, mortality (especially in market weight pigs), poor growth rates, inconsistent market weights, inefficient nutrient usage, unsatisfactory sow performance, decreased carcass value and carcass processing problems. It has been well established that pigs raised under heat stress conditions have reduced muscle mass and increased fatty tissue.

Whether heat stress is directly or indirectly responsible for performance issues by compromising feed intake is not currently understood. Research has shown that post-absorptive changes in nutrient partitioning, as represented in some heat-stress models, do not mirror normal metabolism as observed in thermal-neutral animals on a similarly restricted level of nutrient intake.

To shed some light on this issue, researchers at Iowa State University set out to differentiate between the direct and indirect effects (mediated by reduced nutrient intake) of heat stress on production and post-absorptive metabolism in growing pigs. Their original hypothesis was that heat had a direct (independent of reduced nutrient intake) impact on pig performance and post-absorptive metabolism, and that this change in energy balance may partially explain the increase in carcass fat levels in heat-stressed pigs.

Commonly, the immediate impact of heat stress is a decrease in intake, and this reduced appetite is likely the body's way of minimizing the amount of heat produced by metabolic processes. In the swine industry, limited caloric intake has obvious effects on growth and metabolism. In order to mitigate this effect, and to allow differentiation between the direct and indirect effects of heat stress, the researchers used a thermal-neutral ad libitum model and a pair-fed, thermal-neutral model to eliminate the confounding effects of dissimilar feed intake.

In the study, 48 crossbred gilts (35 kilograms bodyweight) were housed in climate-controlled rooms in individual pens and exposed to thermal-neutral conditions (20 C; 35 per cent to 50 per cent humidity) with ad libitum intake (n = 18) heat stress conditions (35 C; 20 per cent to 35 per cent humidity) with ad libitum intake (n = 24) and pair-fed in thermal-neutral conditions (n = 6).

Pigs in the thermal-neutral and heat-stress conditions were sacrificed at days one, three and seven of the study, while the pair-fed thermal-neutral pigs were sacrificed at day seven of the experiment. Individual measurements of rectal temperature, skin temperature, respiration rate and feed intake were taken daily.

Pigs that were exposed to heat stress exhibited a significant increase in rectal temperature (39.3 C versus 40.8 C) and a doubling in respiration rate (54 versus 107 breaths per minute). On day one, heat-stressed pigs experienced an immediate and significant 47 per cent drop in feed intake, and this decrease continued for the duration of the experiment.

By design, the nutrient intake pattern for the pair-fed, thermal-neutral controls mirrored the heat stress group. By day seven, the thermal-neutral and heat-stressed pigs gained 7.76 and 1.65 kilograms in body weight respectively, whereas the pair-fed, thermal-neutral pigs lost 2.47 kilograms.

By day seven, plasma insulin levels were 49 per cent higher in heat-stressed pigs than in pair-fed, thermal-neutral controls. Reducing feed intake typically results in an abnormally low level of insulin in the blood. Pigs from the pair-fed, thermal-neutral model had reduced plasma insulin compared with heat-stressed and thermal-neutral pigs on day seven.

Both heat-stressed and pair-fed, thermal-neutral pigs were nutrient-restricted by about 50 per cent, and, although both had lower circulating insulin levels compared with the thermal-neutral pigs, the heat-stressed pigs exhibited 100 per cent higher insulin concentrations than the pair-fed, thermal-neutral pigs. This is unusual as both sets of pigs were consuming the same amount of nutrients, although inadequate in terms of their requirements.

Overall, plasma glucose concentrations were 7.1 per cent lower in the heat-stressed pigs compared to the thermal-neutral controls. This decrease appears to be mainly due to decreased feed intake, as no differences between heat-stressed and pair-fed, thermal-neutral treatments were detected on day seven.

On day seven, the level of total protein in plasma significantly increased by 7.4 per cent in heat-stressed compared with thermal-neutral pigs and was incrementally increased in heat-stressed and pair-fed, thermal-neutral compared with thermal-neutral pigs.

Heat stress likely had an impact on protein metabolism, as illustrated by the obvious changes in body weight, and this is played out in how environmentally induced hyperthermia affects a variety of species, not just pigs. This marked change in body weight is further supported by a 31 per cent increase in circulating concentrations of methylhistidine in heat-stressed pigs on day seven. (Methylhistidine is considered a reliable marker of skeletal muscle breakdown.)

When growing animals are undernourished, they typically alter their metabolism to mobilize adipose tissue, a classic glucose-sparing strategy that assigns priority to the growth of skeletal protein. In this study, plasma-free fatty acid concentrations were 110 per cent higher in pair-fed, thermal-neutral pigs compared with thermal-neutral and heat-stressed pigs. However, heat-stressed pigs only had increased plasma-free fatty acid concentrations on day one, returning to similar free fatty acid levels as thermal-neutral pigs on days three and seven. This increase in free fatty acids at the beginning of the study suggests that the mobilization of adipose tissue may have partially contributed to the sudden loss of body weight in the heat-stressed pigs.

It is obvious that heat stress has a significant negative impact on body weight gain in growing pigs. Based on the results of this study, the researchers concluded that, despite similar nutrient intake, heat-stressed pigs gained more weight compared to pair-fed thermal-neutral controls. They noted that the largest metabolic difference between heat-stressed and nutrient-restricted thermal-neutral pigs was the increase in circulating insulin levels. There are definitive differences in response between muscle and adipose tissue (resistant and sensitive, respectively) to these higher insulin levels. Heat-stressed pigs do not mobilize adipose tissue triglycerides, but increase skeletal muscle protein breakdown.

The researchers suggest that these unique changes in post-absorptive fat and protein metabolism likely explain why heat-stressed pigs minimize lean tissue growth and re-prioritize carcass lipid accumulation to a level greater than expected. BP

Source: S. C. Pearce, N. K. Gabler, J. W. Ross, J. Escobar, J. F. Patience, R. P. Rhoads and L. H. Baumgard. 2013. The effects of heat stress and plane of nutrition on metabolism in growing pigs. J ANIM SCI 91:2108-2118.BF

Janice Murphy is a former Ontario agriculture ministry swine nutritionist who now lives and works in Prince Edward Island.