By P.E. Urriola, G.C. Shurson and L.J. Johnston, University of Minnesota
Summer is rapidly approaching and this means hot and humid days. Heat stress causes losses estimated at $300 million for pork producers in the United States. Therefore, it is important that we review the impact of heat stress on physiological functions, describe its implications to animal productivity, and suggest nutritional strategies to mitigate these negative effects. There are numerous management practices that also play important roles in helping relieve heat stress on farms. However, in this column we will focus on nutritional interventions that are useful when heat may overwhelm other management practices.
Effect of environment on pig performance
The ability of pigs to regulate body temperature depends on the balance between heat production and heat losses. As ambient temperature decreases below the lower critical temperature, heat production and heat conservation increases to maintain body temperature.
As ambient temperature increases above the upper critical temperature the ability of pigs to dissipate heat decreases and body temperature increases. The temperature between the LCT and the UCT is the thermoneutral zone. In the thermoneutral zone, pigs can adequately balance heat production and heat losses so that they remain comfortable. At temperatures in the thermoneutral zone, heat production is the consequence of using metabolizable energy for maintenance and productive processes such as growth.
Muscle and fat deposition occur as the pig grows. These productive processes generate heat and some of this heat must be lost for the pig to remain comfortable, especially under heat stress conditions. The amount of heat generated is dependent on the process for which energy is utilized (greater in muscle deposition than fat deposition) and the dietary origin of the energy. The efficiency of energy utilization (ratio between metabolizable energy and net energy) is lowest for muscle deposition (about 60%) and greatest for fat deposition (about 80%). Likewise, dietary source of energy also impacts energy efficiency. Efficiency of utilization of dietary energy is greatest for dietary lipids (90%), intermediate for starch (82%) and lowest for crude protein (60%) and dietary fiber (60%). From the pig’s perspective, greater efficiency of energy utilization means there will be less heat that needs to be lost and greater chances to remain at comfort in hot conditions.
Dietary interventions to help pigs cope with summer heat
The practical implication of these differences in dietary energy sources is that during summer when heat dissipation is compromised, pigs benefit from modifying the source of dietary energy so that they have less heat to lose. Ideally, summer diets would contain greater energy from lipids and starch and less energy from protein and fiber. This can be accomplished by using highly digestible sources of protein that closely match the amino acid requirement of pigs and decreasing use of low energy density ingredients (Table 1).
Heat stress is the condition when pigs cannot balance heat production and losses. The physiological effects of heat stress are diverse and still under investigation. There is evidence that heat stress increases permeability of the gut to pathogens, decreases insulin sensitivity, and has multiple effects on amino acid metabolism. Heat stress of pregnant sows can have long-lasting effects on the resulting piglets after farrowing.
Therefore, mitigation strategies which include modifying the pig’s environment and specially formulating diets for summer heat stress conditions can be helpful. Dietary modifications for summer include shifting ingredient choices as shown in Table 1. Other dietary modifications that show potential to help pigs cope with heat stress conditions include addition of selenium or zinc.
A new technique of liquid chromatography mass spectroscopy used at the University of Minnesota has demonstrated that there are additional changes to metabolism with potential for development of more effective interventions. However, the effectiveness of these newer dietary manipulations is still to be proven. The University of Minnesota has also developed a model of environmental heat stress in near-commercial conditions that is effective in simulating summer growth conditions and can be used to test manipulations to help pigs cope with summer heat.
In summary, hot days are approaching; diet modification is one tool that should be paired to other tools such as ventilation, water supply and cooling systems for a comprehensive management of heat stress.
Baumgard, L. H., and Rhoads, R. P. (2013). Effects of Heat Stress on Post-absorptive metabolism and energetics. Annual Review of Animal Biosciences, 1, 311–337.