Simulating Heat Stress on Finishing Pigs

Lee Johnston, Swine Scientist

About one year ago, my newsletter article was entitled “Heat Stressed Pigs in Winter?!”  The topic of that article was a heat stress study we had under way in our swine finishing barn at the West Central Research and Outreach Center (WCROC).  Heat stress of pigs is a common problem on Minnesota pig farms and leads to poor growth performance, compromised reproductive performance, and can increase sow and pig mortality in extreme cases.  Finding ways to mitigate the negative effects of heat stress on pigs would improve pig welfare, enhance pig performance, and increase the efficiency of pork production systems.  The study was funded by the Minnesota Pork Board and was designed with two main objectives.  First, we wanted to document the magnitude of negative effects on finishing pigs that were exposed to heat stress.  Pork producers know there are negative effects of heat stress on pigs but the magnitude of depression can be hard to measure on commercial farms.  Secondly, we wanted to establish that we could re-create heat stress in pigs using our mechanically-ventilated barn.  If we could demonstrate that our approach would re-create heat stress, then we would have a system that could study approaches to control heat stress.  

Our study used two mirror-image rooms located in the confinement swine finishing barn.  One room was designated as the “Cool” room.  This room was our control for comparison.  The second room was designated as the “Heat” room or the room in which we would try to heat stress the pigs.  We achieved these different environments within the rooms by manipulating the furnaces and ventilation fans in each room.  We were able to consistently maintain a much higher temperature in the Heat room compared with the Cool room (Figure 1).  We were able to maintain a significant difference in room temperature between the Heat and Cool rooms because the outdoor temperature was fairly cold through most of the study period.  

Ave daily room temperature

Figure 1. Average daily room temperature for both Heat and Cool treatment rooms and outdoor temperatures.

Pigs in the Heat room did suffer from heat stress because their resting respiration rate was consistently higher than pigs housed in the Cool room (Figure 2).  Elevated respiration rate is a very sensitive indicator of heat stress in pigs.  One of the first responses of the pig’s body to heat stress is to reduce their voluntary feed intake.  Pigs in the Heat room had significantly lower average daily feed intake than did pigs in the Cool room (Figure 3).  This response further documents that pigs in the Heat room suffered the negative effects of heat stress.  However, this reduction in feed intake had only very small effects on daily weight gain of heat-stressed pigs.  Heat-stressed pigs gained 2.03 lbs. daily compared with pigs in the Cool room that gained 2.09 lbs. daily.  This difference in average daily gain was smaller than we expected.  Nonetheless, pigs housed in the Heat room weighed 4 pounds less at the end of the study and gained almost 6 lbs. less over the entire study compared with pigs housed in the Cool room (Figure 4).  

This study demonstrated that we can successfully simulate heat stress in finishing pigs under controlled conditions in our research barn.  The heat stress that we imposed on the finishing pigs did reduce pig performance as expected.  But, the magnitude of that depression was not as large as we anticipated.  With a few modifications of our heat-stress protocol, we can increase the magnitude of performance depression.  Now that we know we can successfully impose heat stress on pigs under controlled conditions, we can investigate methods of reducing heat stress on pigs in future studies.


Respiration rates

Figure 2. Respiration rates of randomly-selected, resting pigs recorded at four times throughout the study in Heat and Cool rooms.

Average daily feed intake

Figure 3. Average daily feed intake for pigs in the Cool and Heat rooms throughout the experiment.

Average body weight

Figure 4. Average body weight of pigs housed in Cool and Heat rooms.

Want more info?

Check out our Swine Papers page, or our Swine research pages.