By Brett C. Ramirez, Iowa State University Department of Agricultural and Biosystems Engineering Graduate Research Assistant
As we begin to enter the summer months, it is essential we revisit the importance of identifying and reducing the impacts of heat stress. Failure to recognize, delay or correctly implement the best strategies to alleviate heat stress can result in reduced performance and a loss in profit from low market weights.
Every year, the U.S. swine industry experiences substantial seasonal productivity and economic losses from heat stress. Finishing weights and average daily gain dip well below average during late-summer months because of exposure to warm/hot conditions. This column will discuss what can be done to ensure that the implemented heat stress alleviation strategies in your barn are working correctly and if needed, how to improve them. These approaches use data already being measured and recorded with modern barn control systems and can be used to assess the thermal environment the pigs are experiencing to guide management decisions.
Let’s first start with a brief introduction to how pigs lose their excess generated heat to the environment, in order to later understand how our cooling systems remove generated heat. Pigs can release excess heat several ways — first, the pig can dissipate heat to the surrounding air. Skipping all the physics, what’s important is the temperature and speed of that air. A difference between the pig’s skin temperature and the air temperature must exist; hence, as the air temperature approaches the skin temperature of the pig (which is roughly 90 degrees F in warm/hot conditions), very little to no heat can be removed (regardless of airspeed). In addition, low airspeeds do not remove as much heat as higher airspeeds. This has to do with the pig’s body shape, direction of moving air, etc.
Evaporation is the next key mode of heat loss for a pig. This occurs via respiration (why we see heavy breathing during extreme heat stress) or by applying water to their skin via sprinklers (since they cannot sweat) and allowing it to evaporate. What’s important is the air temperature, moisture level and airspeed. The more saturated the air is with moisture, the longer it takes for pooled water to evaporate and the amount of heat that can be lost via evaporation decreases. Throw in low airspeeds, and evaporative heat loss loses much of its power.
Lastly, as long as there is a temperature difference between the pig and a surface, heat can be lost to that surface by directly laying on it or via radiation (think standing by a cold window). Key parameters for cooling success: air temperature, relative humidity, airspeed and surrounding surface temperatures.
The goal of tunnel ventilation is high airspeeds with less than a 3 degree F to 4 degree F rise down the barn length. So, we should not add too much additional heat above what is outside and make sure the airspeeds are sufficiently high down the length of the building. Just to reiterate, airspeed alone can help cool, but only if there is a temperature difference.
Evaporative coolers reduce air temperature, but cause an increase in moisture; however, they are rarely used in finishing. The cooler air alone is often not sufficient to cool the pigs, requiring elevated airspeeds as well. Sprinklers can be very effective with high airspeeds. However, both cool cells and sprinklers lose effectiveness in humid environments as the potential to evaporate water decreases.
As briefly mentioned in previous columns, today’s ventilation controllers have the capability to measure and record much of the data needed to assess the effectiveness of your ventilation/cooling system (except surrounding surface temperatures). For example, we’ll look at some data from an on-going research project in a standard, deep-pit, wean-finish facility that is about 200-feet long. It does not use any kind of evaporative cooling. The first example (Figure 1) demonstrates a common problem of not enough airflow. Looking at temperature (Figure 1, left), we see a large (~7.5 degree F) temperature gradient down the length of the barn creating a cold environment at one end and a much warmer environment at the other. Looking at airspeeds (Figure 1, right), we see a large gradient down the length of the barn creating a drafty environment at one end and stuffy/warmer environment at the other.
While this is a single snapshot in time, prolonged exposure to this situation will cause performance and finish weight differences for pigs at each end of the barn. The second example (Figure 2) demonstrates a well-designed tunnel barn where only a small temperature gradient exists (Figure 2, left) and airspeeds are elevated at recommended levels throughout the room (Figure 2, right). While this tunnel barn cannot do anything to lower the ambient temperature, it can reduce the heat accumulation down the length of the room and sustain sufficient airspeeds to help remove some heat from the pigs.
One thing to point out is, often times fans are sized for tunnels to achieve 300 to 400 feet per minute but don’t account for the number of pigs; hence, not enough airflow and heat accumulation occurs.
There are four key controllable parameters for cooling success: air temperature, relative humidity, airspeed and surrounding surface temperatures. All four interact with one another and can be leveraged to help reduce the impact of heat stress; however, we must understand how they work and how our ventilation system uses them in order to be successful. With data becoming readily available, analysis like this can help determine if more airflow is needed (i.e., inspect poorly performing fans or adding more), if the tunnel curtain is open enough, etc. Similar investigations can be performed on all different types of buildings, provided they have good and sufficient data.
Since we skipped the science behind how direct (sprinklers) and indirect (cool cells) cooling works, next month’s column will get into a detailed discussion on the science of cooling pigs and how that can lead to better management decisions.