Heat stress has negative effects on sow production and reproduction performance. Heat stress has multiple impacts on animal metabolism and physiology. These include: increased body core temperature, homeorhetic adaptations that increase blood flow to the skin to dissipate radiant heat, increased respiration rates to remove the excess heat, and reduced feed intake as a strategy to reduce heat production. As a consequence, sows under heat stress have reduced milk production, decreased litter weight gain, body weight loss during lactation, increased days from weaning to estrus and lower subsequent reproductive performance.
During the last three decades, continuous selection for increased litter size and milk production has increased heat production of sows and reduced their upper critical temperature. This selection has also resulted in decreased piglet birth weights and less body energy reserves at birth, requiring greater farrowing room temperatures, and the use of heat pads or heat lamps to provide local heat for piglets. Daily feed intake, milk production level and subsequent reproductive performance of sows are limited by the need to have higher room temperatures for increased piglet survival. If the anticipated environmental warming continues to adversely affect ambient temperatures, it will become more important that lactating sows be provided with some form of active cooling to maintain productivity and animal welfare.
The Animal Sciences and Agricultural Biological Engineering Departments at Purdue University have been working together for the last three years to develop a cooling pad able to remove locally and efficiently the excess of heat from lactating sows. First studies were focused to estimate the daily heat production of lactating sows. A heat production model was developed using the energy efficiency equations from NRC, (2012). The model estimates the heat production of sows with different daily feed intake and milk production levels during lactation. A lactating sow produces approximately between 350 to 650 watts of heat as a daily average and heat production increases as the parity and lactation progresses (Figure 1). Heat stress results when the heat energy produced by the animal is greater than the amount dissipated.
Figure 1. Predicted daily heat production of lactating sows by parity
Cooling pads technology is based on the conduction of the heat from the source (the animal) to the sink (chilled water running through the pipes). The thermodynamic laws govern the rate of heat transfer by temperature differentials. The design and the materials used in the cooling pads should be selected in order to increase heat removal. Aluminum and copper have a low heat capacitance, which means that rather than storing the heat, they are able to transfer it. The cooling pad has been built with a heavy-duty aluminum plate in the surface, high-density polyethylene base and copper water pipes attached to the aluminum plate. The first prototype had six rows of copper pipes and then an eight rows pipe prototype was developed to increase heat removal capacities. The eight rows pipe prototype was able to remove approximately 50% more heat than the previous prototype. All heat transfer and removal tests have been performed using an “artificial sow”, a bag with approximately 40 gallons of water heated at 104ºF (Figure 2). The initial pads had numerous thermocouples and were run through heat transfer trials with different cooling water flow options, including constant cool water flow, set times between cool water flushes and alternative pad temperatures to initiate the cool water flush. Pad dimensions were refined and the range of constant flow rates, time between flushes and desired upper set temperatures for different environmental conditions were estimated.
Figure 2. Heat removal test station (“artificial sow”)
The best prototype was then replicated, and 24 cooling pads were built. They have been used to run trials on sows during lactation. One of the trials was performed under severe heat stress conditions (95ºF and 69% RH) to test the cooling effect on behavior and heat stress markers in sows (respiration rate and skin, rectal and vaginal temperatures). Sows with no cooling spent more time in a “dog like posture” to increase body heat dissipation to the environment, less time on the feeder and less time milking in comparison to the sows with an active cooling of 0.85 L/min. After 80 min of cooling, sows with active cooling had lower respiration rates (45 versus 122 breaths/min), vaginal temperature (102.6 versus 104.2ºF), rectal temperature (102.2 versus 104.0ºF) and skin temperature (101.5 versus 102.9ºF) than sows with no active cooling, respectively. Heat removal was three to four times greater than previous cooling pads made of concrete and steel pipes.
Pipestone continues to evaluate new technologies like cooling pads to bring a competitive advantage to our producers. For widespread implementation and use of this technology, long-term studies should be performed in commercial barns to estimate the direct and indirect effects of heat stress, and the benefits on sows’ lactation and reproductive performance of cooling pads technology operation. In the near future the use of cooling pads should be investigated on boars, growing finish pigs and gestating sows.