Keeping swine in the thermoneutral zone

Estimating hot conditions affecting pigs raised inside buildings and making modifications to reduce the risk of heat stress.

Pig in front of fan
Figure 1: Fans can improve air circulation in barns and help reduce risk of heat stress in pigs. Photo courtesy of Casey Zangaro, MSU-Extension

Importance of the Thermal Neutral Zone

Every so often, we are shown that "everything old is new again.” Known for his outstanding research, teaching and outreach programs in farm-animal environmental physiology, behavior, and care, the renowned Stanley Curtis defined the animal’s thermal neutral zone (TNZ) in his classic textbook, Environmental Management in Animal Agriculture. In narrative and graphical ways, Curtis described this zone or state where an animal is neither too cold nor too hot. In its 409 pages, the book provides calculations of effective environmental temperature, or what the animal really feels, which is not the ambient temperature reported by smartphone meteorologists. Quantification or ‘how to measure’ heat exchanged by the animal and its environment via radiation, convection, conduction, and evaporation are explained in this must-read textbook if we are to understand the housing and management changes that may be needed on our farms in response to climate change. Before making any change, the potential impact of heat stress and the amount of benefit that may result from the structural, mechanical, or management modification must be calculated and compared to other changes that may be made. We need to know the most efficient and cost-effective way to keep pigs comfortable in their TNZ. Failure to keep pigs within their TNZ, especially over prolonged periods, can lead to significant losses in performance and health.

Potential adaptations for keeping pigs in their TNZ

There are many possible changes in management and housing that can be made to help keep animals within their TNZ and thereby reduce the risks of heat stress. According to Curtis, some of these changes include increasing ventilation rate, using fans to produce local air movement, reducing relative himidity, use of cooling cells, providing shade, reducing stocking density, providing cooled drinking water, reducing dietary fiber, increasing dietary fat, intermittent water sprinkling, and installation of an  earth-air exchange system.

For most pork operations, including many small-holder farms, it would be impossible to implement more than a few of the adaptive measures described above. So, is the cost of adding fat to decrease the heat increment of metabolism more effective and more affordable than cleaning fan blades? Is decreasing the number of pigs per pen more effective and more affordable than adding a sprinkler system or circulation fans? What unit of measure do we use to assess differences in effectiveness? The ventilation system may be the ‘most’ effective and affordable means of keeping animals cool and productive. That system will involve the most efficient sensors and fans. That system will make sure that the air movement is at the pig level and provide convective heat removal from their bodies. The next ‘best’ may be to cool animals using evaporative heat loss by adding intermittent sprinkling systems throughout the barn. In the future there may be ‘better’ cooling cells put into barns. Perhaps all new barns should have earthen air exchange systems once again because they provide the ‘greatest’ impact per dollar invested.

Comparing the effectiveness of production modifications

Few studies have sought to answer these types of questions in a clear fashion so that producers know what to do as we have more consecutive warm days and persistently warm nights per year. A recent article published by the Intergovernmental Panel on Climate Change compared seven production modifications. They concluded that air preparation systems, evaporative cooling cells and earth-air heat exchangers will reduce heat stress in finishing buildings ‘better’ than, for example, doubling ventilation rate or reducing stocking density. The table below shows a ranking of modifications when heat stress is described using a temperature-humidity index. Based on their analysis and looking at results comparing two popular adaptive measures, stocking 40% less pigs provides a 9% reduction in risk of heat stress, while a 33% reduction can be achieved by encouraging feeding time during the coolest half of day. The report did not include a description of the base feeding system, whether meal-feeding as is more common in Europe or in ad libitum feeding situations as we have in north America.

Table 1. Relative impacts of in-barn adaptations to help minimize risk of heat stress in pigs.


Reduction factor, %

Conventional barn (Control)


Stocking 20% less pigs


Stocking 40% less pigs


Shifting feeding time to coolest half of day


Doubling ventilation rate


Cooling cells plus heat exchanger


Cooling cells only


Earth-air heat exchange system



Although not comparing all possible modifications, these researchers have provided a modeling approach to help producers consider the effectiveness of changes in the barn that can be made. The cost of the modification must be considered next and should factor in the time needed to implement the change and its expected longevity. Expertise required for operation and maintenance will also need to be considered. The authors conclude aptly that “data for the design and planning of modifications have to be known early enough for farmers, consultants, and veterinarians to ensure a high level of sustainability in livestock production.” We need to continue to make quantitative comparisons of all the possible management and housing changes with a goal of preventing heat stress in our pigs.

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