By Zhenbin Zhang

Editor’s note: Zhenbin Zhang is a swine nutritionist for Cargill Animal Health. He can be contacted at

While heat stress is highest in summer, production is affected all the way from conception to marketing. Because pig prices tend to peak in summer, just like temperatures, this is the time of year when producers should be concerned the most.

Many producers know first-hand that environmental temperature can impact pig production and reproduction. The target temperature level, called the ‘thermoneutral zone,’ offers the best production efficiency.

Ideal temperatures vary for pigs of different ages and sizes. For nursery pigs weighing five to 20 kilograms, the optimum temperature is 27 degrees-Celsius; for growing pigs weighing 20 to 55 kilograms, 21 degrees-Celsius; for finisher pigs weighing 55 to 110 kilograms, gestating or lactating sows and mature boars, 18 degrees-Celsius.

Understanding the impacts of heat stress

Figure 1

As shown in Figure 1, feed intake in grower-finisher pigs decreases at the same rate as environmental temperatures increase. However, somewhere around 30 degrees-Celsius, growth rate plunges and feed-to-gain ratio spikes, with a breaking point at the thermoneutral zone.

Feed efficiency as a component of feed-to-gain ratio is not changed when the ambient temperature is below 30 degrees-Celsius. When the ambient temperature reaches above that point, feed-to-gain is compromised by 20 per cent, compared to a temperature of 20 degrees-Celsius. At the same time, modern genotypes could be more sensitive to heat stress than older genotypes of lesser growth potential. 

In sows, heat stress has been reported to affect physiological characteristics, including increased rectal and skin temperature, and breathing rate. Behavior-wise, heat-stressed sows stand less while also sitting and sleeping more. As a result, heat-stressed sows are heavier, with bigger loins and more backfat.

For dry sows, heat stress occurs when temperatures rise to more than 27 degrees-Celsius, which can delay or prevent the occurrence of estrus, increase wean-to-service intervals, embryonic deaths and abortion rates, while reducing conception and farrowing rates, along with litter sizes. Heat stress is also documented to shorten gestation length by approximately one-and-a-half days. Heat stress in gestation can also lower the birth weights of piglets from approximately 1.4 to 1.2 kilograms, but this has no effect on piglet weaning weights compared to when sows are in the thermoneutral environment zone during lactation.

Figure 2

For lactating sows, heat stress reduces feed intake, which leads to negative energy balance, loss of body condition and reduced milk production. As a result, piglet growth is reduced during lactation with lower weaning weight. Figure 2 shows the impact of different ambient temperatures on lactating sow performance. When the temperature reaches 27 degrees-Celsius and above, all parameters are worse than those under 22 degrees-Celsius. When the temperature reaches 29 degrees-Celsius, feed intake, body weight, body lipid and body protein drop significantly, which causes lower milk yield and reduced piglet growth and weaning weights.      

Heat stress not only impacts sows but also their offspring. When sows are under heat stress during gestation, their offspring barrows have higher feed intake but reduced body weight and increased fat deposition at finishing, which contribute to poorer feed efficiency. The offspring gilts have lower litter sizes and weaning performance, while offspring intact boars show decreased sperm production and quality.

Based on everything we know about how heat stress affects all types of pigs at different stages, the impact of year-round heat stress becomes clear. If sows farrow in early July, they will likely experience heat stress throughout summer, into the lactation period, which will result in lighter weaning weights for hogs grown and finished at market weight in January. These sows will again be bred in early August.

Because of the summer heat stress impact, sows will have poor body condition at weaning, which contributes to longer wean-to-service intervals and lower conception rates. In August and September, heat stress on sows will affect embryo survival, leading to reduced litter sizes and fewer repeats. Because of heat stress during gestation, offspring pigs that are farrowed in December will grow slower. This means that the impacts of heat stress in summer will affect production throughout the year, going forward.

Improving conditions when it counts most

Heat stress impacts generate real financial losses for producers, which makes it important to take all possible measures to minimize the negative outcomes.

In terms of barn environment, things like adequate insulation and ventilation are key. From a management perspective, ensuring pigs have enough water to drink, to stay cool, plays a crucial role in reducing mortality. Using misters and cooling pads for lactating sows can help.

In terms of a nutritional approach to mitigate heat stress, the use of high-quality fats and fewer fibrous ingredients are common practices. Adjusting dietary lysine levels during lactation to match feed intake is important for maintaining milk production. Adding phytogenics to feed can drive up intake by seven per cent, which can help prevent lactating sow feed intake from dropping.

Many approaches to reduce mortality can be applied when combatting heat stress. In addition to those previously referenced, Vitality Technology from Cargill has been proven to greatly reduce mortality.

In general, hog prices peak in summer, which also drives up the optimum market weight by two to three kilograms, but summer heat stress can lower market weights by the same amount. Consequently, there will be a deficit of four to six kilograms of weight that needs to be made up through a combination of enhanced nutrition and other strategies. To determine optimal levels of energy and amino acids in the diet, modeling can help producers understand and close the gap, taking advantage of those summer hog prices.

With everything we know about how heat stress affects various production factors, producers should consider comprehensive approaches to address barn temperatures, management practices and nutrition to achieve the best possible production and financial outcomes.