Heat tolerance is already an important consideration for pig herds operating where the climate is hot. We know all about the need to have pigs that will keep performing well, even when temperatures are high. This requirement is sure to increase. The fastest growth in pig production over the next three decades is predicted to occur in areas of the world that have a hot climate, such as in southeast Asia and Latin America. In general, we can expect that the future expansion of the pig industry will move it away from moderate climatic zones and into harsher climates, which are hotter, drier and at higher altitudes.
So it must be a matter of concern that modern pigs have become more sensitive to high temperatures because of the genetic improvement applied to them. The metabolism of the pig selected for faster, lean gain produces more heat within the animal's body, and this means it is less adapted to cope with a challenging environment.
An increasingly important question, therefore, is whether the suppliers of genetics are doing enough to provide the breeding pigs that can tolerate the heat without suffering a significant loss of performance. Some valuable insights have been gained so far. Notably, while some pigs do better than others when confronted by a hot climate, locally-bred genetics are more likely to be adapted to performing under those environmental conditions.
What is hot?
To answer this question, we must study the relationship between the temperature to which a sow is exposed and her farrowing rate, defined as her ability to become pregnant from the first insemination and to maintain the pregnancy subsequently until farrowing. In fact, upper critical temperature thresholds for sows of different breeds or lines can be estimated from real-life production data.
An upper critical temperature is the point at which the farrowing rate of a sow starts to decrease. This point has been defined by our research as a maximum daily temperature of 19.2C. But, for practical purposes, the operator of a sow herd can assume heat stress to occur if the environmental temperature around the sows is at or above 20C.
The temperature sensitivity in the sow's reproduction cycle is not limited to the day of insemination. In fact, the influence of temperature on her farrowing rate begins three weeks before she is inseminated. Most probably this relates to the fact that three weeks is the length of the sow's normal cycle, and therefore the heat stress has occurred just when a new wave of ova was being recruited.
Do genetic lines differ?
To check the heat tolerance component of farrowing rate in sows of two purebred dam lines and their reciprocal crosses, our research examined the records of almost 94,000 first inseminations on nearly 24,500 sows at 33 farms in Spain and Portugal. Results revealed quite relevant differences between families for their ability to tolerate a warm environment.
Figure 1 illustrates farrowing rate results according to the temperature on the day of insemination for Yorkshire and Large White purebred sows in Spain. Production by the Yorkshire sows (dotted line) is better at the lower temperatures, but starts to drop after about 20C. Above this temperature, the farrowing rate decreased by 1 percent for every degree Celsius increase. However, the performance of the Large White sows (black line) is less affected by temperature, and it begins to match or exceed the Yorkshire farrowing rate after 23C.
This is a strong indication that genetic differences exist in how animals respond to increasing temperatures. Some families drop their performance dramatically when the ambient temperature rises above the upper critical threshold, others react more mildly, and some families (such as the Large White in Figure 1) appear not to react at all.
Select for tolerance?
From the results of these studies, there certainly seems to be enough genetic variation to encourage the idea that heat tolerance might be improved by selection. More specifically, the work indicated possibilities for improvement by showing that genetic variation was present in the part of farrowing rate that relates to the sow's ability to tolerate higher temperatures. Although both heat tolerance and farrowing rate are traits with a low heritability, they still exhibited a variance genetically that selection schemes could potentially use to achieve an improvement.
The difference in response to high temperatures shown by Yorkshire and Large White sows in Spain adds support to the idea that tolerance may trace back to the selection strategies applied. For example, the Yorkshire dams above all originated from selection in a temperate climate, whereas selection in the Large White line was based on multiple environments including data from warm and tropical countries.
This example shows the need to base genetic selection on data collected in the environment where the genetics is expected to perform. However, a distinct and separate focus on the particular trait of heat tolerance should be unnecessary provided the other selection parameters are correct. When comparing the genetic relationships behind within-line production results from temperate and hot climates, we had concluded that the pigs selected as the best in a thermo-neutral environment would also be the best at a higher temperature.
Therefore, it will make little difference to the rate of improvement whether you have collected data at a low or high temperature. The data collection could be done at a temperature of 20C, for example, or at 30C.
Which temperature to use? My own opinion, based on the information from our research, is that data from unfavorable conditions are a better option because the genetic variance is higher there. More variation always means more possibilities for genetic improvement. Unfavorable, in this instance, would include the challenging heat of a high ambient temperature.
Heat stress is only one of the four main challenges that can prevent the achievement of excellent performance results in pig production -- the others being disease, poor feed quality and shortage of qualified labor. The sows that are steady producers in the sense of being more heat-tolerant may also be more robust or rugged in their ability to tolerate poorer management or feeding.
The catch is the correlation with production in the thermo-neutral zone. As we saw in the comparison of dam lines in Spain, families that are steady producers by keeping up production under heat stress can have a lower performance level than less heat-tolerant lines when the temperature is not challenging. Perhaps, specialization implies selecting some lines to be aimed specifically at moderate climates or controlled-environment conditions, in which they will achieve a high production level. Meanwhile, there can be other lines in a breeding program that are the steady producers, directed where they gain from their ability to cope better with challenges -- even if this may be at the expense of losing some efficiency when the temperature is less high.