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Sow selection can beat heat stress

By Saskia Bloemhof, TOPIGS Research
The modern breeding of sows are already highly productive, but the potential still exists to achieve even better results for reproduction by selecting lines that are particularly robust and which therefore perform well under a range of environmental conditions.Reproductive performance in sow lines is affected by several factors. In our experience, only one – three percent of the total variation in sow fertility shown by differences between farm-month averages are due to service sire effects. Genetics account for about eight percent. There are also influences from parity, lactation length and nutrition to be considered. However, we have found that some ten percent of fertility variation may be explained by the environmental aspects relating to farm management and factors such as the season of the year.
As all pig producers worldwide, we were also confronted by the reality of seasonal effects when evaluating the reproductive performance of our sow lines in Spain and Portugal. Data from 19 359 sows on 28 breeding farms had revealed an increase of almost one piglet per insemination from 2005 to 2007. However, a seasonal decrease in results during summer and early autumn could be seen in all three years. The highest number of piglets born per first insemination came from sows inseminated in December.
Seasons obviously differ in terms of temperature as well as in number of hours of daylight. One of the challenges of pig production in Spain and Portugal and other countries with a warm climate comes from the high temperatures in parts of the year. Heat stress is a limiting factor, especially for production in these conditions.
Literature and pig farming text books indicate that sows are exposed to heat stress when temperatures exceeds 20º C, which is the upper critical temperature of a sows thermo-neutral zone. It is known that heat stress decreases the expression of oestrus behaviour and also alters how ovulation follicles, compromises oocyte competence and inhibits embryonic development.
Management practices such as cooling, offer one way of protecting performance during hot seasons. An alternative is to select animals for increased heat tolerance. This selection approach has the advantage that the changes in the genetic composition of the pig population are permanent. Compared with the use of cooling, it is also more sustainable in terms of its impact on the earth’s resources.
To investigate the selection possibility in more detail, we conducted a study with the aim of estimating the genetic variation in heat tolerance expressed in reproductive traits.
A previous analysis of data from 11 935 sows on 20 farms in Spain, collected from 2003 to 2005, showned a huge effect of heat stress on farrowing rate and litter size, but also a clear difference between two sow lines represented. These lines were a Dutch Yorkshire (DY), used mainly for production in temperate climates and an International Large White (ILW), producing mostly in warm climates, with their reciprocal crosses.
According to that analysis, the temperature on the day of insemination affected the litter size of ILW-line sows but not their farrowing rate. By contrast, both traits were affected linearly by temperature for the DY-line sows. Another difference was that the decrease in reproductive performance with increasing outside temperature was greater in the DY line than in the ILW line. The result was that, above 22ºC, ILW-line sows had a higher reproductive performance than DY-line sows.
A first trait affected by heat stress could be farrowing rate, so our next study of sows originating from the same two purebred lines also looked at their farrowing rate as well as litter size according to the maximum outside temperature on the day of insemination. Temperature again had an apparent effect – it seemed that inseminating on a day of 30º C might result in 0.4 fewer piglets per insemination. Once more, however, important differences were found in the relationship between temperature and reproductive traits in the two genetically different sow lines. One of the lines showed no influence by temperature on performance, whereas the other suffered a decrease equivalent to 0.1 piglets per degree Celsius rise in temperature.
Apparently, therefore, the lines differed in their genetic ability to tolerate heat stress as measured by differences in reproductive performance. This was valuable information from our point of view, not least because of its indication that there might be a genetic component of heat stress tolerance.
Our estimates of heritability as part of the same study reinforced the idea that genetic selection on sow heat stress tolerance may be possible. We found large differences in heritability between sow lines. At 10ºC above the sow’s upper critical temperature, the heritability of heat tolerance with regard to farrowing rate was 0.06 for line DY, but only 0.02 for line ILW. Heat stress had less impact on the litter size of the sows in the study and the heritability estimates relating to heat tolerance of litter size were only 0.03 and 0.01 respectively, but the results in total clearly indicated possibilities for improving sow performance by selecting for heat tolerance.
We have also examined 93 969 insemination and farrowing records from 24 456 sows of the same two lines that were inseminated between January 2003 and July 2008, on 20 farms in Spain and on 13 farms in Portugal.
The average farrowing rate across all breeds was 83%. On average, crossbred sows had the highest farrowing rate (86%) and purebred DY-line sows had the lowest (82%). Farrowing rates from an insemination day when the maximum temperature was below 23ºC were highest for DY-line sows and lowest for those of the ILW-line. At insemination temperatures above 23ºC, however, the farrowing rate of DY-line sows decreased whereas the rates of the ILW-line and crossbred sows stayed similar to their performance at lower temperatures. Our heritability estimates were lowest for ILW-line sows and highest for the crossbreds.
Overall, the available evidence shows that both farrowing rate and heat tolerance are heritable traits and can be improved via breeding. This raises the obvious question of whether these traits can be improved simultaneously without one upsetting the other.
The answer is not yet straightforward, although my previous studies had shown a negative genetic correlation between heat tolerance and production. In the later examination the correlations between farrowing rate and heat tolerance were found to be close to zero.
While we may not be able to aim for simultaneous improvement, however, there remains an important message in this work. I would say that when you improve either farrowing rate or litter size without taking heat tolerance into account, this will lead to animals which have high performance but that are more sensitive to heat stress.
This possibility of selecting for heat tolerance is exciting, not least because most genetic selection of pigs takes place currently in nucleus herds located in temperate zones. Commercial pig production occurs all over the world, including hot climates. Selecting sows to be more tolerant of heat must hold out the promise of a further improvement of reproductive results also under those conditions.
This is currently the approach adopted by the TOPIGS breeding programme, where heat tolerance is considered constantly in the continuous search for even better production. The programme is already applying the knowledge gained from our research to the benefit of our customers.

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