AMES, Iowa – Current trends in litter size for maternal lines of pigs has seen a steady increase, said Juan Steibel, Iowa State University professor of animal science, and Lush endowed chair of animal breeding and genetics.

That’s due to the introduction in 2000 of litter size into the selection index (which combines different sources of information that can be used to predict an animal’s breeding value) for several maternal lines, he said.

“Currently, there is broad recognition from pig breeders that to sustain this trend, more emphasis should be put on other maternal traits, such as the number of functional teats, and to consider using piglets weaned instead of the number of born alive in selection indexes,” he said.

Depending on the population, average litter size has improved by about 0.2 to 0.3 piglets per year over the past decade – and most of that is the result of genetic selection, said Jack Dekkers, Iowa State distinguished professor of animal science.

“While the general thinking several decades ago was that litter size was difficult to select because it has low heritability (only approximately 10 percent), the differences in litter size between sows are due to genetics, the current use of genomics in advanced breeding programs, combined with advanced statistical methods, have allowed effective genetic improvement for even these lowly-heritable traits,” he said.

But, Steibel said, “In my opinion, the biological limit does not matter. What matters is how we envision the production system, and what would be an acceptable range of piglets per litter, considering all the other implications that come with that.

“From my talks to colleagues in genetic companies, I think that the focus now is not so much on increasing the total of pigs born per litter, but on increasing piglet survival and growth (reducing mortality and morbidity in weaned pigs), and on increasing sow lifetime productivity (increasing number of litters per sow), given the current litter sizes, which are already of acceptably large sizes,” he added.

Dekkers said, “Obviously, litter size cannot increase indefinitely, but what the limit will be is impossible to predict,” he said. “At present, however, there’s no indication that current rates of improvement are going to slow down in the near future.

“However, selection for litter size should not be the sole goal,” he added. “We also need to ensure that most of the piglets born are born alive and are viable to go on to become market pigs.

“This requires close attention to traits such as piglets born alive, the ability of sows to raise these larger litters, resulting in reduced mortality prior (to) weaning and adequate weaning weights, as well as the ability of sows to produce and raise large litters for multiple parities (the state or condition of being equal).

“Current modern breeding programs are continuously monitoring these other traits and combine selection for litter size with selection for these traits,” he added.

Michael Langemeier, Purdue University professor of agricultural economics, said in his Feb. 3 Farm Doc Daily article that production performance, measured using pigs per litter, has been particularly strong in the last few years.

Steibel said, “It is recognized by most pig breeders – especially by those in genetic companies – that litter size (number of born alive or total number born) has been included in the selection indexes of pig maternal lines for the past 20 years.

“Current genetic evaluations rely on analyzing large datasets that include: 1) extensive and accurate phenotypic (the set of observable characteristics or traits of an organism) records; b) large pedigrees; and c) genomic information,” he added. “All this results in very effective selection for whatever trait is included in the index. In this case, traits related to litter size.”

Dekkers said, “The implementation of genomics and advanced statistical methods for genetic evaluation has resulted in more effective selection for litter size.”

He said if pigs per sow per year increase, then fewer sows will be needed to produce the same number of pigs for fattening.

“The important result of this improvement, however, is that cost per pig produced will decrease, as the cost of the sow is spread over a larger number of piglets,” he said. “This will ultimately result in lower pork prices for consumers, with potential impacts of the total size of the market for pork.

“The net effect of these two counteracting effects (fewer sows needed to produce a given number of pigs and increased demand for pork) on the overall number of sows needed is, therefore, hard to predict,” he added. “Regardless, we shouldn’t sacrifice potential increases in efficiency of pork production (lower cost price) in order to try to maintain the number of sows needed.”

He said management and selection have both played a key role in improving performance at the whole system level for pigs, and for other species.

“From the genetic improvement side, the adoption of genotyping technologies and the ability to measure new traits such as feed intake has definitely resulted in great improvements of the genetic merit for economically important traits in maternal and terminal pig lines,” he added.

Likewise, he said, improvements in swine nutrition management, health management, as well as in ventilation and housing systems, have all resulted in improved pig production performance.

“In fact, something that pig producers, companies and professionals do great is to work together, or work in their field, while being aware of progress in other fields,” he said.

“For instance, breeders are always looking for new traits that can be improved through selection to facilitate management (i.e., disease resistance, feed conversion ratio, heat stress tolerance, etc.), while welfare experts, nutritionists, veterinarians and engineers consider the genetic backgrounds of the herds for which they develop and prescribe management solutions.”