Selection is the most powerful tool in making permanent genetic improvement in swine. There have been no long-term selection studies targeting the subject of this Blueprint — pigs weaned/sow/year (P/S/Y) — although there has been considerable research on selection for the major components of this popular measure of productivity. In turn, much of this information has been transmitted to the breeding stock currently available to the industry from the major genetic suppliers.
The primary component traits of P/S/Y include ovulation rate, farrowing rate, number born alive, preweaning mortality, number weaned, wean-to-service interval, litters/sow/year and longevity. As we examine each of these important traits, there are opportunities and limitations to each for improvement through selection.
Ovulation rate — Research has shown that ovulation rate is controlled by genetics to a small degree, but there are some severe limitations in
measuring the trait for use in selection programs. Therefore, it has not been utilized to any extent by genetic suppliers for improving reproductive performance.
- Farrowing rate — A very economically important trait, but research has shown very low genetic control and distribution is not “normal,” so few if any genetic suppliers include it in their genetic improvement programs.
- Number born alive (NBA) — The most researched of any of the reproductive traits, NBA has been shown to have a low heritability (genetic control) and did not respond to initial selection when selection indexes were used. However, with the advent of best linear unbiased prediction (BLUP)-based selection, adopted by most genetic suppliers in the 1990s, NBA has shown tremendous improvement.
Pureline populations from most genetic suppliers are now exceeding 12 pigs born alive/litter, bumping the genetic potential to over 13 from crossbred females. But as litter size increases beyond 12 pigs/litter, the average pig weight generally decreases. When pig birth weight drops below an optimal level, the probability of pre-weaning death and/or lower weaning weight averages increases. Both of these correlated responses have negative economic impacts.
- Preweaning piglet mortality — This variable has been shown to have limited genetic control and does not have the classic (normal) distribution of traits, such as litter size. Consequently, selection for this component has been limited in most genetic populations.
- Number weaned — This trait is a function of litter size born alive, number of pigs fostered and preweaning mortality. Because of the crossfostering effect (a non-genetic influence), the heritability of this trait has been very low, and genetic suppliers have had little success in selecting for this trait.
- Wean-to-service interval — This measure of non-productive days was first examined in the late 1980s and, with its low to moderate heritability, has been included in many genetic improvement programs since. However, the overall potential genetic improvement is fairly small, as most sows are cycling within four to seven days after weaning.
- Litters/sow/year — This trait is closely related to farrowing rate. Recent research has shown that while the heritability is low, it may be of adequate genetic control and normal in its distribution to respond to BLUP-based selection.
- Longevity — This trait increases P/S/Y due to the lower number of gilt litters farrowed when longevity is improved. However, it has been a difficult trait to include in genetic improvement programs. Recent research has shown it has adequate genetic control to respond to selection, but since the animal does not express the trait until it exits the sow herd, it remains difficult to improve via traditional selection.
Litter Size Focus
Swine breeders have been successful in selecting for increased litter size. Today, it is not uncommon to see groups of sows averaging 13-16 total pigs born/litter and 12-14 pigs born alive/litter.
Managing these larger litters presents some new challenges. Within-litter variation remains, but as the size of the litter increases, average birth weights tend to decline and the number of pigs that are too small at birth increases. Preweaning mortality rates often increase in the larger litters.
Birth weight can impact postweaning pig performance, particularly when pigs are born weighing less than 2 lb. If the low-birthweight piglets survive to weaning, they have higher grow-finish mortality rates when compared to their heavier littermates.
Research at North Carolina State University (NCSU) suggests that pigs weighing less than 2 lb. at birth have a grow-finish average daily gain of 1.6 lb., while their heavier littermates gain 1.8 lb./day. Often, these losses are not recognized until the barn closeout is complete and the producer examines grow-finish performance data more closely.
Some losses are less apparent. For example, pigs weighing less than 2 lb. at birth have slower growth throughout the grow-finish period and, when harvested, these pigs have less carcass lean.
The NCSU research has shown that pigs weighing less than 2 lb. at birth have less than a 30% chance of being “full value” pigs at market weights. Generally, these pigs are discounted because they fall short of preferred market weights.
Unfortunately, aggressive crossfostering does not decrease the impact that low-birthweight pigs have on pre- and postweaning piglet performance. Iowa State University (ISU) research suggests that there is variation in fat and muscle accretion rates caused by physiological differences.
In addition to having more small pigs in large litters, stillborn rates typically increase, too. With large litters, it simply takes longer for all pigs to be born, which is taxing to the sow and often contributes to higher stillborn rates.
But just because a pig is born heavier does not guarantee it will survive. Slow, difficult deliveries can result in dystocia, even though birth weights fall within a normal range. Some farrowing managers have had success in saving more pigs by staggering the staff’s working hours to ensure the majority of farrowings have someone present to assist sows when difficulties arise.
Striving for Balanced Litters
A shift from selecting for number of pigs born alive to a more balanced selection for both number born alive and some measure of piglet viability at birth is needed. Many genetic suppliers are taking this more balanced approach, which also factors in birth weight variability and/or piglet survival. Some breeders select on total litter birthweight; others adjust for uniformity. Still others utilize a curvilinear factor to select against litters that have a higher proportion of piglets that weigh less than 2 lb. at birth.
As the proportion of large litters increases, breeders are examining ways to keep more of these piglets alive to weaning. To address this, some breeders are using the number of piglets alive at 5 days of age as a measure of piglet vitality.
It is important to remember that many of these traits are heavily influenced by management and the environment. For example, farrowing crate design has not changed in a number of years. We still provide the same amount of creep area on each side of the sow, even though litter size has grown from 11 or so to 13-15 pigs born alive.
It will take a multi-faceted approach to address the challenges presented with higher reproductive levels.
Traditional selection for increased litter size is responsible for the large gains pork producers have seen in maternal lines in the last 20 years. To continue this trend, swine breeders and pork producers now have relatively low-cost gene marker tools available.
Today, it is relatively simple to determine if an animal has a beneficial genotype by using a small blood sample on a blotter card or collecting a small tissue sample using one of the new ear-tagging technologies that collects a sample as a part of the ear tagging process (i.e., Typifix; IDnostics AG, Switzerland; or Allflex USA, Dallas, TX).
The commercially available gene markers to improve litter size are estrogen receptor (ESR) and erythopoietin receptor (EPOR). The ESR gene marker is associated with litter size in pigs and was first discovered in the Chinese Meishan breed. Sows that are homozygous (two copies of the same gene) for this marker would add almost one pig per litter, compared to sows with no copies of the beneficial marker. The ESR test has been shown to be effective in breeds or lines that include the Large White and Yorkshire breeds and crossbred females of those breeds.
Sows with increased ovulation rates have been shown to have the favorable allele for the EPOR marker. These sows have increased uterine capacity, which ultimately is associated with more pigs born alive. The marker information can be combined with traditional quantitative selection methods to speed genetic progress for any trait, in this case litter size, using this process more commonly referred to as marker-assisted selection.
Improving litter size is one way to increase production efficiency
using fewer sows and less feed. Commercially available marker genes also exist for a variety of other traits, such as feed efficiency, growth rate, backfat and pork quality.
Molecular Advances Continue
Use of more sophisticated molecular technology, such as the 60k single nucleotide polymorphism (SNP) chip and even larger chips, allow breeders to dissect complex traits that are economically important in the pig. The SNP (snip) chip information identifies single base-pair differences or polymorphisms among a group of animals. These differences can represent variation for a given trait within a group of animals where the breeder is charged with identifying the “best” animal(s) to keep in the breeding herd.
In recent years, molecular advances have allowed breeders to look for the SNP across the animals’ entire genome through a process called genomic selection. Simply put, this process is marker-assisted selection using SNP information from the whole genome.
It is clear that these types of molecular advances will increase litter size and improve other reproductive traits. The improvement will result from increased selection accuracy (identifying animals with the most desirable genotypes) and measurement of traits that are sex limited (i.e., measuring litter size or milk production on sires directly or late in life) or cannot be measured on the live animal (i.e., meat quality). The molecular information can be combined with traditional quantitative measures to continue reproductive trait genetic improvement in swine.
Maternal Index Emphasis
In pork production, we want to improve more than one trait at a time and improve the traits that influence the profitability of a maternal animal. The role of the sow is to have large litters of profitable pigs.
We know that sows will not be productive forever, and since selection for longevity is still evolving, we focus on trait-based maternal indexes that include measures of her performance over a set period of time. The reproductive traits, such as litter size, wean-to-service interval, preweaning mortality, litters/sow/year or pigs weaned/mated female/year, are used to express a sow’s genetic merit or breeding value (BV) for each trait.
For traits expressed by each pig in the litter (i.e., weaning weight, post-weaning growth rate, percent lean, feed conversion), the sow contributes 50% of the genes for each trait. Therefore, the maternal index used to guide selection should include both the traditional reproductive traits (weighted by economic value and accounting for the sow’s BV) and the traditional post-weaning traits (weighted by economic value, accounting for 50% of the sow’s BV, and accounting for the expression of the trait by each pig in the litter). The resultant maternal index will have about 35-40% of its emphasis on the traditional reproductive traits and 60-65% of its emphasis on the postweaning traits that influence profit.
Fetal programming refers to acute or chronic stress stimulus that occurs while piglets are developing in the sow’s uterus. In other words, the stresses that occur during embryonic development can impact physiologic function later in life. This process is more commonly called prenatal programming, and its effects are not thoroughly understood in swine.
Early research suggests there are associations between within-litter variation in birth weight, preweaning survival and postnatal growth in the pig. This is particularly evident among pigs that are substantially smaller than their littermates at birth. Most producers would refer to these as runts.
Research at Kansas State University and ISU has attempted to identify the birth weight threshold where the pigs that fall below some predetermined weight are euthanized because the likelihood that they will develop into full-value pigs at weaning or as market hogs is relatively small. These runt pigs typically do not grow well between birth and weaning, and they never seem to catch up to their contemporaries after weaning. These fallback pigs require more time to reach market weights and disrupt normal closeout of grow-finish barns. These pigs are often sold at lighter market weights and draw substantial weight penalties levied by the packer. Finally, because these small pigs are often physiologically challenged, they can harbor pathogens that constantly challenge their healthy contemporaries.
These growth differences cannot be fully explained using birth weight alone. Research suggests that these pigs can be identified as early as Day 27 to 35 of gestation. It has been suggested that selection for increased litter size (pigs born alive) in modern commercial dam lines has resulted in some developing piglets experiencing intrauterine growth retardation that results in their small size at birth.
Further, some older-parity sows from the highly prolific dam lines can ovulate 30 or more eggs at each estrus. The increased number of eggs ovulated results in a greater number of developing piglets that survive to the critical Day 30 of gestation. A higher number of developing pigs at Day 30 of gestation can negatively impact placental development and contribute to uterine crowding after the pigs have implanted into the sow’s uterus.
Large numbers of developing pigs in early gestation require a great deal of nutrients. Some may experience a limited nutrient supply. Not only does the reduced nutrient supply contribute to lower prenatal growth rate, it also appears to reduce the number of muscle fibers of all pigs in the litter, not just the runt pigs.
These research results suggest that as we continue to push number born alive higher to achieve greater reproductive efficiency, we may reach a point where further increases in the number of developing pigs may have detrimental effects on the postnatal performance of the entire litter. This issue is magnified by the high feed costs experienced in recent years.
Sires Matter, Too
The sire of a litter has been shown to have a small but significant impact on litter size, pig birth weight, pig viability and postnatal performance.
This influence includes the breed of the sire and the sire within the breed. Research has shown, for example, that using a Duroc sire has small advantages in pig birth weight, preweaning survival and postnatal performance. Within a breed, research has also shown that the service sire can account for approximately 3% of the variation in litter size born alive. This influence on litter size is generally expressed by certain boars having a genetically lower litter size (perhaps due to abnormal sperm cell frequency) without a corresponding, genetically caused increase in litter size. Therefore, the opportunity to select for this trait in terminal boars is somewhat minimal.