2024
A Holistic Approach to Breeding for Enhanced Sow Longevity
by Dr. Jenelle Dunkelberger
As an industry, we have realized the steady increase in sow mortality throughout the past decade and employed various efforts to respond to this critical production issue.
Despite the number of hours and dollars invested in addressing sow mortality, identifying solutions continues to be a challenge. This is likely due to many factors, such as inconsistencies or incomplete recording of removal reasons at the farm, the multifactorial nature of this trait (including both genetic and non-genetic causes), and numerous confounding factors across farms that severely limit, or even disable, the ability to properly analyze farm data.
At Topigs Norsvin, we are passionate about exploring the genetic basis for (general) sow longevity as well as the leading, specific causes of sow death loss.
While numerous reasons for sow death loss are reported, 83 percent of sow mortality events can be explained by one of the following reasons: 1.) unknown/other; 2.) feet/leg issues; or 3.) vaginal/uterine prolapse (Ross, 2019).
Addressing these issues through the breeding program, however, is not trivial.
Unknown/Other
Genetic improvement starts at the top of the production pyramid.
At this level, extensive data is collected on the most genetically elite animals. This data includes information for a diverse set of traits as well as genetic information for each individual.
Sow longevity, however, is not among the traits that are measured at these nucleus locations.
Due to the need to continuously evaluate the latest generation of animals, the sow replacement rate at nucleus farms is intentionally higher than standard production practices.
For this reason, an individual animal’s genetic potential for longevity cannot be evaluated.
As an alternative, Topigs Norsvin has a partnership with a large commercial farm in the US, where detailed removal information and genetic information are captured for each female.
By linking these animals to others in the database via their genetic relatedness, data collected at the commercial level can be used to estimate the genetic merit of individuals for longevity and related traits at the nucleus level.
This is the process that Topigs Norsvin uses to drive genetic improvement in sow longevity.
Selection for enhanced sow longevity has been underway for over a decade, facilitated by selection based on two different traits: longevity to parity 2 and longevity to parity 5.
This approach of selecting for enhanced (general) longevity is the only way to address the number one sow-death-loss reason when the cause of death is not known.
Feet/Legs
While selection for enhanced general longevity is used to reduce genetic susceptibility to any or all sow mortality reasons, we also select against the second and third-leading causes of sow mortality—feet/leg issues and vaginal/uterine prolapse, respectively—to complement this approach.
At Topigs Norsvin, data is recorded for more than 30 structural traits. Of these traits, special emphasis is placed on phenotyping and selecting against osteochondrosis, given that osteochondrosis is a main cause of lameness.
Selection against osteochondrosis has been underway since 1990. At that time, osteochondrosis was scored by evaluating joints post-mortem. This process involved scoring both the medial and lateral condyles of the distal humeral and femur joints (of both legs) to obtain osteochondrosis scores at a total of eight different locations.
In 2003, this method was replaced by assigning osteochondrosis scores based on CT images. Now, selection against susceptibility to osteochondrosis is facilitated using a combined score, calculated as the sum of scores assigned using CT images across all eight locations (Aasmundstad et al., 2013).
It is important to realize that, because pigs are most susceptible to osteochondrosis at a young age, an unfavorable relationship exists between fast (early life) growth and susceptibility to osteochondrosis (Figure 1).
Figure 1. Antagonistic relationship between early life growth (quantified as the number of days in the growing phase) with osteochondrosis.
This natural, antagonistic relationship presents a problem for breeders. If selecting for an increased growth rate, susceptibility to osteochondrosis and, therefore, lameness will also increase.
For this reason, if selecting for faster growth, explicit selection against susceptibility to osteochondrosis must also be performed.
Vaginal/Uterine Prolapse
The third-most common sow removal reason is vaginal/uterine prolapse, commonly referred to as pelvic organ prolapse, or POP.
There are many potential causes of POP, which may be classified as being either environmental (non-genetic) or genetic in nature.
To investigate the potential role of genetics in susceptibility to POP, we launched an investigation in 2020 using data collected from more than 16,000 sows from a single commercial multiplication farm in the US.
This was an extremely valuable dataset, consisting of complete pedigree information, genomic data, and detailed removal information for each female in inventory for the past decade.
As a first step towards deciphering the potential role of genetics in susceptibility to POP, a genetic analysis was conducted using three generations of pedigree data. The results of this analysis showed that POP is heritable at 22 percent (Stevens et al., 2021; Dunkelberger et al., 2022).
In response to these findings, POP was added to the selection index in January 2021. Therefore, explicit selection against susceptibility to POP has now been part of Topigs Norsvin’s breeding program for the last three years.
This was the first study to show that POP has a genetic basis.
To investigate this further, a follow-up study was conducted in partnership with Vishesh Bhatia, Dr. Jack Dekkers, and Dr. Jason Ross of Iowa State University. The same dataset that was used for the initial analysis was also used for this follow-up evaluation, however, this time using genomic data rather than pedigree data to account for relatedness between individuals.
Results of this follow-up study showed that POP was even more heritable (35 percent) than previously realized (22 percent) (Figure 2) (Bhatia et al., 2023).
Figure 2. Heritability of pelvic organ prolapse estimated using pedigree (A) vs. genomic (B) data.
Next, an analysis was conducted to characterize the genetic basis of POP. Results of this exercise showed that there are, likely, a few major genes that impact susceptibility to POP, including genes with roles in pelvic organ support, smooth muscle tissue, calcium level, and even susceptibility to uterine prolapse in other species.
However, in general, genetic susceptibility to POP appears to be due to many genes, each with small effects on POP (Bhatia et al., 2023). Taken together, the results of this collaborative study validated the decision to add POP as a new trait to the selection index, which is an encouraging opportunity to address a major reason for sow death loss through the breeding program.
The heritability estimate of 35 percent, referenced above, indicates that 35 percent of the total variation impacting susceptibility to POP is due to genetics. This estimate also implies that the remaining 65 percent of the variation in susceptibility to POP is due to environmental or non-genetic factors.
Therefore, genetic solutions alone will not solve POP.
Rather, reducing the incidence of POP will require addressing both genetic and non-genetic factors impacting this trait. Although numerous non-genetic factors have been proposed and discussed by industry experts, determining the effect that these factors have on susceptibility to POP is complicated.
Many suspected non-genetic factors are explained in the Pelvic Organ Prolapse Project Final Report (Ross, 2019), a summary of analyses conducted by the Iowa Pork Industry Center, led by Dr. Jason Ross.
Results from this project showed that of the 20+ variables evaluated using data collected from more than 100 US farms, only water treatment, bump feeding strategy, perineal score, and body condition during late gestation were significantly associated with susceptibility to POP (Ross, 2019).
We opted to select two of these variables, the perineal score and the body condition score, for further evaluation in a designed research trial.
For this research trial, data was collected on approximately 4,000 TN70 F1 parent females from two different sow farms within the same system. Data was recorded for each female upon entry into the farrowing unit, including the perineal score, body condition score, and caliper score. Each instance of POP was also recorded.
However, due to the limited number of POP observations, the perineal score was analyzed as the response variable for all analyses conducted. Results show that body condition score and caliper score were highly and significantly associated with a perineal score, where the thinner the sow, the higher the incidence of the non-ideal perineal score (i.e., moderate to high risk of developing POP) (Dunkelberger, 2024).
This finding agrees with the results of the Pelvic Organ Prolapse Project (Ross, 2019), where thin sows were more susceptible to POP. Ultimately, non-genetic solutions, such as proper management of body condition during late gestation, must be used to complement genetic selection for reduced susceptibility to POP.
Overall Conclusions
Sow mortality continues to be a major concern for the US swine industry, likely because sow mortality appears to be due to any number of complex, underlying causes.
At Topigs Norsvin, we take a holistic approach to improving sow longevity by addressing the leading causes of sow death loss through the breeding program.
We address the top reason for sow death loss (unknown/other) by breeding for enhanced (general) longevity. The second and third-leading causes (feet/leg issues and vaginal/uterine prolapse, respectively) are addressed via explicit selection for structural traits, osteochondrosis (a leading cause of lameness), and susceptibility to vaginal/uterine prolapse.
Results presented in this article show that overall sow longevity, as well as these leading, specific causes of sow death loss, while heritable, are only partially under genetic control. This means that the majority of variation in these traits is due to environmental factors.
Therefore, genetics is only one piece of a strategy needed to improve sow longevity. Genetic selection, in combination with non-genetic solutions, will be required to address this critical, multifactorial issue in US swine production.
Dr. Jenelle Dunkelberger
Jenelle holds a Bachelor’s degree from Northwestern College in Orange City, Iowa, and earned a PhD in Genetics from Iowa State University as a USDA National Needs Fellow. She has papers and conference proceedings on animal genetics topics, such as the role of host genetics in response to disease challenge. She continues this area of research as leader of the Global Health & Behavior Platform for Topigs Norsvin.