Sow mortality continues to frustrate U.S. pig farmers. One of those key causes of sow mortality is pelvic organ prolapse (POP). Previous studies on purebred sows show that there is no silver bullet to prevent POP, but susceptibility can be reduced by genetic selection because it has a substantial heritability. A new study shows the same correlation exists among the crossbred (F1) commercial sows, but that’s not the only factor contributing to prolapse incidence.
A recent study led by Tricia Haefner, a master’s degree student at Iowa State University, evaluated genetic and non-genetic factors associated with susceptibility to vaginal/uterine POP in crossbred sows. She wanted to validate the non-genetic factors tested in the Iowa State University POP project led by Jason Ross, including the association of body condition score and perineal score measured during late gestation with POP in crossbred sows.
Data were collected on 3,983 crossbred sows during late gestation on two commercial farms in the Midwest during the summer of 2023. The data include:
- Body condition score (1-5)
- Caliper units (5-24)
- Perineal Score (PS, 0=low risk; 1=moderate/high risk)
- POP – defined as vaginal or uterine prolapse (0/1)
All sows were housed in group pens 35 days after breeding, except for a subset of sows at one farm, which were placed in groups immediately after weaning and managed according to California Proposition 12 regulations and moved into farrowing within 5 days prior to her due date.
The “Thin Sow” Problem
Using body condition score and the sow caliper tool, this study confirmed a “glaringly obvious” linear relationship between body condition score during late gestation and POP.
“Sows that are too thin during late gestation are at a substantially higher risk of prolapse,” says Jenelle Dunkelberger, a geneticist at Topigs Norsvin USA.
She says that’s an important message for the industry to hear right now.
“Late gestation is just a snapshot in time, but we should consider what her body condition at this time point might imply about her development prior to that point,” Dunkelberger says. “For instance, we need to be aware of, and perhaps even implement changes, regarding how she’s managed prior to late gestation to ensure that she is in the appropriate body condition going into farrowing, to maximize her success for longevity.”
Within parity groups, the data showed higher parity animals are also at a higher risk than gilts or Parity 1 sows.
Perineal Scoring as an Indicator
Researchers used a 1–3 scale developed by Iowa State University’s Jason Ross and former graduate student Zoe Kiefer to score the perineal region for swelling and redness. They found a high genetic correlation between perineal score and actual POP, indicating that these are, genetically, the same trait. More specifically, genetic predisposition for an unfavorable perineal score is strongly correlated with genetic predisposition for POP. Because perineal issues occur at a higher frequency (33%) than actual prolapse (2.4%), it can be considered a higher-resolution “indicator trait” for early identification of susceptibility to POP.
“POP is a binary trait – you either have a prolapse or not,” Dunkelberger explains. “Unless you have a high incidence rate, statistically, it’s tricky to analyze these types of traits. That’s why a trait like perineal score, which appears to have a higher incidence rate, may be an attractive indicator trait for POP.”
For this study, they did not see a large incidence of 3 scores, so they grouped together scores 2 and 3 and called them a moderate to high risk.
“We want to use this perineal score to try to develop an indicator of prolapse because you don’t get to see the incidence of it until it actually occurs,” Haefner says. “This helps identify it a little sooner so we can develop mitigation strategies.”
Prop 12 and Group Housing Stress
A significant finding showed that sows in Prop 12-compliant systems (weaned directly into groups) had a higher risk of POP and high perineal scores compared to sows kept in stalls for the first 35 days post-breeding.
“That was a little bit of a shock, but also we know that there’s a lot of stressors occurring around that time frame,” Haefner says, noting that there was limited data for this subset of animals. “It’s a critical time point of breeding and just getting off of lactation, wanting sows to recover their body condition. Even though those sows in that group were at this higher risk, we actually saw higher average body condition scores and caliper scores for them at late gestation.”
The sows were recovering, but Haefner says more research is needed to determine what’s different during that 35-day period as compared their non-Prop 12-compliant sows. She suspects hierarchy, including competition for feed, to be a key factor.
Still More Genetic Work to Do
After compiling this data, Haefner performed a genetic analysis on the tissue samples she collected.
“We were looking at perineal score and actual prolapse incidence,” Haefner says. “We were able to find moderate heritability for both of those traits, validating the heritability of POP in a commercial, crossbred population.”
Haefner says there’s still more that genetics can contribute to reducing the incidence of POP and improving perineal score.
“Perineal score and prolapse together being highly correlated means that we can utilize perineal score to also help mitigate against prolapse and select for perineal score alongside prolapse as well. Or, potentially, use perineal score in place of POP, to select for reduced incidence of POP,” she adds.
In other words, the same genes that control susceptibility to an undesirable perineal score are the same set of genes that control POP.
A Multi-factorial Approach is Needed
“Genetics can be part of the solution, and should be part of the solution,” Dunkelberger says. “But moderate heritability indicates that the majority of phenotypic variation in that trait is actually influenced by non-genetic factors. Therefore, you need to address both the genetic and non-genetic factors influencing POP to make a substantial reduction in the incidence rate of POP.”
Topigs Norsvin USA has been performing direct selection against susceptibility to POP since 2021. Dunkelberger says that should continue to reduce the genetic susceptibility to that trait, but effort should also be invested into looking at these non-genetic solutions as a way of complementing that approach.
“The lowest hanging fruit is body condition,” Dunkelberger says. “There are different ways to go about managing body condition and different things that can influence it. Having awareness of the relationship between body condition during late gestation and susceptibility to POP is important. I really believe that proper management of body condition will pay for itself in terms of sow retention.”
