Differences in design, size, feeding system and the number of sows in group pens may have an effect on stress levels — and, therefore, reproductive performance — but no one has successfully identified the factors that have the biggest impact.

In a new article, Janeen Salak-Johnson, an associate professor in the University of Illinois department of animal sciences, takes a closer look at group housing.

“Reproductive performance has always been a metric that people have been concerned about with housing,” Johnson said. “You can’t just look at individual versus ‘group' housing, because group housing means something different to every producer. There is no one set standard. Nobody has done direct comparisons between things like floor space or group size to determine the best designs.”

In her article, Johnson examines the effects of group housing on reproductive success for pregnant sows in the context of stress, touching on competitive versus non-competitive feeding systems, group size, floor space and social rank.

The relationships between group housing variables are complex, but Johnson said maintaining reproductive performance boils down to social rank and feed intake.

Aggression is strongest when animals are first mixed into groups and during feeding, especially if competitive feeding systems such as floor feeding are used, she said. However, if too much emphasis is placed on protecting submissive sows during feeding — for example, by using long feeding stalls — dominant sows can experience negative reproductive outcomes.

“If dominant sows are not able to assert their dominance during feeding by displacing submissive individuals, they get frustrated,” Johnson said. “That can lead to low-birthweight piglets. Short feeding stalls offer some level of protection but also allow dominant sows to exhibit normal behavior.”

Eleven states and several large pork producers are moving away from gestation crates for sows, but the effects of alternative housing designs on the sows’ reproductive performance are unclear, Johnson said.

Most previous research suggests that reproductive performance in group housing versus individual stalls is approximately equal, but in real-word scenarios, many producers have noticed compromised reproduction in group pens. Most sows are put into group housing after pregnancy is confirmed, so the effects of the transition usually manifest in low-birthweight piglets or fewer piglets, rather than an impaired ability to become pregnant or stay pregnant, she explained.

“That’s one of the big reasons people don’t see effects of group housing on reproductive success — the sows are already pregnant, but that doesn’t mean there aren’t potential effects on the litter during the course of the pregnancy,” Johnson said.

Her article concludes that the wide variation in group housing designs makes it difficult to provide research-backed recommendations for producers, but she does have some ideas.

“Maybe we start grouping them by bodyweight, parity or speed of feed intake,” Johnson said. “Heavier sows eat much faster. Maybe put them all together? You’re still going to have a dominant sow and a submissive sow, but if you bring your composite groups closer together, I think you have an opportunity to do a better job in reducing this variation that occurs in group housing.”

The article, “Social Status & Housing Factors Affect Reproductive Performance of Pregnant Sows in Groups,” was published in Molecular Reproduction & Development. The work was supported by the National Pork Board, grant No. 12-200.