Impacts of group housing on hog farms

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. In a new article, an animal welfare expert from the University of Illinois takes a closer look at group housing.

“Reproductive performance has always been a metric that people have been concerned about with housing,” said Janeen Salak-Johnson, an associate professor in the Department of Animal Sciences at U of I.

Most previous research suggests reproductive performance is approximately equal in group housing versus individual stalls, but in real-word scenarios, many producers notice 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.

“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.

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

“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,” Johnson said. “And 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. 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.”

Johnson’s 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 body weight, parity, or speed of feed intake. Heavier sows eat much faster. Maybe put them all together,” Johnson said. “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 and housing factors affect reproductive performance of pregnant sows in groups,” is published in Molecular Reproduction and Development. The work was supported by the National Pork Board, grant no. 12-200.

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