The feeder can be made of metal, but more often the sow feeder is concrete. Metal bars are at the front of the crate. In both systems to the left, the sows are chewing available substrate. Sows in the crate chew the bars. The outdoor sows shown here have denuded the vegetation and now they chew and root in the soil.
Some sows show repeated bar biting against the front bars.
The reasons sows bite, chew, lick and rut the bars and feed trough are not known. All sows, even those outdoors, show ONF behaviors. Mechanisms for expression of ONF behaviors are not known. Answers to these questions are not known. Indoor sows certainly can be housed in social groups.