What type of enrichment is best for sows?

For piglets and finishers, many enrichment materials of various sizes have been designed over the years. Now how about their mothers? What is the ideal form of distraction for them? In Canada, rope, straw, woods and chains are being observed closely while being provided to sows. While many different forms of enrichment materials have been studied, most of the research has been done on piglets and growing pigs. Examples are: straw, chains, wood, rope, mushroom compost, wood shavings, gard