It appears fertility is a piece to the puzzle of getting pigs to preferentially graze rather than root.
The photo above shows the winter housing situation for my herd of pigs. The bedding pack leached out down the hill and fertilized the grass on the slope below. A few days ago I turned the breeding gilts and boar onto the spot delineated by the strand of poly wire there. The turned dirt on the left side of the temporary paddock received no bedding pack exudate. The grass that is grazed right down did get fertilized by the effluent. The line of fertility is clearly marked by the pigs rooting. Rooted = poor fertility, Grazed = good fertility (even I can taste how much better/sweeter the grasses are on the fertile ground). The rooted area up against the hard fence had no grass growing on it as it died from fertility and excess moisture leaching down during the winter and spring.
The section here looked basically like this after day one, just with more grass still standing on the grazed part. I thought for sure they'd root everything on day two, but I was wrong. Day two just allowed them to finish eating their greens.
I then rearranged the paddock to allow the grazed section to recover for another round of munching later in the year. The pigs got a new chunk of the super fertile grass once the rearrangement occured... I wish I had more super fertile grass.