Orchard grass is hay so your title doesn't quite make sense. Hay is simply dried up plants. It can be grass hays like timothy, orchardgrass, bluegrass, brome, bermuda, etc..., legume hay like alfalfa and clover, or grain hay like oats, wheat, barley. Then my neighbor grows what they call grass hay and I call weed hay. Most of what is in their hay is the stuff we spend our time trying to kill out of our field.
All grass hays are fine for pigs to eat in unlimited amounts. Grain hays are fine treats in small amounts so long as the grain heads are immature. Alot of grains in their diet can lead to bloat. Legume hays are fine for young pigs but for adults they should be limited to none at all or just a few pieces to small handful every few weeks as a treat. The main thing to watch for is that you are feeding good quality hay with few weeds, no mold, preferably low dust, and very green with a fresh smell. You also want to watch that the hay isn't too stemmy because some guinea pigs have been poked and injured by very stiff, stemmy hay.
