This sod forming grass fills in bare spots, especially if you overgraze. Known as cocksfoot overseas, orchardgrass has been grown in North America since 1760. Because of its tolerance of light shade, it was a common groundcover for orchards and wooded pastures. It tends to mature quickly and is often ready to graze before other grasses in a mixed pasture sward. If not grazed early, it becomes stemmy and unpalatable.
Many graziers find it easier to manage orchardgrass alone or with a legume.
This is less of a problem if you specifically purchase seed of medium or late maturity varieties. Orchardgrass is somewhat susceptible to winterkill which, because it is a bunch grass, can result in a patchy pasture. Quackgrass is another uninvited guest in most pastures, but a welcome one. A productive sod-former, quackgrass is persistent, winterhardy, and drought tolerant. All the features that contribute to its value in pastures make it an invasive weed in crop fields, so you can't buy quackgrass seed.