Oats are commonly used as a nurse or companion crop to established hay and pasture. Assuming your pasture is grass (not alfalfa where the oat can be chemically removed), you have limited options. You can allow the oats to mature, harvest the grain and then bale the straw, or you can cut the oats at the boot stage (right before the grain head emerges) and bale the forage as oat hay (which is a suitable feed for cattle, but may not be for horses due to the high energy and carbohydrate content).
Your other option is to treat the oat as an annual grass and mow it and your pasture grasses several times before you graze. As long as the oats are not heading out, they can be grazed. If you planted your pasture this spring, you will need to mow several times to encourage the pasture grasses to root and establish before grazing. You will likely have to mow 3 to 4 times before the horses can graze later in the summer or early fall. Since oat is an annual crop, it will be killed with the first frost and will not re-grow the following year.
