Can I feed round oat hay bales free choice to horses this winter?

You can feed oat hay, but feed it to mature horses and make sure nitrate levels are at acceptable levels.
Oat hay is not a commonly fed hay but can be an effective hay for older horses. The oat hay needs to be harvested in the soft dough stage when the leaves and stems are still green. If the oat hay is harvested when the plant is more mature and only a small amount of grain remains, the forage has the nutritional value of straw, therefore limiting its value in feeding to horses.
The energy and protein content of good oat hay makes it a suitable forage for mature horses at maintenance and early gestating mares. The hay is palatable; however, horses still will selectively consume the grain and leaf portion of the hay and leave the stems.
Oat hay has the potential for high nitrate levels. Higher levels of nitrate may be present in oat hay if it was grown with high nitrogen fertilization or if the plants have suffered the effects of drought. Either situation can cause the accumulation of nitrates in the feed. You can have the hay tested for nitrate levels to ensure that the forage is safe. The level of nitrate in the horses’ total diet should not exceed 0.5 percent.