If that's what we have over here near Show Low, Arizona (and I think it is) I honestly don't know if it's toxic or not. My horses try to grab bites of it out on trail rides (and succeed) and nothing bad has come of it, but I still worry about them eating it just because it is so brushy and tough.
If you ride through this stuff, you will want good strong blue jeans or chaps/chinks or it will rip your pants up. It's very prone to snagging clothing. It is also almost impossible to kill. We trimmed a bunch of it off our easements and used weed/brush killer and the stuff keeps coming back. We just call it "scrub oak" but I googled "Quercus turbinella" and from what I can tell, it's the same or very similar scrub oak. I would probably cut it down (if feasible) just so the horses aren't eating large quantities of it, but expect it to come back. It's tough as nails. I would really love to know if it's toxic as well. But at the very least, I figure it's a colic risk just because it's so tough and brushy. Let us know what you find out.
PS. Just re-read that you have 6 acres of it. Not sure what to tell you. Feed good quality hay and hope they leave the oak alone? Have a dry-lot area and just use the larger acreage for occasional turnout? We don't have our acreage fenced, but I suppose if I did, I would turn them out despite the oak. Just maybe make sure they have access to plenty of hay.
You need to find other horse owners who have horses on similar acreage and see if they have had problems with it. Everyone I can think of that I personally know doesn't have horses turned out on acreage w/ scrub oak. Because most everyone I know doesn't have the luxury of large turnout.
Although the place I bought my mare from had horses turned out on huge acreage (like cattle grazing land) and although I'm sure they had scrub oak, I think the horses had plenty of forage to choose from.