If you are concerned about colic or founder, you could always be cautious and let her out for increasing amounts of time. Generally 5 to 7 days for one amount before increasing to the next.
You could also get your county extension agent out to get his recommendations specific to your area. It's free, unless you want soil and/or forage testing. Then the cost is really, really nominal.
You may be surprised to learn than many of the grasses that grow in your area cure well on the stem. What you have may be quite nutritious without being too rich. That's one of the pluses in living in, and managing pasture, in areas like yours and mine.