Dirt Lot to Grassy Pasture

I have three horses, 2 mares and one gelding. All three of them have lost a bit of weight over the winter, and since I am about out of hay, I was wanting to move them to my large, grassy pasture in the next week, so maybe I could ration my hay until spring comes along and the grass starts growing.

The grassy pasture has fescue in it. I have heard that if you turn your horses out on fescue after it has frosted, they will get sick from it (colic) or get foundered.

These horses I have are right now on a mainly dirt lot, and they were on a dirt lot at the place they were before I bought them.

My question is:
Will fescue hurt the horses? If it has frosted over, will it make them founder or colic?

If it will, when will it be safe to turn them out on it?

And how should I transition my horses from a dirt lot to a grassy pasture? Should I turn them out for only an hour a day and slowly work my way up to 24/7? Or what?

Any help would be wonderful.

Anyone have any ideas?

At this time of year, the grasses are dormant and nothing more than "hay on the stem". It will not pose the same risks as green growing grasses. Nutritionally they will be high in fiber (mature stems), low in sugars and vitamins. I wouldn't have a problem just kicking them out. I always move my horses to new winter pasture after a hard fall freeze and give the grass some time to use up the stored sugars and die back. I've never had any problems other than the expanding waistline.

If it will put you more at ease, than ration the turnout to 2-3 hours at a time. Stuff them with hay before you turn them out so they're not "starving". Rationing should also make it last until first cutting.. I would want the old growth cleaned off either by horses, cows or mowing to promote a better quality new crop.

Personally I would introduce them slowly to the pasture. Can you use hay cubes to make your hay last longer?

Frosted fescue doesn't cause any problems.
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Not enough information for a simple answer.
Yes, putting horses our on mature grass that has been stressed can have negative effects under the corrected conditions.

The first question is what is the current condition of the grass. Take a close look at it closer to the ground. Is it green near the ground and dryed out as you move away from the ground. The simple answer would be that this grass could create problems if it's consumed to too great a quantity. If it's brown and dry all the way to the ground the simple answer is that it's basically low in nutrients and safer for grazing at this stage.
This is all overly simplified. The digestive system of a horse and good nutrition is not as simple. There's really not time to go into the structural carbs dietary fibre, cellulose, etc..., the non structural carbs starch, glucose, etc..... and how grasses at different stages of developement impact these things. Not to mention the differences between different grasses.
If this is your land and you want to keep it in better condition for grazing never let it get too high so the grass is too mature. Mow it to keep the height down and the grass at a more immature state. Grass changes how it uses nutrients and energy as it matures.

The grass looks to be mostly dead but is starting to get some new growth where it has been so warm over the last couple months where it doesn't feel like winter at all. Haha. I used to give alfalfa cubes a couple times a day but I found that they didn't like them and they ended up going to waste, so I stopped buying them. I will turn them out for a few hours a day, after a good meal of hay, just to get them used to the rich grasses. Thanks for the great answers, I really appreciate it! :)