is it possible to founder a thin horse?

It is possible, since any horse can founder. But the issue that poses a risk of laminitis and founder associated with lush spring grasses is the high fructan levels in those grasses, and the fructan level in the spring grasses drops significantly in early evening, and remains low until late morning.

The exposure to sunshine is what brings the sugars up into the blades of grass. So, by turning him out after 5 pm and bringing him in the following morning, the risk is drastically reduced. You don't have to follow the same time limitations on grazing for ovenight turnout that you would for daytime turnout.

His being thin also does work in his favor, although it will not prevent founder if an inordinate amount of high fructan grass is eaten. When the hooves are burdened by carrying excessive weight, it enhances the risk of serious damage if laminitis does occur. I'd wait until as late as possible to turn him out, like 6pm or later. If you can do it gradually, starting with fewer hours out and increasing it as you observe how he is adjusting, it would be even better. You can also try feeding plain grass hay in addition to the pasture. Some horses will still choose the hay despite being out on pasture, and that also reduces the soluble sugar intake.

The answer to your question is - yes, it is most certainly possible to founder a thin horse.

Statistically the most common cause for founder isn't over eating grain, it's overeating on grass, especially on spring grass or grass under stress. Horses that are most prone to this are obese horses, horses with insulin resistence and horses with a genetic tendacy to founder. Your new horse is not obese, so that is in his favor, but you do not know if he has insulin resistence and you do not know what his genetic background is in reguards to founder. So I would go about things cautiously.

I would start by introducing grass very slowly, especially since he's been on nothing but hay recently. Put him out for an hour the first three or four days, and then slowly increase his time out by a hour or two, giving each increase half a week or more before increasing it again. There are a variety of ways you can limit his grass. The easiest one - simply keep him in a dry pen most of the time, allowing him out two or three times a day briefly (Perhaps 1/2 hour each time at first) Make certain most of his calories come from hay during this time. You can also use portable electric fencing to allow him access to a very small bit of fresh grass every few days. Or you could use a grazing muzzle so he could be out in the pasture but his grazing would be much slower. One way or another, putting a horse that isn't accustomed to grass in a lush pasture for 12 to 13 hours at a time is going to drastically increase the risks of founder and/or colic.

Hang in there. In a few weeks he can have that long term access to pasture, if all remains well. Until then I'd take things slow and easy.

Can you hand-graze him and let him slowly become accustomed to grazing on lush grass? It is possible to founder a thin horse, but overweight horses/ponies have an easier time of it. Is there a pasture where the grass isn't so green as the others?

My first horse had also always been on a dry lot. When I introduced him to grass, it was only for around 2 hours at a time for 3 or 4 days until I felt better about turning him loose in the pasture.

Here's a site about it:

http://www.ultimatehorsesite.com/info/founder.html

It is highly unlikely that a thin horse will founder on grass.

There seems to be a great fear of turning a horse out into grass when it isn't use to it, I cannot understand why? Are there any research papers on this to give details and reasons, facts and figures?

I have racehorses come back from training in the spring, they have been on a high hard feed ration and are not use to being out on grass but I have - for years, just chucked them out in a field and never had the slightest problem with any of them.

There is nothing better for horses than good grass.

The only reason I would keep this horse in for a day would be to worm him.

If you are going to graze him in hand, 45 minutes a horse will eat, after that they start to browse.

Do not feed bran mashes as they are a laxative and bran can cause gut irritation.

It is possible for a horse to get founder (or laminitis) and possibly colic when changing his diet suddenly.

Your best bet would be to turn him out on grass for say two hours, then 3 for the first few days and then you could leave him out all night if he shows no trouble in the stomach department.

You could also try introducing sugar beet to his diet which should boost him up a bit without overheating him (remember to soak it first) and generally increase his feed but in smaller feeds, ie more often. I believe that horses are grazers in nature and do better on more frequent feeders.

Good schooling will also help to build him up.

Yes, any horse can get laminitis. I would introduce him to the grass slowly. Allow him to eat a flake or two of hay before turning him out on the grass. Let him be out for a few hours and then bring him back in. Do this for a week or two allowing him out for longer periods of time each day.

getting too much of anything that they aren't used to could cause the problem--I would start by bulking him up a bit before starting him on green grass

Try a bran mash twice a day as well as a good pellet. keep feeding him hay until he seems to not be excesively hungry. Alot of horses (especially if they are thin or hungry) will gorge themselves on their feed (grass) and that in itself can cause founder as their bodies aren't used to processing that much of something

YES. If he's thin, he'll pig out. Fill him up on dry low-protein grass hay first. Then take him out and hand graze him. Don't let him graze long because you don't want to colic him either and that green feed is going to give him belly cramps if you let him have too much. It's also going to loosen up his manure. So watch out for colic too.

its never a good idea to turn a horse out to lush grass after a dry lot piece by piece slowley does it to avoid a colic be carefull and good luck

Very possible.

A horse who is over weight just increases the risk because they have more weight on them so there is more strain on the ligament that is attached to the coffin bone which will pull on it more if they get laminitis or founder.