How to reduce horses weight on rotational grazing?

My daughter's horse is in with our small sheep flock. We rotational graze them all through paddocks.
The problem is the horse is getting overweight. How can we control its weight while in this system?
Is a grazing muzzle a good option?

Thanks

Can you stall the horse overnight to limit his grazing time?

No, he doesn't have access to a stall.

Can you reduce the size of the paddocks at all? Or restrict the horse to a smaller area with an electric fence the sheep could get under?

Is he getting enough exercise?

I am very interested in this subject as well. I intend to rotational graze horse with cattle.

Problem is that the horse will likely be an "Easy Keeper" which means they will gain weight at the drop of a hat and can founder easily from eating the awesome forage that I am planting out. lol

Another problem is that horses need to eat almost continuously in order to not get ulcers from their stomachs producing a continuous stream of acid... and me having to cut their feed back too much.

Yes... I plan to use a grazing muzzle as soon as I see it putting on to much weight. Founder will destroy a horse. I am also setting up a massive "Sacrifice Lot" or "Dry Lot" as well for two reasons.

1) It rains a lot where I am at... and my pastures/grass will get destroyed in a single day after one of those major rain events that happen almost monthly year-round around here.
2) A horse's hoof is prone to water damage.

So... my backup plan is to move the horse up to the 1-acre Dry Lot for the long term if I cannot come up with a way to control feeding/if it becomes an issue. That dry lot is semi-open canopy but heavily forested with large trees. The trees help pull out the water and add structure to the soil. It is on a little 2' hill and is the driest place on the property by far. The horse will be able to have a little/minimal fresh grazing in there. At that point, I will be feeding the horse hay through a eating speed reducing net sack.  

The cows will be joining the horse every time we get heavy rain... or during the Winter once the stockpiled forage has been depleted back to 3" or so.

I have a horse like this too.  He gets way too fat on the all you can eat pasture.  

If we go out riding every few days he can have the all you can eat buffet.  If I cannot ride him enough I put him out only in the early morning when there is less sugar in the grass.

I would definitely get a grazing muzzle. An ounce of prevention is worth a pound of cure. Grazing muzzles are the most humane option in so many ways. There are a ton of options out there- the feed store special I got gives my horse runs on her nostrils but I got a Greenguard which is much easier on her face. If your horse starts to get a hard crest on their neck, I would just muzzle for safety. I do on my fat mare

From the little I know if rotational grazing plans, the general idea with horses is to have them go later in the order.
So dairy cattle / cows and calves get ‘first bite’
Steers / non-lactating heifers get ‘second bite’
Sheep and horses get ‘third bite’
Chickens last on clean up and poo scatter

So if you only have sheep and horses I would have the sheep in first, and the horses in second. For example if you usually have them all in a paddock for 10 days, well utilise two paddocks at once, sheep in a fresh paddock for 5 days, then the horses go in there and the sheep move onto a new fresh paddock for the next 5 days and so on. That way the paddocks are getting the same grazing pressure but the sheep get the best feed and the horses get the leftovers.

Horses do best on second to third go on pasture. Chickens/ducks go after them to spread manure. Horses that are easy keepers do very well on hay supplement like teff/coastal that is very low calorie but a continuous feed. Unfortunately, there is no way to full out stop a horse from eating the good stuff.
Grazing muzzles in my experience are only effective if your grass gets long enough to pass through the guard. Horses require something on their tummies at all times, like others have posted. What we do is exclude our horses from rotational grazing altogether, and have a constant supply of hay in the winter, and full pasture access in the summer. Their grain is mostly flax, sunflower seeds, beet pulp(lowest sugar content we could find) and teff pellets for bulk. Oats, sweet feed, alfalfa pellets, and other concentrates are only necessary for heavily worked horses (5+ days a week working multiple hours a day)

Best of luck and I hope you find an easy keeper. Horses are not easy, but very rewarding.

Grazing muzzle on 24/7 is effective

Put out loose salt as the horse won’t be able to lick a block