Pastures, paddocks are small. 2 larger ones of about 3 acres, 2 smaller ones of about an acre and 3/4, two of about half an acre, plus 2 “barnyards” of a quarter acre each. Barnyards are pretty bare most of the year, with horses in them daily as access to the other pasture areas.
I am in Michigan where we usually have decent rain to keep things growing, though it has been droughty since winter ended.
The 9 horses we presently have are ususually divided 4 and 5 in the pasture. Or 3 and 5 if one goes out for training. I do put 2 on the acre paddocks now and again. This keeps them trimmed down. We get 2 days grazing there, then grass is gone. After mowing to cut weeds it rests until grass is back up about 6 inches. Reduces pressure on the other fields a bit.
Grass and other plants grow daily, though maybe just a small amount. Rotating so often gives each field that bit of growth time to “apparently” recover enough that things stay productive, keep their roots busy growing leaves. Just saying the horses here are well rounded on just grass.
They are easy keepers, though not small. Everyone is 16 to 17H, big bodied, good big bone. They get minimal grain, a literal handful if not in work. Add in some wet beet pulp with vitamins and Selenium once a day while stalled. Any more and they could easily be obese!
I took a Master Gardener class a while ago. One of their classes was on growing grass, mostly lawns. However by asking specific questions, staying late to talk to the speaker, I got more pasture information and tips. He was part of MSU’s team studying rotational grazing for dairy cattle, so was very informative about keeping pastures productive. The Dairy system was experimenting with a new grazing paddock opened daily. Cows took it right down on their turnout time, got a new paddock the next day. They were measuring grass regrowth, to see if they did better with 15 paddocks or 30, to keep the cows fed. He was the one who said “Mow high, mow often”, which has been very beneficial for us. He explained about the plants not getting “shocked” with lots of leaf removal at one time. Losing so much leaf prevents roots getting fed. Takes time for plant to recover, swing back into growth and production. So plants in shock are more vulnerable to being overgrazed, dying more easily. We want good roots under the plants to help grow leaves, survive drought times, resist erosion in heavy rain times.
Adding organic matter helps the fields. Horse manure is not the greatest fertilizer, especially compared to cattle and sheep manure. I spread our stall cleanings on the pastures daily. Sawdust bedding acts as a mulch. Covers dirt between plants, protects dirt from hot sun to prevent drying, holds in moisture, slows rain runoff that can cause erosion. The micro organisms in soil pull the sawdust down into the soil, making it “lighter” soil that absorbs water more easily. Soil uses nitrogen to break down the sawdust, so I am always adding nitrogen in my fertilizer applications, with appropriate lime to aid plants to use the nitrogen. Nitrogen never actually “leaves” the soil as some folks believe, but some is busy breaking down the sawdust, not available to the plants.
The sawdust bedding is dragged after spreading, manure piles get broken, spread thinly over the fields. I use a chain harrow for that.
This system works here. Of course I wish I had a bit more acreage, but we have to work with what we have. Pastures have improved a great deal with these methods. No bald spots, though grass is often short on their favorite spots. No one here has health issues, so sugar in grass is not a concern. They are out at nights, when things are cooler, less bugs, so less sugar.
They are out at night now (deer too 
), rotated between 3 pastures, generally one week on/2 weeks off. We have had neither a particulary rainy nor particularly dry summer, southeastern PA, with predominantly clay soil.
It’s lush grass in the parts they’re not eating, definitely not just weeds. I did not do any fertilizing or herbicide this spring - busy with other stuff, and honestly, I thought with a reduced group of horses maybe it would be ok to skip it this year. It’s definitely always hit or miss though with getting any of that done - not on a great consistent schedule, really just as I have time.
He would MUCH prefer to stand closer to the gate with a pile of hay than go out of sight of the barn door for grass 