Grass clippings, even a small amount, can founder a horse, that's because grass clippings start to ferment almost immediately, even when not put in a pile.
If the pasture is eaten down with 2 horses, it is not enough for 4 horses. And in general, even with two acres of grass and two horses, people need to feed supplemental hay by August.
In most places, on good land where it is not too dry (like say, Arizona is dry in areas) an acre a horse is the rule, but depending on location and climate that can go up to twenty acres per horse.
One acre per horse is assuming the climate has sufficient rain and the pasture is in good shape and is grassy, and not taken over by weeds and shrubs or full of bare spots after years of overgrazing. It also depends on the size and breed of the animal. Some breeds are called 'easy keepers' - such as Morgans and pony breeds.
There is a system for 'scoring' a horse's body fat and condition that a vet or trained person can use. Horses look poorly when muscle and fat are lost. Muscle is affected by lack of feed as well as a lack of activity. Muscle and fat are usually lost together. Thin horses show their ribs, their hip bones, and the top line of their neck looks hollow instead of rounded upwards. The head may seem large in proportion or the belly may look 'tucked up' too much. Horses in good condition have 'round' lines. Horses that are too thin have a ragged, sharp outline with lots of angles.
Horses whose feet have not had care often limp or move about very little. Foundered horses have uneven, deformed looking hooves. The hooves may curl up in the air at the toe or have an unnatural, long shape. Horses that have foundered often will have a typical way they stand - front legs angled forward and hind legs scrunched forward, to try and take the weight off the front feet.
It's not unusual for some animals in the group to look better than others. When there's not enough feed, the more aggressive or the smaller animals look better...at least for a while. Older or sick animals, those with injuries or neglected teeth, and certain breeds, may be the ones that look worse.
You may also see that trees have been chewed on, fences have been chewed on, barn siding is chewed up. Wood in stalls may be chewed on. It can be an indication of chronic hunger and over crowding. You may also see that even manes and tails have been eaten off. While horses often chew on things just to amuse themselves, a group of starving horses will do a lot of damage trying to find something to eat.
I think animal control in most places, has to make a visit if they get a complaint, but I also think the complaint can be made anonymously.
Often if the animal control just TALKS to people they will do something, such as buy some hay or sell some animals. But not everyone responds to those visits. Some people will just go buy one bag of cheap feed to show them, and kind of try to fool the animal control people.
I'm not sure if they will give a citation or take animals away unless conditions are very severe, so don't be disappointed if nothing changes right away.
Sad to say, many, many people get horses who don't manage them well. Horses can be 'impulse purchases' just like any other animal, but being larger, the problems are usually larger too.
