promiseacres
Herd Master
A rough stocking rate is 1000# of livestock per acre. If he is having to mow I would consider a rotating system with a dry lot.
My brother-in-law has a horse and two donkeys on 2 acres and they are not making a dent in his grass. He keeps having to now.
Equines are like cows, in that they don't like to eat over their own manure. I suspect that the grass your BIL is mowing is growing in or near piles of "road apples" from the horse and donkeys, and the areas where they are actually eating are chewed down to the height of a putting green.
I guarantee, if you put two cow/calf pairs on a quarter acre, you will need to be haying year-round.
But what goes in must come out, which creates another issue - smell, and flies, and all the other fun stuff associated with manure. When you are making your calculations, you need to figure out how you will deal with that (and any neighbors that may have a problem with it).
The grass was just tall everywhere. Even where they had not defecated.
Horses are surprisingly picky eaters. If they had pooped somewhere in the past, they wouldn't eat there, and the grass would grow tall. Horses don't like to eat tall grass, because it has a lot of the fiber that they can't digest (because cows chew their cud, they can break this stuff down, but the horse's simple digestive system doesn't process it). Horses instinctively prefer the short, tender grass. Once grass gets to a certain height, horses generally ignore it - keeping a pasture in any kind of shape with horses on it pretty much means you have to mow it regularly.
I trust you are doing your research on donkeys/mules as livestock guardians. I have a miniature mule (not as a guardian, but "just because"), and she made my goats' lives living he11 for the first week or two she was here. Once they learned to steer clear of her, they got along OK, but I have heard of donkeys killing goats while "playing" with them. I hear that the donkey/mule is actually protecting its space, not the animals in it, and that seems to be true for my mule. While she has been observed attacking things like foxes and raccoons, she has also gone after chickens and ducks. Donkeys that have been raised with goats are usually safer with them, but there are no guarantees, especially with intact jacks.
Most people are surprised to find out that goats are picky eaters, too. They are browsers, not grazers; given a choice, they won't eat off the ground, either. Put hay in a rack, and they will drag it all out of the rack and refuse to eat what gets dropped. When browsing, they usually eat a few bites here, a few bites there, moving along and eating only a little bit of any one type of plant. One theory is that this is an instinctive behavior, born of the fact that a lot of the plants in their native land have irritating or toxic oils in them to make them less palatable to herbivores. By only eating a little of any single plant, a goat avoids getting enough of its toxin to actually cause illness. Interesting theory, I thought.