Here in Washington County, its a minimum of 5 head (not unit) of cattle, or 7 broodmares, or 10 miniature horses, or 10 goats, or 10 sheep, and then there are the requirements for trees and crops and hay pasture if you chose to go that route.
How do I know that? It was the reason I started, and just a word of caution to you Hipy, it may start out as a tax break, but quickly grows into much more, especially given your propensity for rescuing and caring for unwanted horses.......I would expect you to start out by thinking you're going into it for the ag exemption, but ending up finding out that each cow has her own personality, her own likes and dislikes, and her own funny way of doing things. It'll break your heart the first time you lose a calf, and you'll end up saying, "Not on MY watch, never again!" and do everything you can to learn about the intricacies of cattle diseases and what to look for, how to care for them, what to feed, when and what to vaccinate, when to worm. You'll spend hours just watching them eat grass, and countless hours worrying over why Bessy didn't come up to eat tonight, or why Elsie is being mean to Belle. You'll wear out your vet's ear with real or imagined cattle problems, until they tell you to just quit worrying and let them be cows, which doesn't work, because they're your cows now and you just don't know what to look for with cattle diseases. You'll watch your pasture and the body conditions of the cows wondering if its ever going to rain again, and if so, will it be in time to get the grass started and ahead of the cows so they have what they were meant to eat in front of them. You'll worry over why they're loosing a little weight over the winter months no mater how much good quality hay they have in front of them, and why they're not eating any minerals that you put out for them, and what affect that's going to have on their health. You'll wonder if they're EVER going to calve or stay pregnant until you're old and grey, whether or not you should put them in the pen to calve or leave them out in the clean pasture, and if they have trouble, how on earth are you going to get them up and in the chute to lend them a hand. You'll stay up till all hours of the night reading everything you can about cattle care, diseases and experiences, AND you'll love every minute of it.
That's my prediction......beware!