Cimarron is a good weed killer for pastures, because there is little or no grazing restrictions after you spray. The key thing with pastures is grazing height. Many, many people let their animals overgraze their pastures. If you let your animals overgraze, they will kill off the grass. When grass dies, weeds move in. When grass is grazed or clipped, the grass starts to regrow. Grasses keep all of their energy stores in the crown, so all of the energy then moves into the new growth. Well, if you let your animals graze that new growth, the plant takes a bit hit because now it doesn't have much energy left to put into regrowth. Combine overgrazing with drought, and you'll surely lose your pasture. Weeds are opportunists, so they'll take over an empty spot of ground. They will also shade grasses. If you're interested in a more natural weed control, keep your pastures mowed and prevent the weeds from getting mature and shading out grasses, and prevent them from going to seed.
It is a winter annual, which means it can't take the heat. Of course it will come back next year, because there are seeds in the ground. If you spray your lawn to kill weeds, you better make sure to seed it shortly after. If you leave bare spots in the lawn (from the dead weeds) you'll have new weeds move in. Seeds can stay dormant for many years, and they're just waiting for an opportunity to sprout. That's why it's important to reseed your lawn when you kill weeds. Zoysia isn't the only grass that will choke out weeds. Bermuda grass and Kentucky bluegrass are sod-forming grasses that will form a nice mat to help prevent weeds. Unfortunately Kentucky bluegrass doesn't grow the best around here. Tall fescue also does a pretty good job if you don't mow it too short. Ideally, it should be mowed at a height of around 3" to prevent stress on the plant, and to also shade the ground to prevent sunlight from reaching the ground and helping weed seeds sprout.
I'm just not a fan of using chemicals unless absolutely necessary.
It is very possible to have a weed-free yard without using chemicals, it just takes more intensive management.