We had to plant grass seed in our yard that was a freaking dry patch of dirt. Get a really good pasture mixture from your local feed store. The best time is during late winter. I know in MD it is around Feburary March, but this year probably more towards the end of March. But only if you want to plant more grass where there is already existing grass. If you are trying to replant a whole new pasture ie. Get rid of the other grass, then your best bet is during late summer, August-September. Depending upon when you plant, (late winter) you can let your horses walk on the seed, which can actually help the grass seeding get farther into the soil. But once the grass starts to pop, you need to get the horses off asap. You kinda have to let the grass grow to about 10-12 inches, cut to 3-4, grow to 10-12, and then cut again to 3-4, then let it grow one more time to 10-12 inches. By this time the grass should be ready for your horses to be let out on. Don't let the horses graze the grass down past 3-4 inches, then you have the same problem of not having enough grass. Oh and don't forget to fertilize, every year. This will help the grass grow healthy and regulary.
Hope I helped!
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