Get your priorities straight. You are not trying to make the perfect golf course, lawn or cemetery. You need to maximize your land so that it produces the greatest amount of horse healthy vegetation with the least amount of runoff possible, at the least cost possible.
1. Do a soil test to determine ph level. A correct balance will give optimum growth and will likely reduce the amount of fertilizer that you use.
2. Check with your extension agent for seed planting recommendations for your area, but do not create a monoculture. Plant several different species of plants and encourage beneficial forbs such as dandy lions.
3. Mow instead of using herbicide. Mowing makes grasses flourish and either destroys or minimizes most summer weeds.
4. Use a subsoiling plow to cut through compaction, allow water penetration of the soil and to fully aerate the ground. This radically reduces runoff and mud accumulation. Might be the best kept secret in pasture management. A subsoiling plow is cheap and along with lime are likely to give you the most payoff for the cost.
5. Hire free labor. Worms and beetles do not charge for the work that they do. Composting techniques that involve earth worm production will not only create great compost but will also give you thousands of earth worms that can be gently transferred to the pasture. They will work to create better soil for years to come. Learn all about the dung beetles that live in your area. Encourage their proliferation. They can bury up to 40% of the manure in the pasture, reducing runoff, aerating the soil, turning the manure into a soil amendment, and radically reducing the number of parasite eggs in the pasture by putting them underground where they will not be consumed by the horses. Remove horses from the pasture for about 30 hours after giving them ivermectin wormers. That drug kills dung beetles.
6. Plant diakon radishes in areas of soil compaction. Keep the horses away from them until they are fully grown. Will reduce soil compaction and will bring nutrients up to the near surface where grasses will be able to use them.
7. Plant winter cover which will reduce runoff and provide some grazing over many of the colder months. I like various rye grasses and clover. (Insulin resistant horses should be kept away from clover as should all obese horses.)
8. Research rotational grazing, become an expert on it and use it as much as possible on your land.
9. Be very proactive in fighting erosion. Seed bare spots. Washes and ruts can often heal themselves by simply placing large tangles of natural baling string from your hay in the fastest flowing parts of the gorges. This slows the flow and allows the soil to become sediment, healing the soil's wound by accretion.
10. Cut swales to slow runoff and consider digging a few retention ponds which can be very shallow and can e dug with a hand shovel. If the soil is not too sandy fence those ponds off from the horses and plant native wetland plants in and around the ponds (e.g willows, cat tails, etc--but no red maple. Wilting red maple leaves are deadly for horses.)
11. There are a thousand reasons to refrain from feeding your horse commercial horse feeds but one of the reasons is that doing so produces wastes that are much higher in ammonia and phosphates than a diet of grass, hay and forbs.
12. Wind row composting is often the best way to deal with waste. The compost forms faster than with traditional methods of composting. Feed round bales in a waste area of the pasture. With a tractor blade make wind rows of the manure and leftover hay for this composting. Very little labor involved, great place to grow earth worms, actually reduces the number of flies that hatch out in wet hay and straw.