I think I'd concentrate more on organic matter than a warm season manure crop. You can get a lot of free things to make your soil richer. A manure crop to me is when it's cold out and I don't want to get out there and mess with it. Some ideas would be wood chips from your local city, aquatic weeds, shad, chicken feathers from chicken houses, manure from ranches, spoiled hay from last year, grass clippings and so on. Just keep your ph in check if you add acidic things. In Fall plant a good brasica with a lot of tonnage per acre and get it under before it goes to seed. That way you have a little fresh composting material underground but from the warm months you can already have several tons that has composted instead of several pounds.