Public stables, on not less than 20 acres of land, provided that no building housing animals shall be within 200 feet of any lot line and that no manure or within 200 feet of any lot line and that no manure or dust- or odor-producing substances shall be stored within 200 feet of any lot line. The total number of horses, mules and/or burros shall not exceed one per acre of contiguous usable grazing land.