Composting cow manure has several benefits. In addition to eliminating harmful ammonia gas and pathogens (like E. coli), as well as weed seeds, composted cow manure will add generous amounts of organic matter to your soil. By mixing this compost into the soil, you can improve its moisture-holding capacity.
This allows you to water less frequently, as the roots of plants can use the additional water and nutrients when needed. Additionally, it will improve aeration, helping to break up compacted soils.