Horse and cow manure is not a very good fertilizer. However it does make for good compost to add micro nutrients and good microbes. Just make sure it is well rotted.

Horse manure is great once it is fully decomposed. You can tell it is fully decomposed when you can grab a heaping double handful and plunge your nose into it. It should smell like very fresh soil - like a forest floor after a summer rain. If you use it before that you will smell it for weeks.