Worst thing about horse manure is that most stables use wood chips for bedding instead of hay. If you have sandy soil that can become a problem if used chronically.

Horse manure takes awhile to break down. I think it was about a year after I dug in a substantial amount that I got a bumper crop of green chilies. I thought 3-4 months would be plenty of time for the horse manure to break down and do good things for the plants, but it took longer.

I didn't buy the manure. I approached somebody in the area who has horses and asked if I could go onto their property and collect manure. It was harder work than I expected, even though in this climate the manure dries out very quickly. I collected about sixty or seventy pounds from the corrals and put it in cardboard boxes. The odor wasn't very strong. I gave the owner of the horses a bottle of wine. She was thrilled that somebody else cleaned up after her horses, plus she got a bottle of wine.

Lorna

Thousands of us use well composted manures - horse included - in our gardens. It can be a great soil amendment. But it must be well composted first - 4-6 months minimum - for safety.

You'll find many discussions about its use over on the Soil & Compost forum here at GW. I have linked one of the current ones for you below.

Dave

Here is a link that might be useful: Composted manure discussion

I use it a little different. By putting a 6 inch layer of DRIED horse manure on top of my raise beds,and start it burning gives me a great amount of carbon.The dried manure want blaze,it will burn very slow.Sixteen hours for a 2 foot square area.That is why it turns out to be carbon and not ashes.
Jackman44

I just picked up a load of horse manure over the weekend, a load of well composted horse manure. This stuff had been around for about a year. It was a great amendment to the flower beds. The fresh stuff can be added to your compost pile and used next year.

Just watch for the weed seeds. I had a extensive cover crop of nightshade one year. Pulled it, pitched it in the compost pile and haven't seen any more since then.