I've never used it for cannabis, but did in the past for a year round veggie garden. I didn't use it straight, but spaded an 8" layer of it into 3x10 beds.
Horse manure is awesome. Because it has a lot of partly or undigested straw (carbon) and urine (N) it turns into a perfect compost on its own.
After 4 years it should be broken down nicely.
If you can, I would supplement a little mild organic fert, like a 3-4-3.
While the texture and micronutrients should be there, its probably a little low on the NPK for a fast growing annual.
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