Horse manure is fine for a vegie patch, but best allowed to age a little as it can be 'hot'. It can have weed seed in it too, and hot composting it will largely take care of this problem.

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The secret is the soil.

Aging manure means 6months to 1 year, turning in straw or hay, other browns, make it even better.

If you put it on top of your garden in the fall, then it should be fine come time to plant in the spring, but seeding might not work that great, unless it's worked in real good in spring.

Ditto on the weed seed, cow manure is probably the best bet for 'cleanest poo', and it's not as hot as others are.

You'll learn best after your first oops.

Your horse manure can be ready to use more quickly if it is composted. As little as 6 weeks or so. It's really good for improving the texture of soils and gives crops like potato and pumpkins a good deep base to draw on. It's not so much a fertilizer as a soil improver, so other methods of feeding the soil will probably be required, like adding chicken manure to the compost too.

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The secret is the soil.