Make sure it's hot-composted. Horses are basically a tube, they eat whatever and it comes out digested - unlike ruminants like cows, that thoroughly chew their 'cud'. Horse manure will help the fertility for anything you grow, but you'll have tons of weed seeds to contend with, thus the need for hot-composting. I avoid it and prefer cow, but that's just my take.
I'd be suspicious of it even if it is composted good and hot (since the outsides of the pile do not heat up enough). And I say this as both a gardener and as someone who has worked with horses (and the removal of the poo thereof) for the past thirty-some years
Although if you happen to have a source for manure from horses that eat ONLY alfalfa hay and pelleted food -- i.e., no grass hay (not even mix), no grain, no grazing -- that should in theory be reasonably weed-free.
There actually IS one way that I consider fairly 'safe' to use composted horse manure, if it fits with your particular gardening circumstances. I do this myself sometimes. The weed seeds in the manure will only sprout if they are near the surface (or are brought near the surface by your activities). It assumes you are starting a new bed from scratch, or tearing down an existing bed to totally-unplanted dirt such as you would do yearly in a veg patch, and are digging or double-digging the whole bed anyhow.
So, if you find yourself in that circumstance and have some composted or at least semi-composted horse manure you are dying to use, you can load it into the bed in a deep layer, on or instead of the top of the subsoil, and make sure it's covered by at least 8" of regular topsoil. Then you must never rototill or re-dig it the bed (nor make deep planting holes, although in that case any weed sprouts would at least be very localized). Then the composted manure can just sit down there providing a nice rich retentive lower layer for plants to dig their toes into. Most weed seeds will never get light to spur them into germination, and the ones that *did* get triggered to germinate when you were putting the manure in place will be covered by so much topsoil they will poop out before reaching the surface.
But be warned, if you dig this bed over some years later, all bets are off. Most weed seeds can sit in the soil for a really really long time before exceeding their 'best by' date
Me, I put my composted horse manure back on the pasture in spots that need soil amendment. It's useful, it's closer to the barn anyhow, and I can easily live with weeds there
It helps, but I would not count on it killing everything. Also as I think someone else mentioned somewhere on this forum recently, if you want to kill growing weeds then black plastic is the fastest thing, but if you are trying to semi-sterilize the soil then you want clear plastic (and a fortnight or month of sunny August sun, not March weather!).
As some one who had to build rasied beds from scratch 6 yrs ago beacuse our lot is all stone back fill. I have to say that honestly ALL of my flower beds are horse manure compost. No dirt or topsoil, just compost. And I have had great luck with it. Weather I am suspose to or not is another thing but I still have. I have a mix of woodland flowers, old farm style flowers, roses, flowering shrubs ect. The only thing I would keep it away from if veggies. Mellons seem to do okay but not great in it. Every fall or early spring before my plants come up I add 4 inches or so of "new" compost to the top. Usually this is stuff that has been out of the barn between 4-6 months. if it is steaming then there is no way it goes in a bed that has anything in it., instead to one that has just been built. But if it is just alittle "warm" yet it goes right on top the existing beds, just not touching anything aleady out of the ground. I have never had a problem with it burning anything I have. Even my lilies.
Like I said this may not suspose to be a "good thing" or something that is "okay and works for everyone". But in my experence it has for me. And I have plants everywhere. I see no point in a grass yard, unless it is pasture it is just wasted space.
I don't recommend horse manure becuase it is harder to break down. Non like chicken manure, Horse manure is a stronger and harder to be broken down by worms or beatles.
And if it is not broken down, it will not help the plants at all. :happy_flower
I plan to get some to help build topsoil after some excavator damage in my yard. I have rocks and dirt from 20 feet under in the place of my topsoil. Darned city and their sewer project. I don't mind the weeds so much because I think my chickens ducks or goats should like whatever weeds a horse does. I think the weeds should help break my soil up as well. I hope I am not going to build too big of a weed problem but I think I should be ok in the long run with all the grazing going on in my yard.
Nightshade makes a very good point and I would like to modify what I said previously... while I still would not use horse manure in a garden if I had any good alternative at all, if you are really NEEDING some kind of compost/organic matter substance and horse manure is What You've Got (no real alternative), then it would quite definitely be worth using.