Horses may eat more grass with seed heads depending on how they're fed (outside grazing...hay vs. straw). If un-composted horse manure is used and they eat stuff with seed heads (or composted poop doesn't get hot enough for the particular seed) it may lead to some weed issues.

For the most part a cow's digestive system can take care of anything.

Try to get as few wood shavings as possible, but it's not worth obsessing over trying to get it clean.

I added a pile of the same material a few weeks ago and couldn't believe the weed starts!
I'm not unduly upset as I covered the whole area in black plastic. This will prevent too much compaction and nutrient loss over the winter. Come spring the weeds should all be gone.
Go for it, the addition of all that OM should do your soil a world of good.

Thanks for responses. These animals are on a horse rescue ranch and do not graze. They feed only in the barn which I hope lessens the weed problem.

I used aged horse manure in a raised bed this year and weeds were not a problem, not in the least. This spring, when we were getting a couple inches of rain every week, the soil (1/3 bottom land dirt, 1/3 compost and 1/3 manure) drained extremely good. This summer, when the drought came, it held moisture extremely well. Well, at least until the temps hit the mid-90s - actually, counting the light and heat reflected back onto the bed by a huge, white, concrete block building it was the low triple digits. Then the soil would need watered every three or four days.

But this was aged - I used my bare hands to add it to some containers.

Mike

I would compost any horse or cow manure before adding it to your garden.
I wouldn't worry about the wood products mixed in. A good compost pile needs both "greens and browns" The manure (itself) is green and the bark/sawdust is brown. The weed problem is NOT lessened because the horses are eating hay, in fact it might be worse because of it. Cheap hay usually has a lot of weed seed and good alfalfa hay has few or none.

The composting will not only help heat up your compost pile, but the heat should also destroy the weed seeds. If you don't already have a pile, this would be a good starter for it.

Last year was my first year gardening in raised beds. I filled the beds with a mixture of native soil (clay), compost (till it ran out), and horse stall sweepings from a friend's barns. There was quite alot of wood shavings in it. I did not have alot of trouble with weeds, though I think this had been piled up for "awhile". However, I believe the wood shavings tied up the nitrogen in the soil for that first season. I finally resorted to Miracle Grow to get a crop. By this year, it was all nicely broken down and I had a perfectly wonderful garden. Through a soil test, I learned that my soil is low in nitrogen to begin with, so maybe the shavings were less of a factor than I originally believed. At any rate, I would use it again for the way it conditioned my horribly sticky, tight clay soil. By spring it was positively loamy, with a minimal number of clods in it. I think what I learned is that when you're using manures, time is your best friend.

With either one, I'd at least add it in fall, wait a few weeks in early spring to see what seeds sprout, and remove weed seedlings before planting your own crop.