Well, it's a good question. I read a lot, but this is anecdotal.

A friend obtained horse manure which he composted and applied in spring. Transplant roots developed "corkscrew" roots (his term) before dying.

The stable had sprayed the herbicide Tordon on pasture.

There is evidence that a worm treatment for horses won't do your compost worms any good.
I would avoid any worm treated manures.

In the United States, for animal manure to qualify as an input to a food that will quality as Certified Organic, the National Organic Standard requires that the manure come from animals not given medications "routinely". "Routinely" means given in disregard of medical advice or for non medical reasons. In the US routine use of medications is a concern with feed lot and dairy cattle and poultry manure, but not with horse manure. Horse owners administer drugs to kill intestinal parasites, and vaccines and other medications as needed by the individual horse, under veterinary supervision.

That said, the drugs used to kill intestinal parasites will kill soil nematodes, both good and bad nematodes. These drugs normally are given not more frequently than every 6 weeks and often less frequently, and they clear the horse in 1-2 days. Thus, most horse manure will have no drug in it. If you plan to apply manure directly then you would want to know about the deworming schedule of the facility where the manure comes from. If you plan to compost the manure before use, it doesn't matter.

Only an idiot would let valuable animals graze on pasture sprayed with herbicides. Pastures normally are mowed, not sprayed.

Okay, first off, good question about the meds residuals. I don't know but suspect there would be meds residuals in manures. How much and how strong they'd still be, who knows, too many variables. As far as the meds surviving composting, once again, who knows, too many variables. Testing could probably show residuals but I'm not sure all compost testing would test for all possible meds. I've looked at some (not all) test order forms, med residuals wasn't on the ones I've seen. It's only been recently that tests have become accurate enough to detect low order (PPB) herbicide residuals so maybe in the future other tests will be more common.

Second, Tordon is sprayed on pastures, that is what it is designed for and there are no grazing restrictions except for "lactating dairy animals". Ergo some people would, and do, let valuable animals graze on treated land, it doesn't hurt the animal.

Third, I do not compost manures for some of these very reasons. I don't want to have to deal with this issue so it's easier to just say I don't do manures.

Lloyd

Only an idiot would let valuable animals graze on pasture sprayed with herbicides. Pastures normally are mowed, not sprayed.

That's a pretty strong statement that really doesn't bear much support. Pastures are very commonly treated with herbicides - it is probably the most effective way to keep pasture lands healthy, ecouraging pasture grasses over weeds, plus it kills off a lot of weedy species that are toxic to livestock. And there's more of them than you might think. Obviously, there are some grazing restrictions on herbicide treated pastures but generally, the restricted period is very short, typically a week or less. FWIW, most commonly used pasture herbicides pose NO toxicity to livestock.

As to the use of Tordon -- the active ingredient in Tordon is picloram, a very efficient systemic herbicide for woody plants and most broadleaved weeds and that most grasses are resistant to, making it an ideal herbicide for use on pasture lands. However, picloram is one of a handful of extremely persistant and compost-resistant herbicides, so even if hot composted, manure from livestock grazed on Tordon-treated pastures should be avoided.

I also question the restrictions you assign to "organic" use of manures. NOP is very clear on how/when these can be used but it doesn't make distinctions regarding medications, at least none that I can find with my research. If you have documentation to support your statements, I'd be interested in seeing it.

If the manure is hot composted (NOP preferred), then much of the concerns are moot. Most livestock medications as well as any potential pathogens are effectively neutralized through this process and the resultant composted manure can be used at will. Non-composted manures do have restrictions placed upon them, but these are related to the timing of the application relative to harvest, not because raw or fresh manure cannot be considered "organic" or used in an organic garden. And remember the NOP oversees certified organic farming, not what home gardeners are free to use in their own organic (or mostly so) garden.

Where you get your animal manure may well determine whether there will be problems with it or not. Confined Animal Feeding Operations (CAFOs) routinely give the animals in their charge antibiotics and dewormers because of the conditions these animals are kept in and because they do help those animals gain weight faster. Some smaller operations may do the same.
Since it is easier to spray poisons around than to properly manage a pasture that mayu be the practice some growers follow so some of the feed their animals receive can have residues from those poisons which will be in the manure.
The only way to know whether the manure you can get is not contaminated by poisons or antibiotics is to know the grower.

"The only way to know whether the manure you can get is not contaminated by poisons or antibiotics is to know the grower."

I can't comment on "poisons" or the effect of antibiotics in soil after composting and a growing season.

But knowing and trusting your input source leads to peace of mind. The breeder from whom I obtain manure is a recipient of my produce and though neither of us have an organic soapbox we do minimise worrying over that which we don't know by wise practice; in his case, knowing how to keep healthy horses.

I almost wish I hadn't started googling this subject!

In case anyone wants to read further, I pasted the link to a study that tested whether c.jejuni in cow manure can survive composting. The composting process took 10 months total. The study doesn't give the temps of the windrows, but I imagine things were pretty steamy. Of course, this is only one study. I'd love to hear if anyone else knows of other studies or findings.

Here's the final sentence of the abstract:
The findings of this study indicate that campylobacteria excreted in cattle feces persist for long periods in compost and call into question the common belief that these bacteria do not persist in manure.

Campylobacter jejuni is now recognized as one of the main causes of bacterial foodborne disease in many developed countries [wikipedia]

Here is a link that might be useful: Study re: survival of bacteria in cow manure

Most of the bad stuff has a time date & hot compost will kill it. Plus, the ppm is small in the waste & compost spread it farther. Yes. I went there!
I do not worry, because we do not use these thing on our horses & they are fine. We let BSF live & die in the circle of life. I do not, but some of this pure green horse manure has been side dressing for vegetable plants.
I think some of us go over board on the whole got to be pure thing. It is manure people, it comes out of the end of the horse,cow,sheep,hen,rabbit!I was raised on this compost in the garden, we bought fruit, made biscuits,(no loaf bread),sugar, cooking oil, flour,peanut butter & syrup. Everything, meat,corn mill,eggs,milk,butter,vegetables came off the farm.