[Community_garden] Herbicide in Manure

[gordonse at one.net]

We have a tricky situation here and would appreciate any advice.

Our gardens have access to a horse manure source again.  In the past we
have read of animals being fed herbicide treated gmo grains as part of
their diet.  The herbicide on the grains makes it through the animal and
into the manure.  The herbicide also survives the usual composting
process.

We  would like to use the composted horse manure if we can, so we asked
the horse farm if there are gmo grains in the grain blend they feed their
horses as a treat.  (Horses are mostly on pasture.)  The horse farm didn't
know, but gave me the name of their supplier.  The supplier didn't know
and said they would have the people that blended their horse feed mix call
me.  In the meanwhile we have waited two more weeks for an answer.  (Will
follow up as necessary.)

Here I should probably note that we have a mix of gardeners on the
gardening organic spectrum.  Many won't be using the composted manure
since it's not organic.  Others will use it if it "just" has pesticide
residue and not herbicide residue as they spray their own garden food with
pesticide.  Some don't mind the herbicide if it's no longer active as they
spray herbicides on their own food gardens.  However no one wants to use
it on their food beds or rows if it still contains some active herbicides.

If we can't get an answer or it turns out that there is gmo feed in the
mix, we have several questions:
1) Is there a likely state group who can test for herbicides in the manure
for free or frugally?  (Have already asked master gardeners and they
didn't know)
2) Is there a national testing service that is good and frugal?
3) Is there a canary type test we could use in the garden such as growing
a highly sensitive plant in a mix of our soil and the composted manure? 
If so what soil-manure ratio should we use and which annual vegetable
plants are the most sensitive to the herbicides?
4) If it does contain active herbicide residue, how much additional or
total composting time is it likely to take to break down all the
herbicide?

Sharon
gordonse at one.net