Peggy said:
<We believe it may be clopyralid but we are not sure and testing is continuing.>
Strictly anecdotally, clopyralid (sold by Dow-Elanco as "Lontrel") sounds like a
likely candidate. Lontrel is very expensive "growth regulator" herbicide,
popular because its very effective. Farmers use it to control dificult
broad-leafed weeds like scentless chamomile, perrenial sow thistle, wild
buckwheat & Canada Thistle in broad-leaf crops like flax & canola or in cereals
& hay grasses. It can also be mixed with other herbicides to control a wide
range of weeds. When used at higher concentrations (up to 0.33L/ac) it controls
Canada Thistle for up to a year after application.
The lengthy persistence of clopyralid is a concern. For instance, Saskatchewan
ministry of agriculture & food recommends not grazing or harvesting forage grass
treated with high concentrations of Lontrel for 2 months after application. If
spoiled hay or even cereal straw from treated fields found its way into
commercial compost, the herbicide MAY still have been active. Wheat, barley &
oats may only be treated with half of the above dose in Sask, but that may not
be adequate for thistle control.
Lastly, that 2 month photo-degrading time is for STANDING grass out in the sun &
air. Conditions in a hay bale or in a commercial compost windrow would possibly
require even longer time to biodegrade. The manufacturers warning label says to
replant fields treated with clopyralid the following year only with wheat,
barley, oats, rye, canola, flax or forage grasses & not to allow the product
anywhere near "desirable broadleaf plants".
In short, not an ideal ingredient for a compost pile!
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