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Don’t let manure-based compost ruin your garden!

Hang out with gardeners long enough, and you’re bound to hear horror stories about gardens being ruined by adding compost. ‘How can that be?’, you might ask, compost should enrich a garden, right? Not always!


Horses

As with many aspects of modern life, hidden dangers in the form of toxic chemicals may be found in your compost source materials. Gardeners should exercise caution when using manure-based composts due to the potential presence of herbicides, antibiotics and dewormer residues. Of particular concern is clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), a selective herbicide commonly used in agriculture to control broadleaf weeds. When animals, such as horses, consume forage treated with clopyralid or eat hay derived from fields sprayed with this herbicide, it can pass through their digestive system and end up in their manure.


Composting processes do not break down clopyralid, allowing it to persist in the finished compost. When this compost is applied to gardens or fields, the clopyralid residues can harm sensitive plants, even at low concentrations. Clopyralid can affect a wide range of plants, including vegetables, fruits, ornamentals, and trees. It can lead to stunted growth, distorted foliage, or complete crop failure. This contamination can persist for several years, rendering your garden area useless for growing most food crops. 


Spreading composted manure
Spreading composted manure

To avoid a garden disaster, gardeners should take the following precautions:

  • Source Manure Carefully: Obtain manure from trusted sources that can verify the absence of clopyralid, or other contamination in the feed or forage consumed by the animals.

  • Perform a Bioassay: Clopyralid is easy to test for, no lab needed, simply attempt to grow bean plants in the compost. If you can successfully grow a bean plant that looks healthy after a few weeks (not stunted, curled or damaged looking), your compost should be safe to use in the garden. You can find detailed instructions here.

  • Use Compost Wisely: If using compost suspected of containing clopyralid, consider applying it to non-sensitive areas or conducting a small-scale trial before widespread application.


Bioassay for persistent broadleaf herbicides
Bioassay for persistent broadleaf herbicides

Simple bioassay setup: left tray is control (commercial garden soil), middle trays are two compost samples that successfully grew bean plants, right hand tray was horse manure compost that could not even sprout a bean plant, presumably due to herbicide residue. The trial was monitored for another week to make sure the young plants continued to grow well. This was spread on pasture, instead of the garden…avoiding certain garden calamity.


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