Organic fertilisers, compost and soil contaminated with clopyralid
In the last couple of years, gardeners in Norway, Sweden and Denmark have frequently reported a lack of growth in vegetables and damaged crops. The symptom of leaves curling in led to a suspicion of soil, compost and organic fertilisers being contaminated with pyralid herbicides. Clopyralid and aminopyralid mimic the plants' own hormone, auxin, and disrupt plant development even at very low concentrations.
Clopyralid and aminopyralid bind to the auxin receptors in the plant, and if enough receptors are blocked, the plant will die. Monocot plants, such as cereals, are not affected. Other known synthetic auxins used as herbicides are 2,4-D, mecoprop, dicamba, picloram and triclopyr, but it is primarily the pyralids, especially clopyralid, that are problematic because they are not degraded well in soil/straw/plant residue.
Clopyralid detected in pelleted livestock manure may originate from previously sprayed straw in the manure but may also be due to straw and grass used for feed containing pyralids, as clopyralid in feed mostly goes straight into urine and manure.
Clopyralid in organic fertilisers may come from vinasse. Vinasse is a by-product of yeast, sugar and ethanol production. The vinasse used in the plant industry for sale on the Nordic market comes mainly from sugar beet produced in Germany, France, and Poland. Spraying with clopyralid in sugar beet production causes clopyralid to end up in the vinasse.
What are harmful levels of pyralids?
In 2021, the Norwegian Food Safety Authority tightened its regulations on fertilisers in terms of pyralid contamination and announced that "fertilisers should not be marketed for sensitive crops if the recommended use leads to a content of more than 1 μg/kg of aminopyralid or clopyralid, individually or in combination in a soil/culture medium. For all fertiliser products, recommended use must not lead to damage to the plants for which they are marketed, or to sensitive crops in subsequent growing seasons. During inspections, the Norwegian Food Safety Authority will be entitled to ask for documentation of assessments that have been made and follow up on the products that are sold.”
Plants have different sensitivities to clopyralid. For example, growing strawberries in a soil treated with fertilisers containing a high level of clopyralid can work, while more sensitive plants tolerate less clopyralid and die. Plants particularly sensitive to clopyralid are:
- Beans, clover, lentils, lettuce, peas, tomatoes, sunflowers (can withstand less than 1 µg/kg in the soil)
- Plants slightly less sensitive to clopyralid: Potato and corn (can withstand less than 10 µg/kg in the soil)
- Non-sensitive plants: Strawberries (tolerates 1000 µg/kg) and asparagus (1000 µg/kg).
The list is not exhaustive as there is still a lot we do not know about the tolerance limits of different plants. Different cultivars may also have different tolerances.