Know Your Sources: Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings

— Written By Eli Snyder
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A number of calls have come into the Extension Center over the last couple of weeks regarding vegetable plants exhibiting strange symptoms. Set out plants looked healthy and then they developed twisting, bending leaves. In some cases the whole plant was affected. In others, only a few leaves were affected and the plants seem to be growing out of the odd growth. Viruses, certain insects, and herbicide drift can all lead to these symptoms in sensitive plants.

But in at least one instance observed recently, this damage was the result of herbicide carryover in manure that was applied to the garden. This manure likely came from an animal that consumed forage treated with a herbicide in the pyridine carboxylic acid class. These are highly persistent broadleaf herbicides with active ingredients picloram, clopyralid, or aminopyralid. They can persist in soil, on treated grasses or hay, or in manure from animals who consume treated forage, for an unusually long time.

Treated forage is safe for livestock, provided the applicator observes the proper grazing or harvest interval. But when manures, composts, or mulches with residual herbicide activity are applied to fields or gardens to raise certain produce or flowers, devastating damage can occur.

These herbicides break down through exposure to sunlight, soil microbes, heat, and moisture. They can be deactivated in as few as 30 days, but field reports indicate that complete deactivation and breakdown can take years. Hay has been reported to have residual herbicide activity after three years’ storage in dry, dark barns. Degradation is especially slow in manure and compost piles.

Several common pasture and hay herbicides are in this group, including Curtail, Foregront HL, GrazonNext HL, Grazon P, Milestone, Transline, and Surmount. Lawn herbicides in this group are Millenium Ultra, Confront, and Lontrel. Stinger is used in crop, fruit, and vegetable fields, and its active ingredient also belongs to this group.

If you are purchasing hay, always ask your hay or grass clipping source if it was treated with one of these herbicides. Generally, hay that has a legume mixed in, such as alfalfa, clover, or lespedeza, was not treated. However, the absence of legumes does not mean that it was treated with a highly persistent herbicide. Consider asking for a copy of the herbicide label if you and the provider are not sure. Gardeners should always ask their manure or compost source if the animals were fed treated forage. Do not build in-ground vegetable gardens in lawn area recently treated with a persistent lawn herbicide.

If you are unsure about herbicides in manure or compost, complete a pot bioassay before applying to your garden area or giving it away to other gardeners. Mix a small amount of compost or manure in a 1:1 ratio with commercial potting media, and fill 4-6 small pots with the mixture. Fill a few separate pots with only the commercial media. Plant 3 bean or pea seeds, or one tomato transplant, in each pot. Keep them watered and in a warm place with light, just as you would a normal seedling, for a few weeks. Allow to grow until 3 sets of true leaves develop, or if a tomato, until the first fruit forms. If herbicide is present, the plants in pots with compost or manure will start to show symptoms, whereas those in just commercial mix should not. If you would like to test this in a field where herbicide may have been applied or may still be active, plant a few beans or peas in the area in question, and see if herbicidal symptoms develop. If they do, you can try this field bioassay in the future to determine if the herbicide is still active.

If you find that your manure or compost, hay, or grass clippings still have active herbicide, apply it only to grassy areas that will remain in grass. With any of these amendments or soil, you can encourage chemical breakdown by exposure to sunlight, water. Tilling ground or turning a pile will expose more of the soil, manure, or compost to sun, and encourage breakdown.

If the herbicide damage is minor and your plants grow out of it, the next question is often whether the produce will be safe to eat. These herbicides are not evaluated for residue levels in many garden crops, because they would generally kill them. Therefore, there are not threshold established for residues and human safety. Consume at your own risk. If you are sensitive to chemicals or want to avoid exposure for a particular reason, you probably would avoid eating them. That being said, if the plant grows out of the damage, it is possible that very little residue persists in that plant.

Hay and pasture managers, people who feed horses or cattle, and gardeners should all be aware of the potential for these herbicides to cause damage when used in the wrong setting. With open communication between all parties, they can be used safely. If you suspect you may have herbicide damage or observe other unusual symptoms on your garden plants, you can always contact the N.C. Cooperative Extension, Wilkes County Center to help figure out the problem.