Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings

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Caution to Hay Producers, Livestock Owners, Farmers, and Home Gardeners

Many farmers and home gardeners have reported damage to vegetable and flower crops after applying horse or livestock manure, compost, hay, or grass clippings to the soil.

The symptoms reported include poor seed germination; death of young plants; twisted, cupped, and elongated leaves; misshapen fruit; and reduced yields.

These symptoms can be caused by other factors, including diseases, insects, and herbicide drift. Another possibility for the source of these crop injuries should also be considered: the presence of certain herbicides in the manure, compost, hay, or grass clippings applied to the soil.

The Herbicides Of Concern

Aminopyralid, clopyralid, and picloram are in a class of herbicides known as pyridine carboxylic acids. They are registered for application to pasture, grain crops, residential lawns, commercial turf, certain vegetables and fruits, and roadsides.

They are used to control a wide variety of broadleaf weeds including several toxic plants that can sicken or kill animals that graze them or eat them in hay. Based on USDA-EPA and European Union agency evaluations, when these herbicides are applied to hay fields or pasture, the forage can be safely consumed by horses and livestock—including livestock produced for human consumption.

Herbicides registered for use in NC that contain picloram, clopyralid, and aminopyralid:

  • Pasture and hayfields: Curtail (2,4-D + clopyralid) , Forefront (aminopyralid + 2,4-D), GrazonNext (aminopyralid + 2,4-D), Grazon P + D (picloram + 2,4-D), Milestone (aminopyralid), Redeem R&P (triclopyr + clopyralid), Surmount (picloram + fluroxypyr)
  • Commercial turf and lawns: Confront (triclopyr + clopyralid), Lontrel (clopyralid), Millennium Ultra Plus (MSMA + 2,4-D + clopyralid + dicamba),   Millennium Ultra and Ultra 2 (2,4-D clopyralid + dicamba)
  • Commercial vegetables and fruits: Clopyr AG (clopyralid), Stinger (clopyralid)

All products listed are manufactured by Dow Agrosciences, LLC with the exceptions of the Millennium products by Nufarm Americas Inc. and  Clopyr AG by United Phosphorus, Inc. Herbicide product names and formulations change; always check labels for active ingredients.

These herbicides pass through the animal’s digestive tract and are excreted in urine and manure. They can also remain active in the manure even after it is composted. The herbicides can also remain active in hay, straw, and grass clippings taken from treated areas. The herbicides leach into the soil with rainfall, irrigation, and dew. As with many other herbicides, they can remain active in the treated soil.

Picloram, clopyralid, and aminopyralid can remain active in hay, grass clippings, piles of manure, and compost for an unusually long time. These herbicides eventually break down through exposure to sunlight, soil microbes, heat, and, but some field reports indicate that complete deactivation and breakdown can take several years. Hay has been reported to have residual herbicide activity after three years’ storage in dry, dark barns. Degradation is particularly slow in piles of manure and compost. Depending on the situation, the herbicides can be deactivated in as few as 30 days.

When mulches, manures, or composts with residual herbicide activity are applied to fields or gardens to raise certain vegetables, flowers, or other broadleaf crops, potentially devastating damage can occur  Crops known to be sensitive to picloram, clopyralid, or minopyralid:

Beans, Carrots, Compositae family, Cotton, Dahlias, Eggplant, Flowers in general, Grapes, Legumes, Lettuce Marigolds, Mushrooms, Peas, Peppers, Potatoes, Roses-some types,  Spinach*, Sugar beets*, Strawberries*, Sunflowers, Tobacco, Tomatoes, Umbelliferae family, Vegetables-in general.

*Applies to aminopyralid and picloram only. This information was obtained from product labels of many of the herbicides listed above. filepath=/PublishToInternet/InternetDOWAGRO/range/pdfs/noreg/010-57689 and the DowAgriSciences Manure Matters website.

How To Prevent Herbicide Damage To Non-Target Plants

The label on every herbicide contains detailed instructions, including animal feeding restrictions and safe use of manure or crop residues. When used as directed on the labels, these herbicides should not cause the problems noted above. The manures and plant residues are safe to apply to grass pastures and grass hayfields, effectively recycling them.

Most of these herbicides have a crop rotation restriction of at least 12 months before certain vegetable or forage legume crops can be planted in treated land. The problems arise when the hay, manure, grass clippings, or other affected materials are sold or given to others who have no knowledge of the herbicides used or of the adverse effects their residues can have on other plants.

The information about the herbicide persistence and effects on broadleaf plants does not always follow the hay, manure, compost, or other materials. Every individual in the chain of use of products treated with these herbicides should provide detailed information on the herbicide restrictions to prevent potentially catastrophic problems for other farmers, gardeners, and for themselves (including possible liability).

Farmers and Gardeners Wanting to Use Manure or Compost

Before acquiring or using manure—fresh, aged, or composted—ask what the animals were fed, the origin of the hay, and what, if any, herbicides were used on the hay or pasture. Some livestock owners can tell you this, but many might not know the products used or origin of the hay they purchased. They may suggest the manure is “safe” because their animals have not been affected.

Take great care in using contaminated manure or compost to grow nonsensitive commercial food crops. Consult the herbicide product label to determine if the pesticide is registered for use (legally permitted to be applied) to that crop. If the product has already been applied to the soil, tilling it several times during the growing season, irrigating the area, and planting it into a non-sensitive cover crop for a year or two will help the herbicides break down.

Farmers and Gardeners Wanting to Use Hay or Grass Clippings

If you want to use hay or grass clippings as mulch or in your compost pile, find out what, if any, herbicides were used on the field or turf area. Be particularly careful about obtaining grass clippings from golf courses and other commercial turf fields where these herbicides are commonly used. Most homeowners do not use these herbicides because they are not labeled for use on residential lawns. Be careful about obtaining hay or grass clippings from sites where herbicides of concern may be commonly used. For instance, clopyralid-containing products have not been registered for use in residential lawns since 2002, so if pesticide applicators have followed label directions, clippings from residential lawns should not present a problem to use as mulch around vegetables and ornamentals.

As previously mentioned, the safest practice in residential lawns is to return grass clippings to the lawn. If you find yourself with contaminated hay or grass clippings, spread them on non-sensitive, non-food crop areas, burn them, or arrange to have them disposed of safely. If the hay or grass clippings have already been applied to the field or garden, remove them if possible, till the soil (multiple times will enhance degradation), sow a non-sensitive cover crop, and let it grow for a year or two to help the herbicide break down.

Responsible Herbicide Use = Healthy Farms And Gardens

Animal manures and composts made from them are excellent sources of nutrients and organic matter for growing food crops. Soils mulched or amended with manure and compost become dark, aromatic, fertile, and active with earthworms and beneficial microorganisms. Farmers and gardeners are encouraged to use these products but must exercise proper caution to prevent damage.

Herbicides are important tools that hay producers use to produce quality, weed-free hay. The use of these herbicides is no more likely in North Carolina than in any other state. Many North Carolina hayfields and pastures do not have herbicides applied on a regular basis. Hay and pasture acreage is among the “greenest” in North Carolina, delivering multiple environmental benefits.

Remember that each pesticide product label states, “It is a violation of Federal law to use this product in a manner inconsistent with its labeling.” Everyone should read an herbicide’s product label instructions before use. All parties need to be aware of the possibility of residual herbicide activity.

Farmers and gardeners need to be fully informed about what they are applying to their soil because the results can be disastrous for a farm business or gardener if one of these herbicides has been applied. Much of the information for this article came directly from the herbicide product labels and the United Kingdom DowAgrisciences website devoted to this issue.

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