Herbicide contamination of garden amendments is not a new problem, but it’s one that impacts growers across Missouri every year and has the potential to cause major losses for those not aware of the issue. The problem originates with the use of several common broadleaf herbicides that are sprayed onto pastures (and sometimes fields of small grains such as wheat) to control broadleaf weeds. The active ingredients that are the most persistent and will be the subject of this article are picloram, aminopyralid and clopyralid.

Tomato plant from southeast Missouri showing signs of severe herbicide damage. This plot was amended with manure that was contaminated with persistent herbicides. Photo Credit: Katie Kammler

When hay or straw from fields treated with these products is harvested, the herbicide residue persists on the plant material, in some cases for several years after harvest. When hay is fed to livestock such as horses and cattle, the herbicide passes through the animal’s digestive tract and is excreted in urine and manure. Even if the manure is composted thoroughly the herbicide can still persist. If manure or compost containing these active ingredients is applied to the soil, any broadleaf plant (most vegetables and all fruits) can suffer damage or total decline. In some cases, these herbicides can persist for several years in soil after the application of contaminated manure or compost. This can be a devastating situation for a grower, especially when growing space is limited or the application is made in a fixed structure, such as a high tunnel.

Close-up image of leaf distortion on a potato plant. The potatoes were planted in soil where contaminated manure had been applied. Photo Credit: Katie Kammler

There have also been incidents where hay or straw applied as mulch has caused damage to vegetable crops. One notable example in Missouri occurred when wheat straw and hay, applied as mulch in several high tunnels, released these persistent herbicidal compounds in a gaseous form (outgassing). The damage was noted several weeks after planting, when the high tunnel environment grew much warmer and expedited the outgassing process. In a field and greenhouse experiment, contaminated straw leached herbicides into the soil, causing dramatic yield reductions in strawberry crops. I don’t know of any colleagues that have encountered issues with herbicide leaching from straw. This might be due to the fact that the levels of herbicide leached might be quite low, and to the untrained eye, no damage may be visible to crops.

Grass clippings from turf farms and golf courses can also be of concern. Products containing picloram, clopyralid, and aminopyralid are no longer labeled for homeowner use but are commonly used in commercial turf settings. However, homeowners might still use herbicidal products that can cause issues, such as broadleaf over the top sprays, or weed and feed products. Herbicides in these products can also cause problems for gardeners and growers.

Pepper plants showing signs of leaf distortion, note the "parallel venation" on the right, and normal venation on the left. Parallel venation is an indicator of herbicide damage. These peppers were planted in a high tunnel where hay was applied as mulch between the rows. The outgassing of herbicide from the hay affected the pepper plants. Photo Credit: Katie Kammler

It’s hard to understand the true scope of the problem of herbicide contaminated inputs, but every year MU Horticulturists get reports from across the state from home gardeners and growers experiencing issues with herbicide contamination in amendments. The issue has been highlighted in many publications by various state’s Extension services as a concern for both growers and home gardeners.

This issue of herbicide contamination might cause growers to question the use of inputs they have come to rely on. However, manure and compost remain valuable sources of plant nutrients and organic matter. Organic mulches such as straw and hay can moderate soil temperatures, retain moisture, provide weed suppression, and build soil organic matter. Use of these agricultural by-products can be a great way to reduce costs, eliminate waste, and retain valuable plant nutrients that take a lot of energy to mine, synthesize and transport.

There are several steps growers can take to protect themselves. First, ask questions of the suppliers of inputs and amendments such as; Do you know what herbicides were sprayed on this straw or hay?, Do you know if the hay fed to the livestock producing this manure was sprayed with herbicides, and if so what kind of herbicides?, Is your compost produced with manure, and do you know if the livestock producing the manure were fed hay sprayed with herbicides?. It should be noted that poultry litter and manure are generally free of persistent herbicides.

In some cases, suppliers of inputs and amendments might not know the answers to your questions about the use of the herbicides of concern listed in this article. If they don’t know or are unwilling or unable to answer your questions, consider finding another source. Many herbicide applicators will know the brand name of the product, such as Grazon used on hay fields or Curtail used in wheat crops. However, they might not be familiar with the active ingredients found in a branded product. You can use a searchable database to quickly identify the active ingredients found in a product or contact your local MU Extension office for assistance.

Searchable databases can allow you to quickly search the brand name of a product and identify the active ingredients. The above example contains the active ingredient picloram, one of the persistent herbicides highlighted in this article.

The second way growers can protect themselves is to utilize a green been or pea bioassay test. Plants in the legume family, such as green beans and peas, are particularly sensitive to broadleaf herbicides and can be used to indicate whether they are present in amendments. To perform a bioassay test you need to have a sample you want to test (compost or manure) and a control (unamended peat-based potting mix). Fill several pots with only potting mix, and several pots with 2/3 potting mix and 1/3 compost. Plant the legume seeds into the pots and place a saucer under each pot so no water running through one pot contacts another pot. If herbicide residue is present in the sample, visible symptoms of damage will occur within 2-3 weeks. You can compare the plants in control and the sample pots for any morphological differences such as stem curling or leaf cupping. From my review of literature, there has not yet been developed a standard procedure to perform a bioassay of hay or straw mulches.

Peas showing symptoms of varying severity of herbicide damage from contaminated compost used in a bioassay test. Photo Credit: Washington State University

The last source of contamination I will touch on doesn’t involve the use of inputs, but the use of land previously tended for crops, hay, or forage. Herbicides used on row crops and hay or forage crops sometimes have what is commonly called a “plant-back” or rotation restriction. These rotation restrictions are found on an herbicide label and indicate how long you should wait before you plant certain crops in a field previously sprayed with certain herbicides. For example, one of the herbicides highlighted in this article and labeled for wheat crops lists an 18-month rotation restriction for potatoes, beans, or peas. The restriction is in place to avoid crop injury and the potential for illegal herbicide residue levels in a harvested crop. If you are looking to rent land or purchase land for growing vegetables, ask the landowner questions about the use of the herbicides of concern on the land.

This tomato was planted in a high tunnel that was sited on land that was previously in pasture. The herbicide "carried-over" in the soil, causing the herbicide damage. Photo Credit: Katie Kammler

The moral of the story is to investigate the source of your amendments to protect yourself from the risks of herbicide contamination. If you know a grower that might benefit from this article, please share it with them to help protect them from the risk of herbicide contaminated inputs and amendments.