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Herbicide damage from composted ditch hay

By Robin Trott - U of M Extension Educator

This past summer, a friend and market farmer who sells produce at the local farmers market came in with a problem. Her high tunnel planted tomatoes had heavily veined, cupped and wrinkled leaves…all the typical signs of herbicide damage.

The problem: She used no herbicides near these plants, and the high tunnels they were grown in protected them from any possible drift issues.

It was a costly mystery. The plants never produced, so she had no tomatoes to sell out of the high tunnel last year.

Further investigation turned up the culprit. Ditch hay. This family feeds ditch hay to their goats. The manure is then added to the compost pile, and the compost is used to amend the soil in the high tunnel.

Seems pretty basic. Goats thrive on ditch hay, it’s cheap and easy to harvest, and it can be used twice as feed and compost. So what gives?

Picloram (commonly sold as tordon, grazon and pathway) and clopyralid (commonly sold as stinger, curtail and transline) are often used to control unwanted broadleaf weeds along roadways.

These herbicides are especially popular with local, county and state highway departments because they control hard-to-kill, noxious weeds like thistles and leafy spurge but do not kill beneficial or planted roadway grasses.

When animals are fed ditch hay that has been treated with either picloram or clopyralid, these chemicals pass quickly through the animal without significant degradation and end up in the manure via the urine, usually within a day or two.

Good for the animal, not so great for the compost pile. Sensitive crops (e.g. soybeans, lentils, peas, legumes, potatoes, tomatoes or peppers) planted in fields where contaminated manure has been applied can be injured or killed.

Injured plants may exhibit twisting, leaf cupping, stunted growth and abnormal side shoots.

Well-managed composting may not be enough to get rid of these herbicides. It can take 18 months for the herbicide residues to dissipate to tolerable levels.

If you already have had a problem with composted areas of your family vegetable patch, deep till that area, spreading out the manure that’s already there as much as possible.

A simple bioassay test devised by Washington State University can be done at home before adding suspect compost to your garden.

● Fill three 3-inch pots with potting soil. Fill three more pots with a mixture of two parts compost and one part potting soil. Mark the pots.

● Plant three pea or bean seeds per pot and keep them watered. Capture any water that drains from the pots so it doesn’t contaminate soil in other pots.

● Put the pots in a sunny, warm place. Once the seedlings have three sets of leaves, compare the plants growing in the compost mix with the control group in potting soil. Unusual cupping, thickening or distortion of leaves signifies the possibility of herbicide contamination in the compost.

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