Experienced gardeners and farmers know that healthy soil is the key to healthy plants, and one way to improve soil health is to add nutrient-rich organic matter.

One favorite and highly sought-after source of organic matter is animal manure, but if you use it you need to ask questions about it and the animals from which it came.

A vegetable farmer in southeast Douglas County, who asked not to be identified, learned the dangers of using animal manures the hard way. After incorporating cattle manure into areas where tomatoes and potatoes were later planted, the vegetable producer is losing the crops in these areas because of herbicide carryover in the manure.

The cattle from which the manure came had eaten hay from a pasture where an herbicide had been used. The herbicide passed through the cattle’s bodies and did not break down even though the manure was composted. Thankfully, the vegetable producer only used the cattle manure in a small percentage of her fields.

This local vegetable farmer is only one of many farmers and home gardeners nationwide who have experienced similar problems. North Carolina State University Extension produced a publication in 2010 warning gardeners and fruit/vegetable producers of the possibilities of herbicide carryover, and there were a few articles about it in trade magazines.

We are still learning, though.

There are several herbicides used for weed control in pastures, typically for control of noxious weeds like thistles and field bindweed. The herbicides are selective to broadleaf plants, so they can be used without damaging the pasture grasses. Some of these herbicides take a long time to break down and can be passed through a cow, horse or other animal’s body without harm to the animal.

The USDA-EPA and European Union say it is OK for livestock and horses to consume grasses produced in pastures treated with these herbicides. Unfortunately, however, the herbicides are passed through animals in urine and manure and can remain active through the process.

In this case, the damage was even easier to diagnose because the vegetable grower used the cattle manure in every other row in the potato field.

“We wanted to do an experiment with the manure,” the vegetable farmer said. “Manure is hard to get and hard to haul. We were wondering if we had improved our soil enough by using it in the past that we could just rely on our cover crops. We amended every other row so we could see the difference.”

They also incorporated the manure into the soil in a high tunnel where tomatoes were planted.

When the leaves on the potato plants in the rows with manure began to curl and look distorted, the vegetable farmer called me.

The tomato plants were showing the same symptoms. Leaves were cupped and curled, veins appeared to pull together and grow in a parallel fashion, and the stems elongated.

“In the high tunnel, we tried watering,” the farmer said, “and then we put shade cloth up. It just kept getting worse.”

Herbicides can be carried over in any kind of hay, manure, compost and grass clippings.

Specialists at Kansas State University say the actual concentrations of herbicide are probably very low and will continue to break down with exposure to sunlight and moisture.

Research has shown that these herbicides can break down in as little as 30 days up to several years depending on environmental conditions. Dry, dark barns — great for storing hay — also allow herbicides to persist the longest.

The vegetable farmer and I suspect the dry summer and winter also aided in the pesticide’s persistence.

However, the entire situation could have been avoided if the two farmers had communicated better.

If you want to use manure, ask questions! Hay and livestock producers may not even be aware that their practices could lead to potential problems.

Livestock producers might purchase hay from other producers. If the producer does not know or is not forthcoming with information about their practices, avoid taking the chance with your crop or garden.

Also, if you are a horse or livestock producer who has purchased hay for which you do not know the history, avoid putting someone’s crops in jeopardy by giving it away.

Although little can be done other than waiting for the herbicide to break down, this vegetable farmer is planning to use a Japanese product called EM™ that is marketed to improve microbial activity in the soil. The farmers will also plant buckwheat as a cover crop to hopefully help the herbicide break down. In late summer to early fall, they will plant a few tomatoes to see if the herbicides are still present and inhibiting plant growth.

Pesticide residues in manure and soil can be tested by private regional labs, but the tests are expensive and less reliable than bioassays.