Herbicide Carryover: Bioassay Results

About a month ago I reported on a farmer who had received manure that was contaminated with the herbicide picloram and applied it to his vegetable production fields. We conducted a bioassay on the contaminated land to see if the soil was safe to plant. Last week we assessed the results of the test.

The first site we tested was "Moses". This field had been contaminated by the manure, however the manure was only applied in a small amount in each of the holes where tomatoes were planted. From the test, it looks like this site should be safe for planting this year.
Side note - The greenhouse where the seedlings were located was very hot, so some of the "crispy" edges are more than likely due to the temperature because some of the control plants also were affected.

Beans are sprouting in soil that was contaminated with manure containing the herbicide picloram.

Control test using soil that was not contaminated.

Next up was the field we called "Fred". Manure was applied to this entire field. The field appears to be unsuitable for vegetable production at this time. The grower will not be producing on this plot this season.

Bean seedlings did not emerge from the soil collected from field "Fred".

Control soil taken from part of field where no manure was applied. Seedlings look pretty good.

We were also concerned about the field below "Fred". There were a few spots where the grower's Austrian winter peas did not emerge. So we tested these areas to see if runoff from "Fred" was the reason. Looks like that field should be okay.

Bean bioassay of soil taken from a field below "Fred" where some water runoff from "Fred" flows.

The final plot that was tested was a field that is located next to the growers house. This field had the most manure applied and there was no soil available to test as a control. As you can see, this soil appears to be unsuitable for vegetable production.

No bean seedling emerged from the soil taken at the home site. This plot had the most manure applied to it.

Overall, we were not surprised by the results. All of our seedlings from the control plots emerged successfully. The seedlings planted in soil from the field with the least amount of manure applied ("Moses") emerged and there appeared to be no damage. The grower is planning on producing vegetables in this field this season. Luckily, the grower did not lease field "Fred" this season due to other reasons.

The most unfortunate situation is the field that is located next to the house. The grower will continue to work this land to allow for the elements, sun, rain, etc to break down the herbicide. He is considering growing a less susceptible crop in this field, such as corn or a grass cover crop.

To read more about the bioassay and herbicides that may be a problem in manure, read Dr. Jeanine Davis's article "Herbicide Carryover in Hay, Manure, Compost and Grass Clippings".

**UPDATE May 2010**
New Herbicide Carryover publication available!

Herbicide Carryover: Bioassay

Last year, a grower that I work with was noticing some unusual growth habits and symptoms of his various vegetables. Many of these symptoms look almost like viral diseases of vegetable crops. However, the problems were seen on many different crops and they were widespread - nearly every plant was affected. This is not common of plant diseases caused by viruses.

It was discovered that he was using manure that he received for free from a neighbors horse operation. The manure was determined to be contaminated with the herbicide picloram (trade name Grazon), that is common in pasture management.

Here are some pictures of the damage.

Chlorotic (yellowing) lettuce damaged by herbicide.

Chlorotic (yellowing) peas damaged by herbicide.

Spinach leaf distortion and puckering due to herbicide damage.

Discoloration of tomato leaves caused by herbicide damage.

There is documentation that shows that this active ingredient can remain in the soil, causing problems for three years!

Today I went out and we took some samples for a bioassay to determine if there are still damaging levels of picloram in the soil.

First we walked through the contaminated field and took soil cores every few feet throughout the plot.

Then we mixed the soil all together and put it in small pots.

We planted three bean seeds in each pot.

Of course we included a control. Some soil from a field where manure was not applied.

The pots were set on separate trays and placed in different spots in the greenhouse.

Now, we wait. In three weeks, we will inspect the seedlings. If they do not germinate or the true leaves show symptoms we will know there is still residue of the herbicide present.

For the grower's sake, let's hope that these beans grow perfectly!
I will keep you updated on the progress.

Before you plant your garden this year, please be aware of the possible dangers of using manure. Question your source about herbicide use. To learn more about herbicide carryover, read about. Dr. Jeanine Davis's extension article, Herbicide Carryover in Hay, Manure and Grass Clippings.

**UPDATE MAY 2010**
New Herbicide Carryover publication now available!

Dr. Jeanine Davis just posted an article about this topic on her site, NC Alternative Crops Blog.

**I'd like to thank Amanda and Phillip for their assistance with the diagnosis of this problem last summer. You two are the greatest!**