Ideally, herbicides simply kill weeds during a crop’s season and then after harvest degrade without harm to the following crop, but in reality, the breakdown depends on time and certain conditions in the soil.

With that in mind, Steve Fennimore, University of California vegetable weed specialist at Salinas, Calif., has been charting the fate of some new herbicides. What happens to the compounds in the soil is the basis for the plant-back restrictions carried on their labels.

At a recent weed school for growers and PCAs in Salinas, he gave some guidelines on how to minimize carryover of herbicides, particularly pre-emergence types.

“With a number of herbicides you have to be more concerned about carryover, particularly in a sensitive crop like spinach. Using less of these materials with banded applications helps avoid carryover, and it is also important that the banding be accurate to prevent overdosing or overlapping the application,” he said.

The next step is to till the field after the crop is harvested so that the herbicide is diluted for ready access by water and complexes of bacteria and fungi that break it down with the crop residue.

Following with an irrigated cover crop or a rotational crop not affected by the particular herbicide will help continue the degradation.

“And then just good cultural practices, like maintaining the soil pH near the optimum for the crop, will also help. If it gets too far out of balance, some herbicides will have a longer residual,” Fennimore added.

“If your rotational scheme is tight, you want to select herbicides that are less persistent. Prefar, for instance, has less carryover than Treflan.”

Fennimore said growers at times call him for advice on how to speed up an herbicide’s degradation. A typical example might be a grower wanting to plant lettuce after a Treflan application. A potential, generalized prescription for such a situation might be application of 300 pounds per acre of activated charcoal.

“But my recommendation for anyone with a problem like that,” he said, “would be to seek some expert advice and soil bioassays to guide you in the amount to apply. Otherwise, you might spend a lot of money on something that may, or may not, do you any good.”

In the case of lettuce, he added, transplants tend to be more tolerant of herbicides than direct-seeded stands.

Other conditions that discourage herbicide carryover are plentiful organic matter and higher clay content in the soil.

Some herbicides break down because of a variety of physical events, including adsorption to clay particles in the soil, leaching through the soil, volatilization, or being taken up and diluted by a high density of weeds. Others are destroyed by microbial activity, exposure to sunlight or by water.

Fennimore did trials on vegetables in 2005 with Chateau, a tree and vine herbicide having promise for use in vegetables but not registered for those crops in California. It is in IR-4 Program residue tolerance field trials.

He applied it at two rates, 3 and 6 ounces/acre, at two, three, and four months before planting broccoli, carrot, green onion, lettuce and spinach. He found no harm to any of the crops from any of the application timings.

Although he noted it contradicted what he found in his earlier trials, Fennimore said, “I have learned since that when Chateau receives about 8 inches of moisture, either by rain or any type of irrigation, you’re going to get rid of this product.” Chateau is degraded by a combination of soil microbes and hydrolysis.

“I think this material will work on rotational or fallow ground just as well as products that are available now,” he said.

Fennimore also reported during the weed school on his recent research with precision cultivation. He said he has been drawn to experimenting with robotics in broccoli and lettuce because of the lengthy process for registration of new herbicides and the shortage of labor for hand weeding.

He also said several innovative Salinas Valley growers have been doing their own trials with robotic equipment.

For the past two years he’s been working with Eco-Dan precision cultivation equipment being used in cotton, tomatoes and other crops. He has tested it in applying banded herbicides to accomplish the twin objectives of greater efficiency in herbicides and reduced labor costs.

The Eco-Dan guidance system employs digital cameras, a computer, and hydraulics to track exactly along the plant row while the tractor is in motion. The driver’s only function is to turn the tractor around at the end of a row.

Fennimore said he reasoned that if the system could place cultivators precisely, perhaps it could also direct nozzles for herbicide bands.