Setting a Threshold for Weed Tolerance in Vegetable Crops

Setting a Threshold for Weed Tolerance in Vegetable Crops
Setting a Threshold for Weed Tolerance in Vegetable Crops

Date: May 20, 2003

URBANA -- According to a University of Illinois researcher, setting a threshold for weed tolerance in vegetable crops is the key to helping farmers decide when or even if they should apply herbicides.

"The threshold for controlling insects is determined from performing sweep nets or other means to collect and count insects," said John Masiunas, "but there isn't the same widely-accepted threshold for weeds."

Masiunas is a U of I professor in the College of Agricultural, Consumer and Environmental Sciences. Over the years he has studied a number of non-chemical techniques for managing weeds in vegetables including living mulches and cover crops. This latest study was based on farmers' observation in the field.

"We wanted to develop decision tools that rely on the number of weeds that are present in the field," he said. "If there aren't enough to worry about, the farmer will save money on herbicides as well as the labor cost to hand remove the weeds. And, if we can help reduce the labor costs in particular, that will help the vegetable industry become more competitive."

The study looked specifically at redroot pigweed and large crabgrass in fields of snap beans. The goal was to find the point at which managing weeds was necessary in order to avoid economic losses. Masiunas found that if there were more than four weeds per meter of row they needed to be controlled.

Surprisingly, the study also found that snap bean fields with large crabgrass actually had fewer potato leafhoppers. Researchers believe that the leafhopper doesn't like the feel of that particular weed. So a presence of large crabgrass may actually be a deterrent to at least one pest. Conversely, redroot pigweed appeared to encourage the population of bean leaf beetles, by providing a better, shaded habitat for them to thrive.

Masiunas pointed out that vegetable farmers do not have the advantage of corn and soybean farmers who may use genetically-modified seed. "There are no Roundup Ready vegetables," he said. Instead, vegetable growers rely on selective postemergence herbicides and hand-weeding.

If the threshold guidelines are followed, consumers and the environment would both benefit by having fewer herbicides entering the soil and water.

The study was funded in part by the North Central Integrated Pest Management Program.