How Weeds Build Herbicide Resistance

JOHN LOVETT

FAYETTEVILLE, ARKANSAS

Herbicide-resistant weeds are the most problematic and expensive management issue in row-crop agriculture, according to Nilda Burgos, professor of weed physiology and molecular biology with the Arkansas Agricultural Experiment Station.

A field infested with water and nutrient-stealing Palmer pigweedand barnyardgrass can become unproductive because those weeds can’t be controlled by herbicides, Burgos said. She is studying the broad-spectrum capabilities of a non-chemical solution – microwaves – to decrease the weed seeds in the ground and potentially curb herbicide-resistant weed evolution.

Synthetic chemical herbicides are popular, Burgos said, because they are usually effective on a broad spectrum of weeds and are more effective than natural herbicides. However, the chemicals generally target just one enzyme to disrupt a weed’s ability to function. Palmer pigweed, for example, builds resistance to the herbicide glyphosate by producing dozens of copies of the gene that produces the enzyme targeted by glyphosate.

“The recommended dose of glyphosate is not sufficient to inhibit all the extra copies of the target, so the pigweed harboring this capability survives,” Burgos said. “In most cases, weed populations become resistant to herbicides due to selection of plants with a mutation in the herbicide target that prevents interaction with the herbicide.”

Herbicide-resistant mutants are extremely rare, Burgos adds, but the possibility of selecting them increases with the number of plants being exposed and the frequency of herbicide use.

The risk of selecting such mutants also increases with genetic diversity of the weed, Burgos noted. Some plants in a farmer’s field could receive a sublethal dose of the herbicide due to various reasons – variable weed density, shading by adjacent plants, or weeds larger than optimum size. In that case, Burgos said the plants would survive by natural protection from herbicide damage or minimal ability to detoxify the herbicide.

“Repeated exposure to sublethal doses favors plants with higher tolerance, eventually resulting in a resistant field population,” Burgos said. “Palmer pigweed has all the ideal traits for resistance evolution. It has high genetic diversity and can grow 2 to 3 inches daily, produce up to 1 million seeds per plant and emerge throughout the growing season. Thus, Palmer pigweed has become resistant to many herbicides, including glyphosate.”

The study exploring microwave technology as a non-chemical solution is part of a larger weed science investigation supported by a nearly $300,000 Agriculture and Food Research Initiative grant from the U.S. Department of Agriculture’s National Institute of Food and Agriculture, with additional support from The Cotton Board and Cotton Incorporated. ∆

JOHN LOVETT: University of Arkansas