Scientists can determine whether a weed will resist glyphosate by measuring the amount of a compound known as “shikimate” in its tissues. Glyphosate kills weeds by interfering with production of aromatic amino acids, and shikimate plays a key role in producing those amino acids. It is the “shikimate pathway” that glyphosate disrupts, causing shikimate to accumulate. Plants susceptible to glyphosate will have high levels of shikimate, while resistant plants will not.

Existing methods for detecting shikimate in plants require sophisticated laboratory equipment, such as spectrophotometers that can measure ultraviolet light. Test results can take weeks.

Now, Monsanto has developed a method for detecting shikimate in just 24 hours, using a dye that changes color. Shaner plans to help Monsanto fine-tune the technology so that it’s ready for use nationwide.

Weed management is a key part of the research conducted at the Water Management Research Unit.

“We study how best to ensure high yields with limited water, and critical to managing water is managing weeds. You want a weed-free field for your crop so there’s less competition for available water,” Shaner says.

As part of his work with Monsanto, Shaner is growing glyphosate-resistant and glyphosate-susceptible crops and weeds in a greenhouse. He will spray some of those plants with glyphosate and place leaves from others in glyphosate solutions and then determine the levels and rates of shikimate accumulation.

The goal is to evaluate different methods for assessing shikimate levels and to determine the most effective way for growers to collect plant material for testing with Monsanto’s system. The test kit’s design has yet to be determined.

Looking at other alternatives

Soybean growers in Georgia have been using the herbicide fomesafen for years, and now with weeds developing glyphosate resistance, cotton growers have been using it as an alternative. It was approved for use on cotton in 2008 after glyphosate-resistant forms of palmer amaranth were discovered in the region.

But concerns about potential adverse environmental impacts were noted at the time, particularly its effects on runoff into surface water. Growers also needed more information on how to use it when practicing conservation-tillage.