California is faring better than other parts of the U.S. and world in the number of weed species resistant to herbicides.

However, that does not mean the nation’s No. 1 agricultural state will be spared the challenges of a growing number of weeds resistant to herbicides.

It will take a change in management strategies to mitigate those challenges, according to a pair of weed control specialists speaking at the California Weed Science Society annual meeting in Visalia.

While much of the focus over the past decade has been the rapid emergence of weeds resistant to glyphosate, 21 weed species have been identified as herbicide resistant to seven herbicide families.

By comparison, there are 125 weeds resistant to 15 herbicide families in the U.S., according to Brad Hanson, the new University of California, Davis weed management specialist for tree and vine crops.

Herbicide resistance was first validated in 1957 in the U.S. California rice has been the hardest hit by herbicide resistant weeds. There are nine broadleaf and grassy weeds resistant to one or more herbicides.

“Three out of four California rice fields have at least one weed species that is herbicide resistant,” said Hanson, imposing “tremendous” impacts on yield and production costs.

Rice growers are trying to meet this challenge with a wide array of chemical control strategies, as well as alternative, “dramatic” new production systems like stale seedbed planting.

Glyphosate has yet to be implicated in the rice weed herbicide-resistant scenario; however, it is the No. 1 herbicide for the emerging worldwide weed resistance issue.

Steve Duke, research leader of the Natural Product Utilization Research Unit located at the National Center for Natural Products Research on the campus of the University of Mississippi in Oxford, has called glyphosate “a once-in-a-century herbicide” that for almost 25 years after its introduction had no weed resistance issues. However, since the introduction of transgenic crops there have been 16 weed species develop resistance to the herbicide.

Compounding this scenario is the fact glyphosate is off patent, making it not only one of the safest herbicides ever developed, but also one of the cheapest.

Almost 90 percent of all transgenic crops grown worldwide are glyphosate resistant.

Duke said it has been estimated that the economic benefit of glyphosate-resistant crops worldwide has been more than $44 billion due to such factors as less soil loss, lower overall herbicide use, less soil compaction and less fuel use due to fewer trips across fields for mechanical weed control.

Roundup-resistant soybeans alone in the U.S. have reduced carbon emissions equivalent to removing 2 million cars from the road.

Duke and Hanson do not recommend growers stop using glyphosate or quit planting glyphosate-resistant crops, but rather that growers start using well-known diversification weed control strategies to reduce selection pressure favoring glyphosate-resistant weeds. “Put simply, glyphosate can only be sustainable in the long term if there is sufficient diversity in weed management practices,” Duke said.

This diversity includes gene-driven and/or new modes of action herbicides and innovations like novel full-dose herbicide mixtures.

Mechanical and precision application technologies offer the potential to reduce reliance on glyphosate as well.

Hanson called these approaches “preservation” tools. Use them and glyphosate will be around for years. Ignore them, and resistance will be “off the chart.”

Hanson also pointed out that glyphosate’s favorable environmental profile is increasing its use in trees and vines in California with stricter groundwater protection regulations. Older herbicide compounds are use-restricted in many groundwater protection zones.

Postemergent use of glyphosate in stone fruit has gone from 81 percent of the herbicides used in 1981 to 110 percent in 2007, reflecting multiple applications. In tree nut crops, use has gone from 116 percent to 144 percent in the same period.

Hanson and Duke said it is not the use of glyphosate that is threatening its long term future, but the lack of stewardship of product that is the concern.

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