Weed Resistance To Glyphosate Is A Growing Problem

Earlier this year, Dr. Jason Norsworthy, a University of Arkansas weed scientist, told farmers attending the Iowa Soybean Association's On-Farm Network Conference in that glyphosate is the world's greatest herbicide. For more than a decade, it was a cheap, effective way to grow weed-free soybeans. But those days are over.

Extensive use of glyphosate, especially after Roundup Ready soybeans hit the market in 1996 and Roundup Ready corn came on the market a few years later, led to weeds developing resistance. For many farmers, Norsworthy said glyphosate was the only herbicide they used, which sped up and compounded the emergence of resistant weeds. In some cases in the South, he said, it put farmers out of business.

"We are quickly approaching, and not only in the South, the day when there will be very little value in the use of glyphosate," Norsworthy said. "U.S. farmers are heading for a crisis. It's unfortunate that we are not being proactive and addressing this issue."

Why Iowa farmers need to use best practices for weed control

That's not the case at the Iowa Soybean Association. The organization's research teams support Iowa State University and other university weed scientists in conducting trials and outreach to help farmers implement best management practices for weed control, says Ed Anderson, ISA senior director of supply and production systems. "Raising awareness to the potentially devastating realities of this weed resistance problem is important," he said.

Glyphosate resistant horseweed or marestail was first discovered in 2000 in Delaware. It gradually spread to other parts of the country and weeds like waterhemp, Palmer amaranth and several others also developed resistance to glyphosate.

Waterhemp, giant ragweed, pigweed and horseweed are the most common herbicide-resistant weeds in Iowa, says ISU weed scientist Mike Owen. Other weeds to watch include velvetleaf and lambsquarters.

More than half of U.S. farms are battling glyphosate resistant weeds

A survey by Stratus Ag Research shows glyphosate resistant weeds were found in 33 million acres in the U.S. in 2010. The acreage jumped to 61 million in 2012. Today, more than half of U.S. farms are battling the problem, the survey shows. "The reason we have an issue and will continue to have an issue with weed resistance is because U.S. agriculture as a whole lacks diversity," Norsworthy said.

While there is no "silver bullet" in the product pipeline that will be released soon to make herbicide resistance a non-issue, Norsworthy said there are many steps farmers can take to curb the problem and effectively control weeds. He said it will take intense management and several applications of different chemistries, which will cost four times as much or more than a single pass of glyphosate like in the past.

What farmers can do to keep resistant weeds from growing worse
The weed scientists who presented at the February 2014 On-Farm Network meeting listed more than a dozen ways farmers can move forward. Some include:

1) Use multiple, effective modes of action.
2) Identify the enemy. Know the weeds in your fields, and which ones are prone to resistance to formulate an effective management plan.
3) Emergence and growth rate. Stop weeds before they start. Treat weeds at 2 inches or less. The bigger the weeds, the tougher they are to kill. Some weeds grow 2 inches a day.
4) Find a weed's weakness. For Waterhemp and Palmer Amaranth, small seeds are susceptible to tillage and burial. Norsworthy said spot tillage may not eradicate a problem but can bring populations down.
5) Start clean and stay clean. No weeds at planting and overly residual herbicides. Multiple applications will be needed until canopy, possibly every two weeks. Post emergent herbicides are only used to control the few weeds that come through residual applications.
6) Use full, lethal doses of herbicides. No "setup" or "foundational" rates.
7) Apply herbicides, if possible, in a timely manner over the entire field. Complete, timely coverage is essential to prevent weed escapes.
8) Premixes. If a rate is not effective alone, two half rates of two modes of action won't work.

Don't let your weed management build weed resistance

Anderson said farmers need to take this advice to heart about the serious need to have a more diverse weed control program on their farms. This is similar to what Iowa State University weed scientist Mike Owen has preached for years. "There are steps farmers can take to fight weeds while extending the life of glyphosate and other herbicides," says Owen. "Cheap weed control is no longer sustainable. Farmers are going to have to invest in a program that uses residual herbicide treatments and other tactics. The problem is, many growers still think they can get another year out of using only glyphosate. Once you start seeing weed escapes, you can quickly lose a technology. Be proactive and don't let your weed control program build weed resistance."