Battle continues to prevent resistance to herbicides

Frances Gould, McClure, Olivia J.  |  10/14/2016 5:52:56 PM

Donnie Miller, LSU AgCenter weed scientist and research coordinator at the Northeast Research Station, talks to the group gathered for the station’s annual field day on June 16. Photo by Johnny Morgan


As farmers continue to fight weeds that are resistant to commonly used herbicides, LSU AgCenter weed scientists are working to learn more about new control technologies and the best ways to use them in Louisiana.

Two new herbicide systems — Xtend, a technology providing plant resistance to dicamba, and the Enlist system, which offers resistance to 2,4-D — should be on the market soon, offering producers additional modes of action to tackle glyphosate-resistant weeds, including Palmer amaranth and other broadleaf weeds.

AgCenter weed scientists Donnie Miller and Daniel Stephenson have been conducting trials to find out more about the systems, which should provide farmers a boost in fighting some weeds that became resistant to glyphosate in recent years.

Farmers in other states are now seeing resistance to PPO herbicides, such as Valor, Reflex, Ultra Blazer and Cobra, which exhibit the most common alternative mode of action used on Louisiana soybeans after glyphosate. Though PPO resistance has not yet been documented in Louisiana, the issue highlights the importance of using multiple weed control products, Miller said.

“Producers have really gained an appreciation in the past several years for this resistance issue,” he said. “They’re understanding that the more modes of action you have available, that alternately kill by impacting different target sites within the plant, the better.”

Using different herbicides, however, can be a problem if farmers do not apply them carefully or clean their spray tanks, Stephenson said.

Heavy rains caused flooding that delayed many farmers this spring, forcing some of them to make herbicide applications on corn and soybean crops at the same time, he said. Not taking time to clean spray tanks ended up costing some farmers.

Timing of herbicide applications, which is key to a product’s effectiveness, also was a problem this year.

Stephenson has been studying how to optimize the application timing of both pre- and post-emergence herbicides so yields can be maximized. For example, late-season grass weeds are one of the biggest issues in grain sorghum production, so he’s trying to determine if making both pre- and post-emergence applications will offer better control than pre-emergence.

Stephenson also is doing a four-year study to find out if high-intensity herbicide treatments in corn and soybeans — pre-emergence followed by a post-emergence program as opposed to post-emergence alone — are worth the extra cost.

“We want to see what, in the end, reduces the soil seed bank the best,” Stephenson said.

He and Miller are also working on a weed control project in winter cover crops, whose popularity has grown considerably in the past five years in Louisiana. Cover crops improve soil health and help maintain soil integrity during the winter, but can pose issues with winter-spring weed control, Miller said.

“These weeds can harbor insects and compete with the winter cover crop, and potentially offset the benefits of the cover crop,” he said.

Most herbicides for winter weeds will also kill cover crops, Miller said. He’s working to identify herbicide options that don’t cause much damage to the cover crops but still offer effective weed control. Olivia McClure

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