Weeds will decrease crop yield if they are not properly managed, but successfully tackling weeds means keeping pace with their developing resistance to herbicides.

LSU AgCenter researchers have been conducting field tests in different regions to evaluate weed treatment programs for new cropping systems in unique soil and climatic conditions.

Tests conducted in 2019 at the Northeast Research Station in St. Joseph showed good results for transgenic Xtend and Enlist cropping systems, as well as the HPPD system, which stands for 4-hydroxyphenylpyruvate dioxygenase.

“The Balance Bean herbicide utilized in the HPPD soybean weed control system gave good to excellent control of most broadleaf and grass species and should be a safe, useful tool in a complete weed management system,” said AgCenter weed scientist Donnie Miller.

Post-emergence applications of glyphosate in combination with dicamba in Xtend or 2,4-D choline in Enlist soybean weed control systems, double-cropped, following wheat, provided excellent control of most grass and broadleaf weeds common to the region, Miller said.

Miller still recommends including residual herbicides with both weed management systems to prevent weed seeds from germinating and competing early season and to aid in resistance management.

“The ultimate goal is to stay weed free to allow the soybean plants to provide natural residual control in the form of shade later in the season,” he said.

“We found that the addition of commonly used insecticides did not cause crop injury when co-applied with glyphosate plus Xtendimax or 2,4-D choline,” Miller said. “This offers producers the opportunity to reduce production costs associated with separate applications.”

At Dean Lee Research Station in Alexandria, research has been focusing on programs in Xtend, Enlist and GT27 technologies in soybeans.

“Some of the most difficult weeds now are Palmer amaranth and waterhemp,” said AgCenter weed scientist Daniel Stephenson. “Although it has been problematic in the past, prickly sida, or teaweed, has become more prevalent, leading to more control issues.”

Stephenson said regardless of row spacing and time planted, a two-pass herbicide program is needed in soybeans.

“Growers should spray a residual herbicide pre-emergent. When it starts to fade after three to four weeks, tank-mix a residual herbicide with your nonselective like glyphosate, glufosinate or others,” he said.

A new technology in the battle to stop weeds is harvest weed seed control (HWSC). AgCenter agronomist and weed scientist Lauren Lazaro has been testing two of the six types of HWSC at the Central Research Station in Baton Rouge.

“We have done two years of narrow windrow burning and one year of chaff lining,” Lazaro said.

Narrow windrow burning funnels the chaff and straw from the combine into a windrow where it’s burned, while chaff lining sends only the chaff fraction into the windrow, allowing natural elements to break down the seeds through decay and predation.

“General observations show that both options reduce the amount of seeds that enter back into the soil seedbank,” she said.

“We are always trying to layer alternate modes of action and expose weeds to as many modes of resistance as possible,” Miller said.

“We’re trying to put together programs for our farmers, but it’s not a one size fits all,” Stephenson said. “Every year, it’s just another change; there’s never an identical year.”

This story is featured in the Louisiana Soybean and Grain Research and Promotion Board 2020 Report.

Weed management with glyphosate plus Xtendimax following residual herbicides at planting in Xtend soybeans, Dean Lee Research Station, Alexandria, Louisiana. Photo by Marcie Mathews