Herbicide-resistant weeds have been causing havoc in crop fields across the South, and they appear to be “just an eyelash away” from being confirmed as a problem in Louisiana, an LSU AgCenter weed scientist told the participants at the at the Aug. 4 field day at the Dean Lee Research Station in Alexandria.
Daniel Stephenson said the three weeds Louisiana farmers are starting to have trouble controlling include Johnson grass, waterhemp and Palmer amaranth, also known as pigweed – all vigorous growers and prodigious pollen producers.
These weeds have been found resistant to glyphosate herbicide in Arkansas, Georgia, Mississippi and Tennessee. Glyphosate is safe to use on Roundup Ready soybean varieties, which are grown on more than 95 percent of the soybean acreage across the country and on nearly all of the million acres of soybeans planted in Louisiana.
Alternative weed control programs – should weed resistance occur here – are under investigation in tests being conducted by LSU AgCenter scientists.
“We don’t have a lot of control options right now, but we’re working on that,” Stephenson said.
He outlined for them some steps to take to prevent glyphosate from becoming ineffective, which would be a severe blow to the soybean industry here. One of his recommendations was to go back to applying pre-emergence herbicides with soil residual activity.
“This allows you to expose weeds to another chemistry,” Stephenson said.
Before Roundup Ready soybeans became ubiquitous, farmers used pre-emergence herbicides to control weeds in soybean fields.
“We’re recommending going back to using soil residual herbicides,” Stephenson said.
Some of the pre-emergence herbicides being tested in the Dean Lee research plots are Authority MTZ, Boundary, Enlight, Prefix, Fierce, Sharpen and Valor.
Farmers will still have to apply glyphosate to weeds that emerge, but the pre-emergence herbicides will have slowed down weed growth and bought time for the soybean plants to grow taller and outcompete the weeds for water and light.
Another technology that would extend glyphosate effectiveness is to plant Liberty Link soybeans, which are resistant to glufosinate, the active ingredient in Ignite herbicide. The only problem is Liberty Link beans are just not available in large quantities.
“Mississippi soybean producers are trying to buy every bag of Liberty Link seed they can find,” Stephenson said.
Linda Foster Benedict
(This article was published in the summer 2010 issue of Louisiana Agriculture.)
The LSU AgCenter and the LSU College of Agriculture