A farmer can end up throwing a lot of grass control money down the drain with little to no results if he uses poor application methods.
At the 2019 Mississippi State University Extension Row Crop Short Course, Daniel Stephenson, weed scientist from the LSU AgCenter, Alexandria, La., talked about simple solutions to common grass control problems in corn, cotton, and soybeans in the Mid-South.
"Grass control has always been a multi-step process, and grass was a big problem prior to Roundup Ready crops, which is where the revolution started in the mid-1990s. That's when the old saying came out about a pint of Roundup every Monday morning for three weeks, and you were good to go. Then resistances started to show," he said.
Common grass problems
"Resistant johnsongrass was confirmed in Louisiana back in 2010 and then in Mississippi," he said. "Italian ryegrass popped up, which Mississippi was a leader in Italian ryegrass resistance. Glyphosate-resistant goosegrass has been confirmed in Tennessee, and barnyardgrass/junglerice is a grass weed that farmers have been having problems with this year."
Stephenson said johnsongrass in Louisiana, although it is resistant, is becoming an old problem, except some farmers can't kill it with graminicides.
"Select is beginning to fail for us in Louisiana, and we don't use Liberty as much as some other areas. The literature shows that separating applications by three weeks with three applications was better. Liberty works well on johnsongrass, and we found, surprisingly enough, the bigger the johnsongrass the better it looked as far as burning it down goes. Also, we found that this works well in combination with a graminicide," he said.
Recommendations looking ahead
Stephenson has a few recommendations for mixed grass problems.
"I would say to stop tank-mixing dicamba with clethodim or any graminicide to avoid antagonism," he said. "If you've got glyphosate-resistant Palmer amaranth, you need to spray dicamba alone and come back about five to seven days later with glyphosate and clethodim.
"I tell this to my growers all the time when they say they have broadleaf weed and grass problems, and they want to know which one to take care of first since they can't tank-mix the two. I ask, 'Which one is worse?' If the broadleaf weed is worse, spray it and come back in seven days with your graminicide. If the opposite is true, spray your grasses first and spray for the broadleaf weeds in two days. Use the seven and two or two and seven rules to avoid antagonism because, if we don't do that, we run the risk of not getting expected control or possibly building resistance."
Some research shows a significant decrease in control when dicamba is tank-mixed with glyphosate.
Stephenson said another problem in grass control this past year was poor coverage due to nozzle issues as well as poor boom height and poor coverage.
"I understand we have to cover thousands of acres in a very short window of time to get herbicides applied, but we are sacrificing something for speed; we are losing efficacy.
Stephenson said most of this isn't that new. Antagonism and resistance have been problems, but we do have the tools and the knowledge to solve these problems.