Glyphosate-resistant Palmer amaranth, first confirmed in Louisiana in 2010, continues to spread south in Louisiana, said LSU AgCenter weed scientist Daniel Stephenson. But Louisiana farmers have learned from their peers in other states that early control can be the answer to preventing spread of this persistent weed.
“Palmer amaranth can be controlled, but success takes persistence,” Stephenson said. “The only practice that’s foolproof is zero tolerance. Any plants that aren’t killed by pesticides must be removed and burned.”
One plant can produce as many as 7 million seeds, he said. And because resistance is spread by pollen, nonresistant plants can produce resistant offspring if they’re pollinated with pollen from a resistant plant.
“Even leaving a single weed is critical,” Stephenson said. “Why run the risk?”
The best control is using residual herbicides that are soil-applied and water-activated to prevent germination or are absorbed by the plants at emergence. This should be followed by a foliar herbicide application when weeds have emerged at 1-2 inches.
Early foliar application is important, Stephenson said. Palmer amaranth seedlings can grow as much as 2 inches a day in Louisiana, and waiting to apply herbicides can result in poor control.
“This approach has been proven to mitigate resistance in weeds,” he said.
Glyphosate-resistant johnsongrass was confirmed in 2010 in Rapides and Pointe Coupee parishes, Stephenson said. “It is continuing to spread and can be found in numerous parishes in the state.”
Johnsongrass was once called the “world’s worst weed.” Even prior to glyphosate, it was tough to manage. It has spread slowly in Louisiana because growers manage it well through proper sanitation and other herbicides, although they have fewer options.
Johnsongrass is spread by both seeds and rhizomes, so growers need to make sure they don’t carry the weed from one field to another – or even within a single field.
"If a grower comes upon a patch of Palmer amaranth or johnsongrass during harvest, it may be a good idea to go around it and not drive the harvester through it,” Stephenson said. “It may be better to lose a few bushels rather than spread weed seed throughout the field, which will just cause more of an issue next year.”
He suggests Corvus pre-emerge and Capreno postemerge in corn, but no soil-residual products are available for soybeans. One way to treat johnsongrass is to spot spray when escapes are discovered.
“Thankfully, johnsongrass isn’t spreading like Palmer amaranth,” Stephenson said.
Glyphosate-resistant Italian ryegrass was documented in 2014 in Louisiana. It has been in Mississippi since 2008.
It is a winter annual that emerges in October and November, so the time to control it is in October to mid-November.
The AgCenter is adopting recommendations from research at other land-grant universities. “Use residual herbicides from about Halloween to mid-November – a non-selective such as paraquat tank-mixed with Dual Magnum,” Stephenson said.
One plant in 7 inches of row can reduce yield by as much as 70 percent in corn, he said.
Louisiana growers have to deal with two to six grass species and two to six broadleaf species, so these treatments control more than one problem weed, Stephenson said.
Glyphosate is still valuable, but growers need to use other herbicides to provide residual control.
Crop rotation can help control these weeds, and using different herbicides rotates chemistries and mitigates against herbicide resistance, Stephenson said. The best practices include chemical control, cultural control and equipment sanitation.