By Dr. Daniel Stephenson, LSU AgCenter Weed Scientist
We have all seen or heard about the tremendous troubles glyphosate-resistant Palmer amaranth is causing producers in Arkansas, Georgia, Mississippi, Tennessee, and other states as well as the steps they have to take to manage it. Applications of residual herbicides preplant, preemergence, early-postemergence, postemergence-directed, and post-harvest in addition to hand-hoeing have been become a requirement. In Louisiana, the LSU AgCenter confirmed the presence of glyphosate-resistant Palmer amaranth in 2010. Prior to 2012, we knew it was primarily located in Concordia, Madison, and Tensas Parishes.
Unfortunately, Louisiana is experiencing an explosion of instances where glyphosate is not controlling Palmer amaranth in 2012. Whether I have personally seen locations or had numerous calls from producers, consultants, or industry representatives telling me about the failures, the problem is ballooning. Locations where I have received calls in 2012 include Northeast, Northwest, Central, and South-central Louisiana, so it isn’t just a problem for a few Mississippi River parishes anymore.
Although corn weed control in-crop is over, producers need to utilize post-harvest weed management techniques. Considering the early corn crop Louisiana will have this year, we will be left with many months of excellent growing conditions for Palmer amaranth and all other weedy species. Post-harvest weed management techniques include multiple tillage operations, applications of a non-selective herbicide plus a residual herbicide, or a combination of both tillage and herbicides. The goal is to prevent weeds from producing seed. Another consideration is sanitation during and after crop harvest. Harvesting and tillage equipment are excellent tools for spreading weed seed. All equipment should be thoroughly cleaned to remove weed seed before moving to the next field.
Hand removal of weeds that escaped herbicide applications is very important also. For example, a soybean field has lapped and you spot a couple of pigweeds still growing out in the field. It is not that difficult to walk out in the field, pull them up, take them out of the field, and burn them. The old saying is “an ounce of prevention is worth a pound of cure”. With potentially glyphosate-resistant weeds, prevention is worth much more than a pound.
LSU AgCenter weed scientists feared that we’d have a year were pigweed populations exploded. Well, 2012 is that year! If you suspect a problem, call your local county agent for help and remove the weeds from your field. Don’t just ignore this issue. It must be taken seriously.