Louisiana Confirms First Case Of Roundup-Resistant Palmer Pigweed, Water Hemp Issue Also Suspected

Roundup-resistant Palmer pigweed (aka Palmer amaranth) has been confirmed for the first time in Louisiana, according to a memo from Daniel Stephenson, LSU AgCenter Weed Scientist.

The memo was included Sunday morning, September 5, in AMS AG REPORT, the weekly newsletter published by consultant Roger Carter with Agricultural Management Services, Inc., a firm that covers several parishes in east-central Louisiana.

Carter said that Tim White, a consultant in the firm, "has worked on this farm with Dr. Stephenson for several years trying to manage this weed. This particular farm is near Lake St. John."

Stephenson noted in the memo that Roundup-resistant Palmer pigweed already had been confirmed in Alabama, Arkansas, Georgia, Mississippi, Tennessee and several other other states.

Stephenson reported that he had now "confirmed that Palmer amaranth collected from a cotton field in Concordia Parish is resistant to glyphosate."

According to his analysis, 50% of the Concordia Parish population "is approximately 54 times less sensitive to glyphosate compared to the susceptible standard. In other words, if the labeled use rate of glyphosate is 32 fluid ounces/Acre, a producer will have to apply 118 oz/Acre of glyphosate to control 50% of the Palmer population or 723 oz/A to control 90% of the population.”

In a followup memo Sunday afternoon, Stephenson noted that other cases of hard-to-control pigweed were being investigated.

"Unfortunately, LSU AgCenter weed scientists are following up on many other locations throughout Louisiana where Palmer amaranth or waterhemp, another Amaranthus species, have not been effectively controlled by glyphosate," he noted. "Any producer or consultant in Louisiana should be aware of the potentially devastating effect herbicide-resistant weeds can have on their crop. If you suspect that a weed population is resistant to a herbicide (not just glyphosate, ANY herbicide), please contact your local LSU AgCenter county agent.

"AgCenter personnel need to know of these issues so we can help you avoid or mitigate weeds resistance from spreading throughout Louisiana."

- Owen Taylor