The first documented case of waterhemp that is resistant to a number of postemergence HPPD-inhibitor herbicides has been discovered in a seed corn production field in central Illinois.

Greenhouse tests conducted by Syngenta confirm that the waterhemp population is resistant to several postemergence-applied HPPD-inhibitor herbicides including mesotrione. Field studies also showed ineffective weed control with post applications of HPPD inhibitors and ineffective control with post applications of some ALS and triazine herbicides applied alone to the same population.

“This particular field was used for seed corn production, and for seven consecutive years there was overreliance on certain postemergence HPPD-inhibitor herbicides to control key weeds,” says Chuck Foresman, manager of weed resistance strategies for Syngenta. “Hybrid seed corn production systems often preclude the use of important broad-spectrum herbicides like glufosinate or glyphosate. The lack of diversity in both herbicide modes of action and crop rotation in this field led to the development of resistant waterhemp.”

Syngenta field studies showed that preemergence applications of HPPD inhibitors Lumax and Lexar (containing mesotrione, S-metolachlor and atrazine) provided effective control of this waterhemp population.

Field tests also showed that PPO-inhibiting herbicides, certain glufosinate and auxin herbicides, as well as glyphosate also controlled the population.

“At Syngenta, we are dedicated to sound weed resistance management and took immediate action to understand this nonperformance issue,” Foresman says. “We are working with the University of Illinois, local dealers and farmers on proper stewardship.”