Studies show waterhemp can reduce corn yields by more than two-thirds and soybean yields by close to half.  Once established, it is difficult to eradicate. Female plants can produce more than a million seeds that remain dormant in the soil for years.  In addition, waterhemp emerges over a more prolonged period than most other weed species, often necessitating use of both soil- and foliar-applied herbicide treatments for effective management.

Unfortunately, waterhemp has evolved resistance to many foliar-applied herbicides, including the ALS, PSII, PPO and EPSPS inhibitors. Now HPPD-inhibiting herbicides have become the most recent addition to the list.

Researchers at the University of Illinois conducted recent field and laboratory experiments involving HPPD-resistant plants found in McLean County, Illinois.  They discovered that regardless of the application rate, HPPD inhibitors controlled less than 40 percent of resistant waterhemp at either one week or two weeks after treatment.  They also found that application timing played a role. 

“Though the HPPD inhibitors offered poor control of resistant waterhemp, we saw the best results when we treated plants only two to five centimeters tall,” said Aaron Hager, associate professor at the University of Illinois.  

Researchers also found that HPPD-resistant waterhemp is poorly controlled by PSII and ALS inhibitors. The PPO inhibitors, including glufosinate and glyphosate, provided the greatest level of control.

Full text of the article, “Responses of a Waterhemp (Amaranthus tuberculatus) Population Resistant to HPPD-Inhibiting Herbicides to Foliar-Applied Herbicides,” is now available in Weed Technology Vol. 30, Issue 1, January-March 2015.