Waterhemp Could Get Ugly this Spring

April 13, 2010 06:30 AM

Resistant to: triazine, PPO inhibitors, glyphosate, ALS inhibitors
First known resistance to glyphosate: 2005 in Missouri
Geography of known resistant populations: Missouri, Illinois, Kansas, Minnesota, Iowa
Conditions look to be optimal for common waterhemp populations in 2010, says Aaron Hager, University of Illinois extension weed specialist. Wet spots and good growing conditions last year left a hefty seed bank for the coming season. Add ample moisture conditions this spring and the stage is set for this problem weed to get a foothold without proper management.
What makes common waterhemp particularly ornery is that it now is resistant to multiple modes of action. Soybean fields that a few years ago would have been clean as a whistle are increasingly polka dotted with the wily weed. Hager and fellow weed researcher Pat Tranel, confirmed last year that at least one waterhemp patch in Illinois is now resistant to four herbicide classes--triazine, PPO inhibitors, glyphosate and ALS inhibitors.
That leaves HPPD inhibitors and glufosinate-based herbicides as the remaining mode of action herbicides left to control the problem. Weed scientists are already starting to worry about what happens if growers start to depend too much on those herbicide classes.
Common waterhemp is a member of the pigweed or amaranth family—which explains some of its stubborn ways. Like corn and sorghum, it is a C4 plant that is very efficient at fixing carbon and well-adapted to high temperatures and intense sunlight. It's capable of producing 500,000 seeds per plant that tend to germinate throughout the summer. Waterhemp has separate male and female plants and cross-pollination between plants increases the genetic diversity of a population and favors development of resistance.
While waterhemp is native to the U.S. it was not considered a major agronomic problem until the 1980's. The native habitat of waterhemp is wet, low-lying areas, but it is quite at home in reduced tillage and no-till environments.
Historically, it's been divided into two species: common waterhemp and tall waterhemp. Tall waterhemp as a smooth, erect stemp that can grow more than 8 feet tall. Common waterhemp has smooth leaves and stems, but doesn't grow as tall. The stem color can range from yellowish-green to red to reddish-purple.
One reason the weed is so frustrating is it can germinate over a relatively long time period and grow rapidly. Iowa State University studies show the growth rate of waterhemp is 50% to 70% greater than that of many other annual weed species.
"The long emergence window means it's unlikely a pre-emergence alone will provide complete control,” says Hager. "A post-emergence treatment will also likely be necessary to manage late emerging individuals.” 
Watch for patches to pop up along field edges or near the field entrance. Waterhemp seed is easily transported when equipment moves from field to field. This easy seed spread also increases chances of it moving long distances.
Weed management programs that use both pre-emergence and post-emergence treatments offer the best control of waterhemp in both corn and soybeans, but you need to consult your state weed specialist for specific programs. "What's really important is to treat weeds at the size specified,” says Hager."Delayed post applications of glyphosate are many times to blame for poor results.”
A variety of pre-emergence and post-emergence herbicide options are available for controlling glyphosate-resistant waterhemp in corn. Fewer alternatives are available in soybeans, where waterhemp is competitive enough that research has shown up to 40% soybean yield loss.
"If you know you have resistance, you definitely need to switch to glufosinate (provided you are in a Liberty Link system) or a PPO inhibitor,” says Hager. If you suspect glyphosate resistant waterhemp in soybeans he recommends the following weed control measures:
-- Apply a full rate of a soil residual herbicide no more than 7 days before planting.
-- Apply post-emergence glyphosate treatment when waterhemp plants are 3 to 5 inches tall.
-- Scout the field 7 days after glyphosate treatment. If waterhemp control is inadequate, consider applying a full labeled rate of a PPO-inhibiting herbicide.
-- Rescout the field within 10 to 14 days. Hand rogue any surviving plants before they reach reproductive growth stage.
Listen in as Aaron Hager discusses potential waterhemp issues in the 2010 crop:

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