As you were guiding your combine through corn and soybean fields this fall, what weeds did you see that concerned you?  You probably thought your weed control program should have taken care of those weeds, but for some reason they escaped, and now their seed has gone through the combine and been further dispersed in the field.  One of those may well have been waterhemp, which Charles Darwin would have used as his best example of survival of the fittest.  The ability of waterhemp to survive herbicides has befuddled some of the nation’s best biochemists in weed control laboratories.  Some of us used to control it with a weed hook, and some of your frustrated neighbors have returned to that in patches of waterhemp in fields where herbicides receive no respect.  As you are planning your weed control program for next year, what are your strategies for dealing with one of your worst enemies?

Waterhemp is legendary in its ability to re-invent its genetics because it is different from many other weeds.  There are male plants and female plants and one male waterhemp plant with a gene that gives herbicide resistant abilities can be spread its pollen to thousands of female plants.  Subsequently, a female with herbicide resistance can create over one million seeds for the next crop year.

Purdue agronomists Bill Johnson and Glenn Nice have created a waterhemp fact sheet in which they say waterhemp is an emerging problem in Indiana, but it is a publication that can be distributed in every other Cornbelt state.  They say waterhemp has been a significant issue for the past 10-20 years, but is quickly “moving up on the radar of concern.” 

Because waterhemp is a cousin to the family of pigweeds—sometimes called amaranth—it resembles many others, and is difficult to specifically identify when it is just emerging in the spring—or year-round for that matter.  Johnson and Nice offer these characteristics:

• First true leaves are generally longer than other pigweeds.
• Seedlings are hairless with waxy- or glossy-looking leaves.
• Waterhemp and Palmer amaranth stems are hairless, whereas other pigweeds have hairy stems.
They say waterhemp can be anywhere from four inches to 12 feet tall at maturity, but are usually in the 4-5 foot range when they hit your combine header.  They will emerge throughout the growing season, so one trick they play is to emerge just after you apply a contact herbicide.  The Purdue weed specialists say a larger percentage of waterhemp will emerge later in the season than other weeds, which is one reason you may not see them when initially scouting the performance of your weed control program.

Since the waterhemp family has both male and female plants whose genes mix annually, the genetic diversity increases every year and an increasing number of plants have become resistant to a wider variety of herbicides.  Johnson and Nice say, “Currently, waterhemp populations resistant to ALS-inhibitors, triazines, diphenylethers (PPO-inhibitors), and glyphosate (Roundup) have been identified.”  They say the first to fail were the ALS-inhibitors, such as Pursuit and Accent.  The next to fail were PPO-inhibitors, such as Reflex, Ultra Blazer, and Cobra.  And with the widespread use of glyphosate, it did not take long for it to fail as a control alternative.  With all of that great news, what can you do to control a nasty waterhemp infestation in your best cropland?

For corn, Johnson and Nice recommend:
1) A pre-plant or pre-emergent to either control or suppress it, mixed with atrazine.
2) A post emergent treatment if the population is dense or continued rain promotes more germination.
3) Post emergent treatments include 2,4-D, dicamba, Status, Callisto, Laudis, Corvus, Impact, or glyphosate if you are using GT corn.  Ignite and atrazine will control small waterhemp in Liberty Link corn.

For soybeans, Johnson and Nice recommend:
1) A pre-plant or pre-emergent herbicide that contains Authority, Valor, or Dual.
2) A post-emergent herbicide that controls later emerging plants should include Ultra Blazer, Cobra, Reflex, Flexstar, or Ignite for Liberty Link soybeans.  Those should be applied when waterhemp plants are less than 4 inches tall.
3) Glyphosate with Warrant or Outlook (for Roundup Ready soybean) can be effective where the waterhemp population is not resistant to glyphosate and provide residual control.

Summary:
Waterhemp has become a headache for many Cornbelt farmers because of its unique ability to become genetically diverse.  That means many herbicides, which initially would control waterhemp will no longer suffice.  A strategic plan has to be developed for farms which have waterhemp populations that are resistant to one or more herbicide modes of action.  For both corn and beans, that includes the use of both a pre-plant and a post emerge treatment.

Source: FarmGate blog