Game changer: Palmer amaranth vs. Cornbelt farmers

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What do these have in common: a grand slam home run, a hole-in-one, a pick six, Palmer amaranth? The answer is that they are all game changers. As many Cornbelt farmers harvested their crops this fall and found the tall spindly seed heads of Palmer amaranth, they realized it was too late in 2013 to do anything about it. After all, weed specialists had been urging farmers to carefully cut the weeds, remove them from fields in plastic bags, and burn them. If that did not get accomplished in 2013, what can be done in 2014 to plan for hundreds of thousands more Palmer amaranth in that field which normal weed control may not touch?

“Among the weedy species of Amaranthus, Palmer amaranth has the fastest growth rate and is the most competitive with the crops common to Midwest agronomic cropping systems.” That is the assessment of Aaron Hager, University of Illinois Extension weed specialist, who says confirmed reports indicate it will cut soybean yields by nearly 80% and corn yields by over 90%. The weed is originally from the southwestern U.S., but in 1957 weed specialists said he was on a northeastward movement. While it may not be in every county, USDA plant ecologist Adam Davis says, “There are few landscape-level barriers to the establishment of Palmer amaranth populations in Illinois, and that these populations, once established, are competitive with crop species. In other words, these results indicate that it’s not a question of if Palmer amaranth will become established in Illinois, but rather when and where it will become established.” While his observation follows research in Illinois, the conclusion could be applied to nearly every state.

"So just spray a little glyphosate on it." While that may control some of the plants the first year, it may have little impact the second year, says Hager, “Palmer amaranth can be effectively managed in Illinois agronomic crops, but the greatest likelihood for successful management is with systems that employ multiple effective management tactics. Palmer amaranth is perhaps the personification of a weed species that requires an integrated management approach.”

The female plants produce an abundance of seeds, which are spread by birds, furry-footed wild animals, dirt clods on tillage equipment, and particularly by combines. And farmers who operate across wide expansive areas are more likely to quickly spread the seed than those who remain close to home.

So how do you control Palmer amaranth? Hager and other weed specialists recommend some specific precautions where the plants exist:

1. Fields with Palmer amaranth populations should be the last fields harvested this fall and the last fields planted next spring.

2. Mark or flag areas where Palmer amaranth plants have produced seed. These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production. 

3. Do not mechanically harvest mature Palmer amaranth plants with crop harvesting equipment. Physically remove the plants immediately prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants from the field. Bury or burn the bags in a burn barrel as soon as possible. 

4. Fields in which Palmer amaranth seeds were produced should NOT be tilled during the fall or following spring. Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.

To address the problems for next year, Hager says include soil residual herbicides at the full recommended us rates within 2 weeks of planting, followed by post emergent herbicides before the Palmer amaranth exceed 3 inches tall. And he says a successful long term program will require more than just herbicides. His colleagues at Purdue, Bill Johnson and Travis Legleiter, have produced an 11-page guide to Palmer amaranth which they say will produce 100,000 seeds per plant when competing with a crop, and a half million seeds per plant when not in competition with other plants.

The troubling part about Palmer amaranth is its resistance to herbicides.  Johnson and Legleiter say, “Palmer populations have evolved resistance to multiple herbicide modes of action, including ALS inhibitors, triazines, HPPD inhibitors, dinitroanilines, and glyphosate. The majority of populations in the South are ALS-inhibitor- and glyphosate-resistant.” Their guide contains formulas for control in both corn and soybeans, but those methods must be carefully followed to achieve success. They say, “Palmer is a very aggressive and adaptive weed, and management programs that rely on a single mode of action (such as glyphosate as the only post herbicide) will typically be ineffective at completely controlling the weed.”

Johnson and Legleiter say the key to success is the use of residual herbicides. “Residual herbicides should be the foundation of all Palmer amaranth herbicide control programs in soybean. There are a variety of residual soybean herbicides that will control Palmer amaranth at its weakest point (emergence) and substantially reduce the number of plants requiring a post-emergence application. Using residual herbicides to manage Palmer will reduce the selection pressure of the few post-emergent herbicide options.”

Iowa State University weed specialists Bob Hartzler and Mike Owen say Palmer amaranth is just moving into Iowa, but has been confirmed in a number of counties. They stress that you need to properly identify Palmer amaranth from other varieties of amaranth. “Knowing how to differentiate the pigweeds is the key to reducing the likelihood of Palmer amaranth becoming established and spreading throughout Iowa. The easiest way to tell smooth and redroot pigweed from Palmer amaranth and waterhemp is that smooth and redroot pigweeds have hairy stems, whereas both Palmer amaranth and waterhemp have  hairless stems.”

While that system may seem simple, it is not always simple.  Hartzler and Meaghan Bryan offer a guide to help identify Palmer from other varieties of amaranth.

Summary:

Palmer amaranth can devastate corn and soybean yields if the weed is competing with row crops. Producing hundreds of thousands of seeds per plant, and resistant to multiple herbicide modes of action, Palmer amaranth requires a strategic approach to control. That includes the way fields are cultivated, scouted, and harvested, in addition to a prescribed treatment with residual herbicides along with a post emerge application.


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