Last Tuesday, Extension agronomists from Penn State University did a “Tour dePigweed” of Southeast Pennsylvania farms infested with Palmer amaranth and waterhemp — pigweed variants. Here’s what they found, as related by Weed Specialist Bill Curran.

“We revisited farms that we first visited in the fall of 2014. Although the problem still exists, these farmers are proactively trying to manage it.

SEED HARVEST TRAIL: Late-emerging waterhemp pigweeds follow the combine harvest trail of last year’s crop — despite three herbicide applications. Bill Curran photo.

“On an Amish farm near Quarryville, the farmer was well aware of Palmer amaranth and trying hard to make sure that it was contained. At least three of his boys had been roguing some of the fields, trying hard to make sure the weeds didn’t set seed.” However, the Palmer was persisting in his corn and alfalfa.

Mowing a perennial forage like alfalfa is better than trying to manage it in soybean, says Curran. But it won’t be the answer alone for this resilient invasive weed.

On another farm, pigweeds were 2 or more feet taller than the chest-high soybeans. The farmer had hired a weed-pulling crew to walk his fields for a few weeks. “While they had missed some, it was certainly better than it would have been. Hopefully,” he adds, “the crew will return and continue the job using ‘pigweed bags’ to burn or bury the plants and seeds.”

Weeds expanding their territory
From Lancaster County, the group traveled west and north to Berks County to fields where Palmer amaranth and waterhemp have been expanding over the last three years. A number of problem fields were near Robesonia.

On one farm, waterhemp was abundant. “You could see the pattern where the combine harvest from last season helped distribute the seeds,” notes Curran. “We were told that the farmer made three herbicide applications to the problem fields — pre and two post applications. You could see where the post contact herbicide had killed the apical meristem. But the plants had recovered and were now flowering.

“It’s obvious that management on some farms isn’t working,” reports Curran, “and the results will likely spread to neighboring fields, farms, townships and counties.”

Over-reliance on certain herbicide families quickly leads to herbicide-resistant weeds. The utility of the Group 14 herbicides is in jeopardy, particularly for these problematic invasive pigweeds, but also for the ragweeds and likely marestail/horseweed. Management needs to be more than simply adding additional herbicides to the regime, concludes this weed specialist.

Common postemergence Group 14 herbicides include Reflex/Flexstar (fomesafen), Cobra (lactofen), Ultra Blazer (acifluorfen) and Cadet (fluthiacet). They often follow a preemergence Group 14 application of a Valor (flumioxazin) or Authority (sulfentrazone) product. The increasingly repeated combinations on big weeds has likely led to natural selection of PPO-resistant pigweeds.

Importing weed seeds onto farms in feed, hay and other commodities can be a serious problem. Once introduced, spreading seeds through equipment and particularly during combine harvest will quickly move these pigweeds into new fields and onto neighboring farms.

Back at Beltsville and USDA
At a recent USDA meeting in Beltsville, Md., Curran’s Midwest colleagues talked about multiple herbicide resistance in waterhemp in parts of the Corn Belt. Resistance to the PPO (Group 14) herbicides is becoming much more frequent, and that’s one of the primary tactics used for control in soybean.

Palmer amaranth is widely distributed in the South. There, growers are looking at alternatives, and in particular, options for controlling weed seeds during or after cash-crop harvest.

Scientists at the University of Arkansas and other institutions are testing “harvest weed seed control” tactics for effective control of invasive pigweeds and other problem weeds. The management tactics being tested include using chaff carts, narrow-windrow burning and using the Harrington Seed Destructor developed in Australia. American Agriculturist told you about the HSD three years ago in “Bust weed seeds with a cage mill?” The HSD pulverizes weed seeds coming out of the combine. In Australia, the HSD was clearly most effective — 95% of weeds seeds were destroyed.

Now, Ag Research Service scientists are beginning to test two of the machines at USDA research labs. The Arkansas team is anxiously awaiting arrival of an HSD component to be integrated into a conventional combine.