Nebraska and Arkansas don't share a border and are separated by about 300 miles, but there's at least one agricultural challenge the Cornhusker State and the Natural State have in common: glyphosate-resistant weeds — in particular, Palmer amaranth.
University of Arkansas weed scientist Jason Norsworthy recently discussed managing the pigweed at a Glyphosate-Resistant Palmer Amaranth Management Day in Carleton, Neb. In the past five years, Norsworthy has made multiple trips to Nebraska, and while helping growers combat Palmer amaranth in Arkansas — where the weed has developed resistance to at least nine groups of herbicides — he's also seen firsthand the expansion of Palmer in Nebraska.
While it's well-known that repeated use of the same modes of action leads to resistance, in many cases, the chemistries being used on Palmer amaranth aren't new. Norsworthy points out growers in Nebraska already use DiFlexx and other dicamba products in corn.
"We're not starting at ground zero with some of this chemistry," he says.
Meanwhile, pigweeds such as Palmer amaranth can develop metabolic resistance — where resistance to one herbicide group can develop cross resistance to other herbicide groups. This year, Kansas State University announced the confirmation of 2,4-D and dicamba-resistant pigweed in Kansas. And Norsworthy notes growers there had never planted dicamba or 2,4-D tolerant soybeans.
That may paint a grim picture, but with the right integrated approach, Norsworthy says growers can be successful in keeping Palmer amaranth at bay. He outlined three "don'ts" — or three lines of defense — when managing Palmer amaranth.
1. Don't let it emerge. Palmer amaranth can emerge over a four-month period, from May through August. Like other weeds, it relies on fluctuating soil temperatures to emerge.
At 90% soybean canopy closure, there's less light interception and not enough temperature fluctuation for Palmer to emerge. However, it takes eight to nine weeks for soybeans to canopy in 30-inch rows in Nebraska. Planting in 15-inch rows and planting cover crops also can reduce light hitting the soil and keep soil temperatures at more consistent levels.
"Weeds do not emerge if there's a cover. Soybean will emerge. You can plant soybean into a cover. It does not need light in order to emerge," Norsworthy says, noting research in Arkansas comparing Palmer emergence on fields with a cereal rye cover crop versus fields without. "I found the cereal rye cover crop reduced the Palmer amaranth over a two-year period by 62%."
However, he notes a preemergence herbicide still is necessary, even when "planting green" into a standing cover crop. Both herbicides and cover crops are part of an integrated management system.
With this emergence pattern, Norsworthy encourages using two effective active ingredients at planting, and following up three to four weeks later with a post and overlapping residual. One program Norsworthy has used successfully is Zidua (pyroxasulfone) mixed with Tricor (metribuzin) at planting, followed by Liberty (glufosinate) and Prefix (metolachlor and fumesin) four weeks later.
2. If it emerges, don't let it produce seed. "If you've got this on your farm, and you've only got a few of them out there, you'd better get out of the truck and pull them and bury them," Norsworthy says.
Norsworthy notes up until it reaches 6 inches in height, Palmer amaranth can grow almost an inch per day. Once it reaches 6 inches tall, it will grow 2 or 2.5 inches per day. Within a 14-day window, it can go from 5 inches to 40 inches tall.
"If you don't know what you have in the field from a resistance standpoint, you choose the wrong herbicide and say I'm going to come back to the field, and 14 days later, I can't kill that with anything at this point," he says. "There are no options for effectively controlling that pigweed. You've got to know what works if you're going to manage this and have success."
Norsworthy says there is value in certain mixes for controlling taller pigweeds (up to 10 inches) — including applying a post of glyphosate plus glufosinate, and coming back five to six days later and applying glufosinate again.
However, keep in mind applying Liberty after soybeans flower is not permitted by the label. Also, while Liberty can be mixed with 2,4-D choline, as with the Enlist soybean system, mixing it with dicamba is considered an off-label use.
3. If it produces seed, don't let it get into the seed bank. Individual Palmer plants can produce up to 1.8 million seeds by some accounts. And research from Wisconsin to Arkansas shows 99% of Palmer amaranth seed is retained in the combine, Norsworthy says. Still, he notes there are some promising alternatives for Palmer control.
In a recent experiment, University of Arkansas researchers removed spinners from the back of the combine and installed a chute to funnel soybean chaff and Palmer amaranth seed into windrows, before setting the windrow on fire, finding they could kill 100% of Palmer seed.
The University of Arkansas also has experimented with an integrated Harrington Seed Destructors —a machine pulled behind the combine to crack weed seeds that come through. In this trial, the seed destructor killed 99% of Palmer amaranth seed.
Of course, most don't have access to a Harrington Seed Destructor. Alternatives include hand-weeding and burning or burying plants, and in some cases, prioritizing fields to harvest, and harvesting fields with Palmer amaranth last to avoid spreading seed.
"We're only going to be successful long term managing this weed if we can keep the seed out of the soil seed bank," Norsworthy says. "We cannot stay in front of this plant that's producing hundreds of thousands of seed otherwise."