“One pigweed in the field is one pigweed too many.”
That’s the heartfelt conviction of North Carolina farmer Landon Moore.
“It’s so competitive; I’ve seen it take fields out of production altogether,” reports Moore, who grows corn, soybeans and wheat with his father, Richard, near Maxton, N.C.
Farmers have reported yield losses from uncontrolled Palmer amaranth of up to 91% in corn and 79% in soybeans, according to USDA.
PIGWEEDS IN FOCUS
Palmer amaranth and waterhemp, both pigweed species, are often cited as driver weeds in the U.S. because of their prolific seed production, competitiveness with crops, genetic diversity and ability to germinate throughout the growing season.
Driver weeds can vary depending on your geography, the time of year and the crops grown.
Tommy Butts, University of Arkansas weed scientist, says Italian ryegrass is his No. 3 weed problem behind Palmer amaranth and barnyardgrass in rice, but it can be a driver weed in soybeans.
“If we don’t get a good burndown on Italian ryegrass, it’s going to be fighting with our soybean crop all season,” Butts says.
Weeds such as lambsquarters, kochia and horseweed (marestail), can also qualify as driver weeds. Moore says he has a “zero-tolerance level” for pigweed.
“I’m using a herbicide program with various modes of action, both pre-emergence products and post-
emergence herbicides, but there can be escapes,” he says. “If there is one, I’ll walk out into the field and get it.”
BASF is leading an industrywide initiative, Operation Weed Eradication, to change grower mindsets on how they address pigweed, particularly waterhemp and Palmer amaranth species, in key U.S. row crops, including corn, soybeans, cotton and rice.
DON’T LET SEEDBANKS BUILD
Having zero tolerance for tough-to-control weed species, especially driver weeds, is a goal Don Porter, Syngenta herbicide technical lead, would like to see more row-crop growers adopt.
Driver weeds, he says, are often resistant to glyphosate and other products, including ALS and PPO chemistries. As a result, they require diversity in herbicide programs and agronomic management practices.
A “start clean, stay clean” approach to weeds won’t allow weeds to produce seed that can disperse and go back into the soil seedbank, adds Nick Hustedde, FMC technical services manager in Illinois.
Hustedde says the number of prevent plant acres in 2019 compounded the problem. Prevent plant acres that August were at nearly 20 million acres, more than double the previous record, according to USDA Farm Service Agency crop acreage data.
BLOCK AND TACKLE
While many farmers sprayed prevent plant acres or even mowed them that summer, Hustedde says their actions were often too little too late.
“In most cases, it was after the pollination period for waterhemp, for instance, so we weren’t able to reduce the amount of viable seed,” he says.
Farmers who prevent seedbanks from building can slowly gain an upper hand over weeds, even herbicide-
resistant ones. Hustedde references work by Iowa State University that shows after four years of concerted effort in test plots to keep waterhemp from going to seed and back into the soil, only 10% of the waterhemp seedbank was still viable.
“If we can do a good job of practicing zero tolerance for three consecutive years, then Mother Nature is going to reward us and start playing in our favor,” Hustedde says.
Integrated weed control practices also minimize the potential for herbicide resistance to build.
However, as farmers reduce the number of driver weeds in the field, they sometimes inadvertently open the door to a shift in species. Hustedde sees that occurring in Illinois with giant ragweed.
“It’s somewhat adapted to our management strategies and has evolved to emerge later in the season outside of our normal tillage timing, as well as when our residual herbicides have degraded,” he says.
This situation takes a new approach and strict management practices.
“To manage all these weed species, it does require a diverse system with multiple different sites of action and residual activity to target the emergence characteristics,” Hustedde points out. “If you chase just one weed, other weeds are going to start chasing you.”
A Weed Numbers Game
The more a weed species is exposed to one management practice or a single herbicide product, the more likely it will select for resistance and populations will build.
“Reducing selection pressure should really be a key priority,” points out Nick Hustedde with FMC.
He offers a hypothetical scenario: A single female Palmer amaranth, which can produce up to 300,000 seeds, escapes control measures. This is how it can multiply over a three-year period.
Rhonda Brooks leverages 35 years of experience and farm roots to report on seeds, agronomy and inputs.
