Kissing Cousins
February 25, 2009 Glyphosate- and ALS-resistant species of pigweed have become nothing short of terrors across the Southeast and Mid-South.
Palmer amaranth (Amaranthus palmeri) is the most notable, but other species – waterhemp (Amaranthus rudis) and redroot pigweed (Amaranthus thunbergii) – are becoming major problems, too. Since the species are from the same genus, they are genetically “kissing cousins.”
All of these pigweed species have individual male and female plants, meaning that a resistant male can pollinate a non-resistant female, producing offspring that are likely to be resistant. The converse of that is a non-resistant male can pollinate a resistant female and that offspring, too, is likely to be resistant. Weeds that have both male and female tend to remain stable in a field, whether resistant or not.
To compound matters, pigweed is a prodigious producer of seed that are so small and light they can easily move from field to field via equipment and water.
Crisscrossing?
To make a bad situation even more volatile, Palmer amaranth, waterhemp and redroot pigweed appear to be crossing with each other. Potentially, that could spread resistance to species that have not been resistant before.
“We believe we are seeing hybridization with the pigweed species,” says Dr. Trey Koger, an agronomist at the Delta Research and Extension Center in Stoneville, MS. “We have Palmer amaranth that we think has crossed with waterhemp, and we are having a hard time killing it.”
Koger adds that extremely dry weather across much of the South made it very difficult to control pigweed with glyphosate. Then, parts of Mississippi and Louisiana got up to 18 inches of rain from late June to mid-July, leading to healthy, vigorously growing pigweed. The Delta normally gets 50-54 inches of rain annually, meaning that over one-third of the annual rainfall came in less than a month.
Koger says he spent one or two days a week in July traveling the Delta and hill areas of Mississippi, doing nothing but studying pigweed: “I’ve been everywhere from inside the levee at Tunica, all the way down to Port Gibson, then east to Winona.” That all but covers the entire Delta and a portion of the hills.
“The worst field I saw was west of Tunica, inside the levee,” he says. This area, he explains, has always had a history of severe pigweed infestations, and high-water events can compound the problem by depositing even more pigweed seed. “When I looked at it, the first thing that came to mind was Georgia.”
If you’ve seen resistance in Georgia – and Koger has – that’s a terrifying image.
Paint it Yellow
Koger says it may be a matter of going back to the basics to get a grip on resistance management. He explains, “We are at the point where we have to fight this with residuals. We have the residuals to help us combat this problem, and they are basically old chemistries. Yes, that is an added expense. But if we are committed to growing cotton, we cannot rely on glyphosate, glyphosate, glyphosate.”
Some of the older products that would be effective now are the “yellows” – trifluralin and profluralin, for instance – and metalachlor.
Valor, Koger says, has “excellent residual activity when used as a layby on fields with glyphosate-resistant and glufosinate-resistant varieties.” Glufosinate is the active ingredient in Ignite.
Captions:
A giant seed head on Palmer pigweed
Tall Trey …
… taller pigweed
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