Glyphosate-resistant pigweeds are changing the way Mid-South farmers approach crop management. What does that mean in terms of dollars and cents?

“I’ll start with pigweeds in cotton because it’s where I’ve spent my time, it’s the crop I know most about and it’s the most interesting, as well,” said Kelly Bryant at the recent Pigposium in Forrest City, Ark.

“In 2008, (University of Arkansas weed scientist) Ken Smith and company worked a field in Lee County known to be infested with glyphosate-resistant Palmer amaranth,” said Bryant, director of the University of Arkansas’ Southeast Research and Extension Center. “They planted Roundup Ready cotton there and came up with about nine different ways to control weeds in that field.”

Bryant calculated the cost for those nine treatments while researchers “took measurements on the percent control achieved.” The cheapest “was strictly Roundup — four shots. Included in the cost are materials plus application. That treatment got 93 percent control. I thought that was pretty good but (Smith) says we can’t live with just 93 percent control.”

The next best “got 98 percent control. But it cost us about $35 per acre to get there.”

In 2008, in that field, “we had to spend $113.91 per acre to get 100 percent control.”

In 2009, researchers repeated the work in a different Lee County field. There were also additional treatments attempted and results were a bit different.

A Roundup-only treatment — with a cost of $60.84 per acre —gave only 65 percent control.

For $9 more per acre, “there was a treatment that provided 98 percent control. We got 100 percent control at a cost of $82.30.

“One of the take-home points I’ll leave with you: in Roundup Ready cotton, we’re looking at an extra $30 to $35 per acre to control glyphosate-resistant Palmer amaranth.”

Researchers also planted LibertyLink cotton and ran some 17 different herbicide recipes on it.

“In 2008, for $58.16 (only Ignite), the test got 78 percent control. Another treatment, used in both 2008 and 2009 studies, got about 86 to 88 percent control for $61.67 per acre. For $67.07, we got 100 percent control.

“So, in LibertyLink cotton, it’ll cost around an extra $10 to get 100 percent control.”

In Arkansas, Roundup Ready cotton varieties still dominate, Bryant pointed out. “You have two options. If you don’t have (glyphosate-resistant) pigweed today, you can go into a ‘preventive mode.’ That’s where some extra herbicides are used to try and keep from having a problem (develop). I estimate that will cost you about $15 per acre because it includes a preplant application and another at layby. And in cotton that has to be done with a hooded, directed sprayer.”

Otherwise, “when you do develop a problem, it will cost an extra $35 per acre to grow Roundup Ready cotton. And that will require two post-directed applications. Anything that escapes is likely Roundup-resistant and must be dealt with through cultivation or hand-hoeing.”

Having spoken about per-acre numbers, Bryant switched to a more “whole-farm perspective” where “to prevent (resistance) or to control it, requires post-directed, hooded spray applications.”

A high-clearance sprayer will cover about 90 acres in an hour while a hooded, directed sprayer covers only about 25 acres per hour. “We’re working to assess the impact that may have on the number of acres you can plant.

“Certainly, when June comes around and it’s time to use the hood, directed sprayers, you’ll have to pull hands off some activity to make the applications.”

Also in need of analysis is the impact of weed resistance on conservation tillage. “I’ll reiterate: we can control glyphosate-resistant pigweed in both cotton and soybeans. And we can do it without tillage. We can do it by including residual herbicides at pre-emergence, with an in-season, and at lay-by.

“However, if there are escapes, we’re down to just cultivation and hand-hoeing. That’s going in the wrong direction when it comes to conservation tillage.”

Besides only increased costs, trying to grow Roundup Ready cotton in the face of resistant pigweed “requires increased effort and the risk of growing the crop. The pre-emergence, for example, means you either spray behind the press wheel (which means hauling water and filling spray tanks during the planting operation) or making an extra trip behind the planter with a high-clearance sprayer.”

In 2010, Bryant followed several farms that did work with a high-clearance sprayer. Even then, “they told the planters to ‘roll on. We can cover 90 acres in an hour and we’ll catch up with you before the crop emerges.’ But complications can set in — windy weather, machinery or personnel issues — and I’m not sure every acre got the pre-emergence application.”

Hooded sprayers are slow and precise and require extra effort. “And residuals may not receive timely rainfall for activation. Numbers show that, on average, about 85 percent of the time you’ll get sufficient rain. And sometimes you can get too much rain. That means there are acres where the pre-emergence herbicide isn’t activated correctly.”