Herbicide Resistance Concerns Growers and Scientists

Southern row-crop farmers are dealing with their own kind of drug-resistant staph, and while it is not fatal to them, it could eat a big chunk out of their wallets.

Instead of bacteria, though, the problem involves weeds — weeds that in a few cases have developed herbicide resistance.

Currently, Alabama growers are dealing with three confirmed species of herbicide resistant weeds.

They have known about the first one for a long time — common cocklebur, whose resistance to a comparatively older herbicide, MSMA, was confirmed roughly 20 years ago in cotton fields in western Alabama and near Harpersville in the central part of the state.

As herbicide resistance goes, this is not a catastrophe for growers, says Dr. Michael Patterson, an Alabama Cooperative Extension System weed scientist and Auburn University professor of agronomy and soils. MSMA, an organocarcenical herbicide, now is used more in the sod industry than with row crops.

Also in the late 1980s, Cherokee County farmers began noticing goose grass that seemed to have developed resistance to Prowl and Treflan, the so-called yellow herbicides —this subsequently was confirmed by Auburn University researchers.

But again, this proved more of an inconvenience than a calamity in the making.

More recently in the early 2000s, researchers also detected glyphosate-resistant mare’s tail, also known as horseweed, in Tennessee cotton fields. Although the presence of the weed has not been confirmed in the Tennessee Valley of Alabama, it has been reported by farmers in the region.

However, Patterson describes resistant mare’s tail as a nuisance rather than as an impending problem.

“We have things we can mix with our burn-down herbicides ahead of planting cotton to deal with this resistant mare’s tail,” Patterson says.

What Patterson describes as the 800-pound gorilla of herbicide resistance is still — fortunately for Alabama farmers — primarily a Georgia problem, though its footprint may have been detected in two southern Alabama counties that share a border with Georgia.

This impending threat — glyphosate- and, in some case, ALS-resistant pigweed — discovered in south Georgia in 2005 already is believed to have jumped the border into Barbour and Houston counties.

It’s a threat because Alabama growers by and large are deeply invested in a weed control strategy built on the herbicide to which this pigweed has developed resistance — glyphosate, widely known among farmers and consumers as Roundup, Touchdown and other brand names.

Roughly 95 percent of acreage already is invested in Roundup Ready cotton, Patterson says, adding that growers are similarly committed to Roundup Ready varieties of soybeans and are making a rapid transition to high concentrations of Roundup Ready corn varieties.

Simply put, he says, farmers are moving rapidly to a system in which all of the state’s major row crops, except for peanuts, are based on Roundup Ready technology. In fact, to a large degree, farmers depend on this technology to grow these crops economically.

So what happens when cropland is overrun by pigweed?

“With a glyphosate-resistant weed like Palmer pigweed in a field grown with Roundup resistant crops, farmers face the real risk of having their yields reduced to about zero,” Patterson says.

Patterson and other researchers already have treated two fields in southeastern Alabama believed to be infested with glyphosate-resistant pigweed using considerably higher rates of glyphosate, although with no success.

With the assistance of regional Extension agents, they already have developed a monitoring program to trace the spread of this highly adaptive weed, though Patterson thinks it’s all but inevitable that more of it will begin turning up around the state.

“I think we’re going to continue to see more because pollen from all of the resistant pigweed they have in Georgia may be able to move many miles in a storm event,” he says.

“In cases where pollen from a glyphosate-resistant pigweed lands on a glyposate-susceptible pigweed, you get offspring with some degree of resistance — maybe not as much as the parent resistant plant but somewhere in the middle range.”

For now, Patterson and other scientists, as well as Extension educators, are working to impart some sober advice to growers throughout the state.

“What we’re trying to do is educate people that if this thing is in your neighborhood and you depend totally on a Roundup Ready system, you need to get off that wagon to a degree and add some of the older but still active forms of products in the program.”

This includes products such as Prowl, trifluralin (Treflan and other generic forms), fluometuron (Cotoran and other generic forms), Dual, and diuron, as well as some of the newer products, such as Reflex and Valor, Patterson says.

But what worries Patterson the most, because in addition to being less convenient to growers, they also cost more money, at a time when many farmers throughout the state are frantically trying to cut costs in a highly competitive economic environment.

Posted by Jim Langcuster at November 6, 2007 04:07 PM