HERBICIDE-RESISTANT weeds are increasingly a problem for growers in the Mid-South and Southeast.
When growers began to plant new Roundup Ready crops in the mid-1990s, most were surprised at how easy their lives became. They could fill a spray tank with Roundupand water and, for the most part, just keep adding glyphosate until they laid by their crops.
Fast forward 15 years and farmers are finding not only do they have to add more chemicals to the spray tank, but Extension weed scientists are also beginning to recommend they make wholesale changes to their production systems, often just to deal with one main weed culprit, Palmer amaranth or pigweed.
“There really are no (herbicide) ‘silver bullets’ for dealing with herbicide resistance,” Mike Owen, Extension weed scientist at Iowa State University, said during a recent meeting. “It will require you to change your whole system of operation if you are to keep it from ending your farming career.”
Owen, one of the keynote speakers at a meeting on herbicide resistance in Washington last summer, was speaking primarily about common waterhemp, a cousin to Palmer amaranth. But Palmer amaranth has now been observed as far north as Michigan, so it may be just a matter of time before pigweed resistance rears its ugly head to Midwest corn and soybean producers.
“The message needs to be to increase the diversity — Don’t do what you’ve been doing. You need to bring in new herbicide modes of actions and use new tactics so you can control these weeds that are becoming more difficult.”
Owen and other weed scientists are recommending specifically that growers use multiple modes of action in the residual herbicide mixture they apply at planting.
“I like the pre-mixes that contain two different modes of action against pigweed, such as Prefix, Boundary, Authority MTZ, Valor XLT and Envive,” says Bob Scott, Extension weed scientist at the University of Arkansas.
He is also advising growers to make changes as simple as planting soybeans in narrow row spacings vs. the 30- or 38-inch spacings that held over from the horse- and mule-drawn plow days.
“My first observation in the pigweed fight is that wide rows simply don’t work,” he says. “Whether you are staying in Roundup Ready or going LibertyLink, failure of the soybeans to canopy means that pigweeds will continue to germinate all year long. That may mean you have to change the way you farm — but you must do what you can to narrow your row spacing.”
Postemergence applications of residuals such as Dual or metolachlor and the new hooded sprayers can help. “There isn’t much else to pick from in terms of post residual herbicides for soybeans because most of our pigweeds are ALS-resistant. So, products like Scepter and Classic don’t help much.”
In side-by-side studies in 2012, drilled soybeans were on average 10 percent to 20 percent cleaner at canopy than wide rows with similar or same herbicide programs. “Drilled rows are best, if you must stay on beds,” Scott says, “I would consider trying to get two or three rows per bed instead of one.”
Adopting a “zero tolerance” policy is also helping reduce infestations of Palmer amaranth in Mid-South fields, according to Ken Smith, who recently retired from the University of Arkansas Cooperative Extension Service as a weed specialist.
“We’re doing a better job than we have in the past,” says Smith, who was based in Monticello in southern Arkansas. “Even though the weather has not been good for weed control in 2012, the commitment to reduce that soil seed bank (of Palmer amaranth) has been encouraging.”
Smith and Larry Steckel, Extension weed scientist with the University of Tennessee, say one of the reasons growers are having more success is they are switching to corn or grain sorghum where they can do a better job of managing fields with intense soil seed banks of Palmer amaranth with conventional herbicides.
Other factors include use of other herbicide-resistant traits, better timing of herbicide applications to smaller weeds and the wide-scale adoption of a zero-tolerance policy. That can mean something as simple as walking to a ditch bank and pulling up a pigweed.
The scientists cite an example: a highly infested 60-acre field in a zero tolerance program (where the goal is to not allow a single plant to go to seed). The first year in zero tolerance, it required 100 hours of hand-chopping to clean the field. The next year, the field required only five hours of hand chopping.
“We’re never going to eradicate Palmer pigweed,” says Smith. “But we can get it down to a manageable level.”
He urges growers to start clean, overlap residual herbicides and manage escapes. A Palmer amaranth seed lasts three to four years in the soil. If there is a weakness in Palmer amaranth, which is specifically adapted to the hot, dry climate of the desert Southwest, it is that the seed do not live a long time in the soil.
Producers should plan for a two-week overlap between residuals, even though many residuals may last longer. “If you do that, you’re more likely to get it done on time. It’s not rocket science. Get in there early enough so that the residual has enough time to start working before the other one runs out.”
Mid-South research on cover cropsas a resistance management tool “is promising,” Steckel says.
“If we can get the ground shaded with a cover crop early on, we can really curtail how many weeds we may have to fight with our herbicides later. Pigweeds need three things to germinate, heat, water and sunlight.”
Research on vetch, wheat and crimson clover showed the cover crops saved up to $15 per acre in herbicide costs due to the elimination of one burndown and helped with suppression of horseweed, and pigweed early in the season. The effect of the cover crop “starts to play out in mid-June, and we had to go to post applications,” Steckel notes.
Legume-based cover crops like vetch can put 50 pounds to 60 pounds of nitrogen into the soil, an increasingly attractive proposition given higher N costs.
Cover crops are also showing promise in other areas of the Southeast, including the planting of rye in Georgia.
“I think this will be a significant factor to growers in the next three to four years,” says Stanley Culpepper, University of Georgia Extension weed scientist. “We’re looking at using very large rye — six to eight feet tall. We roll the rye, and then we plant into it.
“There are different systems, but what we can do with the rye is to block the sunlight from getting to the ground. Pigweed will not emerge without sunlight, so we can basically use a rye cover crop to manipulate the amount of emergence, thereby improving overall weed control.
“And we can do it with less herbicide inputs to help offset the cost of the rye and the other challenges of that system,” he says.
Some Extension specialists and growers are questioning whether traditional tillage practices are still warranted given the changing cropping patterns and weed control issues in the Delta and Southeast.
“After a field is harvested, the initial reaction is to get it worked up and ready for next year,” says Jason Bond, a weed scientist with Mississippi State University’s Delta Research and Extension Center. “But in today’s Mississippi Delta, corn or soybeans are typically harvested in July and August, so field operations that took place in October or November are now done in late summer.”
Tillage is more efficient following corn harvest if the stubble is burned. But how many nutrients are in the ash that blows off the field? Could the corn residue potentially suppress weed germination and emergence? It is a fact that disturbing a field again after the beds are pulled does not make sense.
Weeds that emerge, grow, and produce seed after a field is worked and before the first frost will have to be dealt with during the following year’s crop, Bond notes. Nutrients that are removed by burning crop stubble will have to be replaced. Beds that erode over the winter will have to be reformed.
Bond says growers should maintain as much flexibility in their cropping decisions as possible from a weed control standpoint. For example, if a product that contains chlorimuron (Authority XL, Canopy, Canopy EX, Envive, Valor XLT, etc.) is applied for fall burndown, then the re-cropping restriction is a minimum of nine months to any crop other than soybeans.
“These products are all excellent herbicides for fall burndown in soybean, but they remove any flexibility for changing the crop to be planted. Allow a margin of error in planting intentions during the fall burndown.”
Managing glyphosate-resistant Palmer amaranth drives most weed control decisions in the Southeast, but, with the current crop mix, problems with other weeds such as morningglories and annual grasses are intensifying.
Fields and areas within fields that have problems with these particular weeds should be identified and recorded during or after harvest. These records will be a good reference in planning for weed control the following year.