Producers across most of the southern U.S. have been dealing with resistant weeds for a decade or more. Resistant Palmer amaranth or pigweed, for example, has been documented in every single agricultural county in Arkansas. Tennessee now has six glyphosate-resistant weeds. Meanwhile, producers in Louisiana may not fight the evil pigweed in every county, but they have resistant johnsongrass — and like most southern producers, they have a plethora of other weeds with which to contend.
Consequently, nearly every major agricultural company in America has promising new technologies on the horizon to help producers fight herbicide resistance. None, however, will be available in 2014.
But producers are not without current options in the battle against resistant weeds.
“Diversity is a good thing, especially in agriculture,” says Bob Scott, University of Arkansas Extension weed scientist. “We’ve diversified our approach in the last five to seven years using various herbicides and production practices to combat pigweed.
“Since we’ve so intensely managed for this one weed, we have prevented the development of new resistance.”
Due to environmental conditions and a decline in the planting of LibertyLink soybeans this year, Scott says, Arkansas producers didn’t completely control resistant Palmer amaranth.
Overall, he says, producers “survived” because most knew they had resistant weeds and understood how to manage them. However, he reminds all producers that it’s a whole farm approach and that resistance is often spread during harvest.
“If you ran a combine through any pigweed this fall, you will have to manage for it next year,” Scott says. “It’s definitely going to be around in 2014.”
He recommends that soybean growers rotate crops, plant LibertyLink soybeans, or have a strong management plan if they’re going to plant Roundup Ready soybeans.
In addition to using effective herbicides and technologies, he suggests other management practices, such as narrowing seedbed rows, choosing bushier-type soybean varieties, increasing seeding rates and planting twin-row.
“The worst mistake a producer can make is to ignore what he saw when he was sitting behind the wheel of his combine,” Scott says. “It’ll be too late to worry about pigweed after planting next spring.”
The number of spray rigs also continues to rise across the Mid-South and Southeast. Many producers are investing in their first, and in some cases second, sprayers so they can make timely herbicide applications.
Producers also are investing in center pivots for further control of moisture and herbicide applications.
“Thankfully, resistant Palmer amaranth doesn’t cover every acre of the state of Louisiana,” says Daniel Stephenson, weed scientist, Lousiana State University Ag Center.
“Most growers who have identified it have taken quick action, either hand-pulling or quarantining fields to keep it from spreading. We’ve also seen more adoption of resistance practices from growers who don’t have the problem — yet.”
Similar to Scott, Stephenson urges producers to not dismiss herbicide-resistance management practices simply because they don’t have resistant weeds in a particular field or farm.
“We are not a one weed monoculture in Louisiana,” he says. “Money spent on residual herbicides is not just about managing the monster Palmer amaranth — it’s also about managing the many other weed species that reduce profits.”
Soybean producers aiming for maximum yields should use a preemergence herbicide, plant clean, and use an early postemergence residual herbicide three to four weeks later.
“New research reveals that producers will maximize yields if they maintain soybeans weed-free the first five weeks,” Stephenson says. “That’s contrary to the traditional mindset of letting weeds get up and established before spraying them.”
Stephenson’s research shows that if soybean fields are not kept clean, they essentially produce no yield. But if kept weed-free for one week, yields increase by 39 percent; two weeks clean increases yields by 70 percent; three weeks, 89 percent (or a loss of 11 percent of yield potential); four weeks, 97 percent. If kept weed-free for 5 weeks after emergence, yield potential increases to almost 100 percent.
“Soil residual herbicides can buy time and manage weeds to protect yields, regardless of whether producers are managing for herbicide resistant weeds,” Stephenson says.
“If properly activated, a preemergence residual herbicide applied at planting can control weeds to lessen yield loss due to early-season crop/weed competition.”
Stephenson says producers who have utilized a fall burndown have more options in the spring, given the number of effective herbicides on the market.
But it they don’t start clean, they may have the additional expense of making two spring applications to burn down, along with another application to combat early-season weeds.
“After harvest is a pivotal time for producers to manage the spread of resistant weeds through planting cover crops or post-harvest weed control,” says Larry Steckel, Extension weed scientist with the University of Tennessee. “Preventive planting is key to resistance management in Tennessee. A good cover crop not only decreases Palmer amaranth, it virtually controls marestail in early spring, eliminating that first application of herbicides.”
Steckel warns producers to look now for atrazine failures in Palmer amaranth and to be vigilant identifying new species of resistance, such as Italian ryegrass.
Producers can also use the time between harvest and year’s end to implement preventive measures.
“Resistance continues to spread across weed species,” Steckel says. “Resistant marestail and pigweed are pretty much in every field.”
Goosegrass is increasingly problematic and glyphosate-resistant Italian ryegrass also is spreading into Tennessee from surrounding states.
In the fields where producers have resistance problems, their best bet is to start clean by tilling and then applying herbicide applications before planting cover crops.
Since it was so wet this fall in Tennessee, some growers have successfully aerially seeded cover crops. Based on past success, many producers are planting grasses such as cereal rye as cover crops for both cotton and soybeans.
“It’s a seed bank game,” Steckel says. “You can get 99 percent control of pigweed this year and still have a problem with it next year.”