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Drought's Weed Woes

September 29, 2012
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
p26 Droughts Weed Woes
Weeds visible in 2012 crops might not have been herbicide escapes but rather multiple late flushes that normally would have been controlled by the crop canopy.  

Who benefits from drought? Weeds! Step up your management to protect yields next year and beyond

Short crops have long tails in more ways than one. Weed management after a drought year can impact yield and profit for years to come.

An important aspect of your weed control program is shade—the absence of sunlight prevents weed seeds from germinating. But the drought-stunted crops of 2012 didn’t close their canopies as they normally do. In addition, the herbicides that you applied might not have activated or were not degraded into harmless compounds, so the active ingredients are still present in your soil. Those two aspects of weed control can haunt you if you don’t start managing them now at harvest.

"What look like late escapes—such as giant ragweed in corn and waterhemp in soybeans—may actually be three to four late flushes of weeds that normally would have been controlled by the crop canopy," says Farm Journal Field Agronomist Ken Ferrie. "They may have time to set seed, increasing the seedstock for 2013."

Control starts in the combine cab. "Record the location of heavy weed patches," Ferrie advises. "Often, you can record them on your yield monitor. If you can’t, note them in your written records for each field."

Fall treatment. "Depending when you harvest, you may be able to apply 2,4-D or dicamba herbicides this fall, and kill some weeds before they go to seed," Ferrie says. "If you spray, do it immediately after combining. Even if some weeds have gone to seed, controlling the later-germinating ones will reduce the seed bank for next year."

In corn, if you spot weeds that are still young enough to treat, lift your head and run it just under the ear. "The less you mangle the weed with your header, the easier it will be to kill it with herbicides," Ferrie says.

In soybeans, you can’t avoid cutting off the weeds. "Let those weeds regenerate, and wait until you see some new growth before spraying," Ferrie says.

Southern growers are used to applying postharvest herbicides to control Palmer amaranth, Ferrie notes. But in all regions, postharvest weed control treatments are especially important in 2012 because, without a normal crop canopy, more weeds than usual will have germinated.

Next year’s weeds. "In 2013, especially with really tough weeds such as giant ragweed and waterhemp, you may want to apply a herbicide late in the season, prior to harvest, using a high-clearance applicator," Ferrie says. "In corn, wait until brown silks appear, then apply a growth regulator herbicide. At that stage, corn will be too mature to be damaged and nearby soybeans will be too mature to be injured."

In soybeans, you can apply a cleanup herbicide after the plants have reached the R7 growth stage with minimal risk of yield damage, Ferrie adds. Before spraying, always check the product

label for harvest restrictions.

For significant weed escapes in soybeans, harvest-aid herbicides such as Gramoxone are an option if you are harvesting late in 2012, or for 2013. "Growers in the South use harvest-aid herbicides routinely to kill their determinate soybean varieties," Ferrie notes. "But they may be new to Northern growers, who don’t normally need them with their indeterminate varieties.

"With indeterminate soybeans, apply the herbicide when 65% of the pods are brown in color and the beans are at 30% moisture, physiologically mature and separated from the pod, rattling around inside," Ferrie says. "With determinate varieties, apply when half the leaves have fallen off and the rest are yellow. If you spray a harvest aid too early, it will shut down pod fill and reduce yield."

Accurate weed mapping this fall sets the stage for your 2013 control strategy. "Don’t just mark the location of weed patches—record exactly what species are present in each location," Ferrie says. "Carry a scouting manual in your combine, and stop the combine to take a close look."

Plan your strategy. "Later, tell your herbicide supplier which weeds escaped your herbicide or outlasted it," Ferrie adds. "He needs that information to help you develop a management plan for next season. Download your maps and share them with whoever will scout your crops in 2013, so they will know where to look for problems."

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FEATURED IN: Farm Journal - October 2012



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