Drought's Weed Woes

September 28, 2012 09:03 PM
Drought's Weed Woes

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."

A more radical option for this fall—because it carries its own set of problems—is the moldboard plow. "Some weeds can be controlled by burying the seed," Ferrie says. "For plowing to be effective, you must understand each weed species because some weed seeds must be buried
longer than others."

Avoid plowing if you can, Ferrie urges. But in a few situations, if the weed seed bank is completely out of control, it might be your only option.

If you spray this fall, check the recropping restrictions (also called recropping intervals) for the herbicide you apply. "A grower in a corn-soybean rotation has more options than one planning to follow soybeans with cotton," Ferrie says. "Consider the restrictions imposed by your rotation or your plan to plant a cover crop.

"Choose the right herbicide, and the right timing, for the weeds you want to control," Ferrie adds. "Think about the restrictions imposed by soil pH. Atrazine and similar herbicides will not break down in high-pH soils, so the herbicide will carry over. It may affect next year’s crop or this fall’s cover crop, or it may leave the field and get into water supplies."

Carryover issues. Similar carryover issues apply to your 2013 crop. "Review the herbicides you applied in 2012, their half-life in the soil and the amount of rainfall you received," Ferrie says. "The lack of rain may have reduced the microbial activity required to break down the products, so the active ingredients may still be out there. This is likely with postemergence products applied later in the summer."

p27 Droughts Weed Woes chart

Discuss potential issues for next year with your retailer. "If you made a late post application of an ALS or diphenyl ether herbicide to non-GMO soybeans, you may have to grow soybeans again next year, unless you get plenty of rain during the off-season."

Some soil test labs can test for specific herbicides, but you might have to send samples to different labs to test for various products. "Each herbicide requires its own test, and each test is likely to cost $150 to $200," Ferrie says. "A cheaper option is to collect some soil and conduct your own grow-out test. Oats can provide a red flag. If oats won’t grow, have the soil analyzed by a lab or rethink your rotation."

Some extra effort to control weeds this fall will pay big dividends in 2013, and maybe even longer.

Carryover Concerns for Covers and Wheat

If you plan to plant a cover crop this fall, consider the effect of the herbicides remaining in your soil because of limited rainfall. Unfortunately, information about herbicides’ effect on cover crops is not widely available.

One way is to look at the restrictions for similar species, say Penn State Extension weed specialists Bill Curran and Dwight Lingenfelter. For example, you would expect legumes such as hairy vetch and winter pea to be sensitive to the same herbicides as alfalfa and clover. Herbicides that affect canola would be likely to affect tillage radish or daikon radish.

Rotating to wheat might help you hold on to unused nitrogen, but carryover concerns exist with wheat, too. "In many instances, the interval between [corn herbicide application] and winter wheat planting will not be long enough this year," warns Aaron Hager, University of Illinois weed scientist. "Small-grain crops can be sensitive to even small amounts of residual herbicides that remain in the soil."

"The carryover potential for herbicides depends on several things, including soil texture, organic matter content, soil pH and rainfall," Curran says. "Lighter soils with less clay and organic matter and an intermediate soil pH of 6 to 7, with ample or average summer rainfall, generally aren’t a problem for many row-crop herbicides.

"Colder climates, like the Upper Midwest and New England, are more prone to carryover than warmer, milder Southern climates. Western U.S. soils, with high soil pH and arid climates, present a particular problem for herbicide persistence."

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