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Keeping weeds down in a wet year
Add weed control to the list of elements of growing your 2011 crop that is being complicated as cool, wet weather continues to delay planting in the bulk of the Corn Belt.
If you're too far delayed in your planting and were originally planning on using a quick tillage trip to knock down early-emerging weeds, you may not be able to pull that off this spring. "Preplant tillage operations can effectively control existing vegetation while preparing a seedbed," says University of Illinois Extension weed specialist Aaron Hager. "However, as weeds become larger, the effectiveness of tillage to control weeds before planting can be reduced."
Even if you are able to squeeze in a round of tillage as things start to dry out, it may lose some efficacy, Hager says. "Reduced weed control may also occur when fields are slightly wet during the preplant tillage operation," he says. "Soil disturbance may not be as extensive when soils are retaining moisture, and clods are more likely to be formed. Weeds sometimes take root again after tillage when soil disturbance is inadequate and soil moisture is abundant."
So, what's the answer? If tillage is already done, you don't have enough time before you plant, or you were already thinking of a burndown application anyway, Hager says you can control winter annual weeds with a little stronger rate of burndown herbicide to "account for the large and dense vegetation."
"Glyphosate application rates (alone or tankmixed with other herbicides) should be in the 1–1.5 lb ae/A range to control the large vegetation," according to a University of Illinois report. "Include AMS at 8.5–17 lb/100 gallons of spray solution and apply in sufficient carrier to ensure good coverage of the dense vegetation. It is advisable to add the full recommended rate of AMS under these challenging conditions."
2,4-D is frequently used in burndown tankmixes prior to corn or soybean planting. Both the amine and ester formulations are labeled for applications prior to planting, but the ester formulation is usually preferred over the amine formulation, according to the university report.
"The low water solubility of an ester reduces the potential for it to be moved into the soil by precipitation, where it could cause severe injury to seedlings," Hager says. "Also, the ability of esters to better penetrate the waxy leaf surfaces of weeds often results in better control of large weeds and better overall weed control during periods of cool air temperatures.”
The labels of many 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow applications of 1–2 pints per acre 7 to14 days prior to planting corn. In addition to waiting intervals, labels sometimes indicate that tillage operations should not be performed for at least 7 days after application, and that the seed furrow must be completely closed during the planting operation or severe crop injury may result, he said.
"Factors that increase the likelihood of the growth regulator herbicide coming in direct contact with the crop seed increase the probability of severe crop injury," Hager says. "If you intend to plant before the labeled interval will elapse, leave out the growth regulator from the burndown application and replace it with another herbicide and/or increase the rate of the non-selective herbicide, if possible."
Hager warns producers to be cautious about which herbicide alternative they include with glyphosate. Contact herbicides can sometimes antagonize glyphosate, especially on large weeds. Alternatively, improved performance of nonselective contact herbicides used for burndown, such as paraquat or glufosinate, can be realized when other contact herbicides, such as metribuzin or atrazine, are tankmixed with them.
Glyphosate-resistant populations of horseweed (marestail) and waterhemp occur across many areas of Illinois and can be present prior to corn or soybean planting. Failure to adequately control these glyphosate-resistant populations before planting could lead to significant challenges after the crop has emerged, especially in soybean where very few alternative postemergence herbicide options exist, he said.
"Tankmix partners with glyphosate or alternative herbicides will be needed to control glyphosate-resistant weeds prior to crop planting," Hager says. "More tankmix partners or alternative herbicide options are possible before planting than after planting."