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Issue No. 6, Article 4/May 1, 2009

Weeds at the Time of Planting

Excessively wet soil conditions have hampered planting progress across most of Illinois. The ample soil moisture that has hindered the establishment of the 2009 corn crop has aided the maturation of many winter annual weed species. Several mustard species (shepherd's purse, pennycress, yellow rocket, etc.) have initiated seed production and soon will complete their life cycle by (unfortunately) returning seed to the soil seed bank. Other winter annual weed species, such as butterweed and chickweed, have recently begun to flower. Early-season summer annual weeds, such as common lambsquarters, smartweed, and kochia, have become well established in some fields, although most are less than 3 inches tall.

When conditions again become favorable, we expect planting to proceed at a very rapid pace. Existing weed vegetation should be controlled before planting by utilizing tillage, herbicides, or a combination of tactics so the corn can become established under weed-free conditions. Following are a few points to consider with respect to managing weeds in corn:

1. As mentioned, some winter annual weed species are beginning to flower, whereas others are setting seed. While applying herbicides to species already making seed may not appreciably reduce seed production, species in the early stages of flowering should be controlled soon to prevent seed production and addition to the soil seed bank. Reduced tillage and delayed applications of burndown herbicides are factors that have contributed to the increased prevalence of certain winter annual weed species.

2. Growth regulator herbicides are frequently included in burndown applications. 2,4-D is the most common growth regulator used, but there is also some use of dicamba. The labels of several 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow preplant applications without a specified waiting interval between application and planting, while other formulations require a 7-day waiting interval between application and corn planting. In addition to waiting intervals, labels sometimes also 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. Factors that increase the likelihood that 2,4-D will come in direct contact with the crop seed increase the probability of severe crop injury. Pay careful attention to label statements of any 2,4-D formulation used prior to corn planting.

3. Soil-residual herbicides can be tank-mixed with most burndown herbicides, and many soil-residual herbicides themselves possess activity against emerged weeds, especially when applied with spray additives such as crop oil concentrate or UAN solution. Utilizing soil-residual herbicides in corn and soybean production provides many advantages, including reduced potential for early-season weed interference that can lead to loss of crop yield potential, reduced weed density and more uniform weed size range when postemergence herbicides are applied, and reduced intensity of selection for herbicide-resistant weeds through exposure of the weed spectrum to multiple sites of herbicide action.

4. Cool temperatures can slow the activity of many burndown herbicides, and translocated herbicides are sometimes slower acting than contact herbicides under these conditions. For example, glyphosate is very effective for control of common chickweed, but symptoms of activity may take several days to develop during periods of cool air temperatures. Contact herbicides may not be as slow to act as translocated herbicides under cool conditions. When the forecast calls for several days or nights of cool air temperatures, symptoms of activity on existing vegetation may develop sooner with a contact herbicide than with a translocated herbicide.--Aaron Hager

Aaron Hager

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