No. 4 Article 9/April 15, 2005

Considerations for Controlling Existing Vegetation Before Planting

The decreased use of soil residual herbicides in soybean, coupled with several recent "mild" winters, has resulted in some changes in the weed spectrum across much of Illinois. One change that is very noticeable at this time of year is the amount of weed vegetation present in no-till fields. Compared with 10 years ago, the amount of existing vegetation to be dealt with prior to planting is often much more dense and composed of species not familiar to everyone ("What Weed Is That?" in issue no. 3 of the Bulletin, April 8, 2005, listed useful weed identification resources). Even though the past winter seemed more normal (although "normal" as it pertains to weather is difficult to define), cursory observations suggest weed growth is abundant. As air and soil temperatures continue to increase, expect these weeds to grow rapidly.

Much of this existing vegetation consists of winter annual weed species, such as chickweed, henbit, and purple deadnettle. These species generally emerge in the fall and over winter but sometimes (depending on weather and soil moisture ) emerge in the early months of the calendar year. Some early-emerging summer annual species, such as prostrate knotweed, kochia, common lambsquarters, and giant ragweed, have already made their presence known as well. This "mat" of vegetation can cause significant problems with planting operations and crop establishment if not properly controlled. In most situations, producers should plan to control existing vegetation before planting no-till corn or soybeans.

2,4-D is frequently used as a burndown tank mix prior to corn or soybean planting. The ester formulation is usually preferred over the amine formulation since the waiting period between application and planting is generally less for an ester formulation. The labels of many 2,4-D ester formulations (3.8 lb acid equivalent per gallon) allow applications of up to 1 pint per acre 7 days prior to soybean planting; increasing the rate to more than 1 pint increases the waiting interval to 30 days. Keep in mind that some 2,4-D ester formulation labels also specify a waiting interval between application and corn planting (for example, 7 days for up to 1 pint, 14 days for rates between 1 and 2 pints). 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 of 2,4-D coming in direct contact with the crop seed increase the probability of severe crop injury.

Several soil-applied herbicides used in corn and soybean have both soil and foliar activity. This foliar activity can provide some control of small annual weeds. In corn, products such as atrazine and Balance PRO, or premixes containing these herbicides, can provide control of small weeds. In soybean, products such as metribuzin (Sencor, in Boundary, Axiom, and Domain), Valor, Gangster, Extreme, and Backdraft SL all have foliar activity and can be applied before planting.

Keep in mind that most of these herbicides work best on small annual weeds, especially when applied with a crop oil concentrate or liquid nitrogen solution (consult the respective product label for additive recommendations). If existing vegetation is larger than 2 to 3 inches, adding another herbicide to the tank can often improve burndown activity. Gramoxone Max and glyphosate have foliar activity, but all lack any soil residual activity. These herbicides are often tank-mixed with corn or soybean preplant herbicides to improve control of existing vegetation.

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 can take many 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 compared with a translocated herbicide.

Table 5 is reproduced from the 2005 Illinois Agricultural Pest Management Handbook. The table includes weed control ratings for several corn and soybean herbicides used to burn down existing vegetation prior to planting.

--Aaron Hager and Dawn Nordby

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