The decreased use of soil residual herbicides in soybean, coupled with several recent "mild" winters, has caused 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, existing vegetation to be dealt with prior to planting is often more dense and composed of species not familiar to everyone. When air and soil temperatures begin 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 overwinter, but sometimes (depending on weather and soil moisture conditions) these species 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, will soon make 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 attempt to control existing vegetation prior to planting no-till corn or soybeans.
2,4-D is frequently used in a burn-down tank mix prior to corn or soybean planting. The ester formulation is usually preferred over the amine formulation since its waiting interval between application and planting is generally less. The labels of many 2,4-D ester formulations (3.8 pounds of 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. Certain labels also recommend that applications should be made 7 to 14 days before corn planting or 3 to 5 days after planting before the corn has emerged. Factors that increase the likelihood of the 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 soybeans 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 soybeans, products such as metribuzin (Sencor in Canopy, Boundary, Axiom, and Domain), Canopy XL, Authority, Valor, Extreme, and Backdraft SL all have foliar activity and can be applied prior to planting.
Keep in mind that most of these herbicides work best on small annual weeds and 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 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 often more slow acting than contact herbicides under these conditions. For more information on the effect of temperature on burndown herbicide activity, see the accompanying article.
Table 2 is reproduced from the 2003 Illinois Agricultural Pest Management Handbook. The table includes weed control ratings for several corn and soybean herbicides used to burndown existing vegetation prior to planting.--Aaron Hager and Christy Sprague