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Issue No. 4, Article 5/April 29, 2011

Delays in Weed Control Before Planting

Wet field conditions across much of Illinois have, in addition to delaying corn planting, provided for substantial growth of various weed species. The "fields of purple" described in issue 3 of the Bulletin are now supplemented by color from white- and yellow-flowered species. When conditions become conducive for field operations, controlling these large weeds could be challenging. The suggestions here might help improve the performance of preplant weed control tactics.

1. Preplant tillage operations can effectively control existing vegetation while preparing a seedbed. However, as weeds become larger, tillage to control them before planting can become less effective. Dense stands of certain winter annual weeds, such as common chickweed, can "ball up" in a field cultivator. Stems of larger common lambsquarters plants bent over but not completely severed from the roots during tillage may spring back upright in a C-shaped or S-shaped configuration. While the winter annual weeds not completely controlled by preplant tillage will eventually complete their life cycles, summer annual weeds that survive preplant tillage are often much more difficult to control with herbicides applied after crop emergence. Weed control may also be reduced when fields are a bit wet during preplant tillage. Soil disturbance may not be as extensive when soils are retaining moisture, and clods are more likely to be formed. Weeds are also more likely to take root again after tillage when soil disturbance is inadequate and soil moisture is abundant.

2. As winter annual weed species approach maturity, they can become increasingly difficult to control with herbicides. Be sure to adjust the rate of burndown herbicides upward to account for the large and dense vegetation. Glyphosate application rates (alone or tank-mixed with other herbicides) should be in the range of 1 to 1.5 lb acid equivalent per acre to control the large vegetation. Include AMS at 8.5 to 17 lb per 100 gal 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. Be cautious about including "replacement" additives or blends that do not provide sufficient AMS. While most glyphosate products are formulated with a surfactant, some require the addition of NIS. Be sure to check the label for additive recommendations or requirements.

3. 2,4-D is frequently used in burndown tank-mixes applied prior to corn or soybean planting. Both the amine and ester formulations are labeled for preplant applications, but the ester formulation is usually preferred. The low water solubility of an ester reduces the potential for it to be moved by precipitation into the soil, where it could cause severe injury to seedlings. Also, esters' ability 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 to 2 pints per acre 7 to 14 days prior to planting corn; Table 1 lists the intervals between application and planting commonly found on many growth regulator herbicide labels. 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. 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. 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 nonselective herbicide, if possible. Be cautious about which herbicide alternative you include with glyphosate. Herbicides that are more contact in activity 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 tank-mixed with them.

Table 1. Application rates and preplant intervals for growth regulator herbicides used to control existing vegetation prior to planting.

Herbicide

Corn

Soybean

Clarity

No waiting interval
Apply 8-16 fl oz/A
Use 8 fl oz/A on coarse soils or soils with less than 2.5% organic matter

8 fl oz/A or less: 1 in. precipitation followed by 14 days
Up to 16 fl oz/A: 1 in. precipitation followed by 28 days

2,4-D estera

Apply 1-2 pt/Ab 7-14 days before planting

Apply 1 pt/A not less than 7 days before planting or 2 pt/A not less than 28 days before planting

2,4-D amine

Apply 1-2 pt/Ab 7-14 days before planting

Apply 1 pt/A not less than 7 days before planting or 2 pt/A not less than 28 days before planting

aPreferred formulation for burndown applications.
bBased on formulations containing 3.8 lb ae/gal.

4. Glyphosate-resistant populations of horseweed (also known as marestail) and waterhemp occur in many areas of Illinois. Both species 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 post-emergence herbicide options exist. Tank-mix partners with glyphosate or alternative herbicides will be needed to control glyphosate-resistant weeds prior to crop planting. More tank-mix partners or alternative herbicide options are possible before planting than after. Products with the active ingredient saflufenacil (Sharpen, OpTill, Verdict) have demonstrated good burndown control of horseweed. Saflufenacil-containing products should be tank-mixed with another broad-spectrum herbicide, such as glyphosate, and MSO should be included.--Aaron Hager

Author:
Aaron Hager

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