No. 2 Article 3/April 8, 2011

Revisiting the Realm of Residuals

Years ago it was very common for most corn and soybean acres in Illinois to be treated with one or more soil-residual herbicides before crop and weed emergence. During the 1980s, commercialization of broad-spectrum, postemergence herbicides began the shift away from widespread use of soil-residual herbicides; products such as Basagran, Classic, Accent, and Pursuit contributed to the early adoption of postemergence weed control programs. The era of total postemergence weed control reached its zenith following the widespread adoption of glyphosate-resistant crops and the concomitant use of glyphosate. However, "recent" changes in weed spectrums and an increasing frequency of weed populations resistant to glyphosate have heralded a shift back to soil-residual herbicides, especially in soybean.

Soil-residual herbicides can provide many weed management benefits, but several factors influence their effectiveness. Product selection, application rate, and timing of application in relation to crop planting are largely under the control of the farmer, whereas soil moisture at application and the interval between application and the first precipitation event are largely not. A few considerations and suggestions for improving the effectiveness of soil-residual herbicides are provided here.

Many different soil-residual herbicides are available; some contain only one active ingredient, while others are premixes of two or more active ingredients. Be sure to select a product that offers the best solution for the problem species in each field. For example, many products containing imazethapyr (Pursuit) can provide excellent control of eastern black nightshade, but not all would provide equally effective control of waterhemp. Much of the Illinois waterhemp population is resistant to ALS-inhibiting herbicides, so if you intend to use a soil-residual premix herbicide that contains an ALS inhibitor and waterhemp is a target weed species, be sure the other premix component(s) have good activity on waterhemp.

Also pay careful attention not only to what products are contained in a premix, but how much of each active ingredient will be applied at the expected rate. For example, sulfentrazone is an active ingredient in several soybean herbicide premixes (Authority XL, Authority MTZ, Authority First, Authority Assist), but the amount of sulfentrazone applied can vary by product. In calculating the amount of sulfentrazone applied at the maximum labeled rate of Authority XL (9.6 ounces) and Authority MTZ (20 ounces), we find it to be 0.37 lb in Authority XL and 0.23 lb in Authority MTZ. If your target weed species is waterhemp and you want to select the product that provides the highest amount of sulfentrazone, Authority XL (in this example) would be your choice.

Application timings of soil-residual herbicides can range from several weeks before planting to after crop emergence. Early preplant (EPP), preplant incorporated (PPI), and preemergence (PRE) surface are the most common application timings of soil-applied herbicides. Early preplant applications largely have been replaced by applications made within several days of planting. Preplant incorporated applications were once very common, but they have declined in recent years with the adoption of conservation tillage systems. The duration of weed control provided by a soil-residual herbicide is influenced by when it was applied to the soil. Generally, herbicides applied close to crop planting (within 14 days) control weeds longer into the growing season than those applied several weeks before planting.

Some residual herbicides that are most commonly applied to the soil also can be applied after the crop has emerged. Applying these products after emergence may extend residual weed control for a few additional weeks. This practice has historically been more common in corn, but it appears to be gaining traction in soybean. In addition to residual weed control, some of these products can control emerged weeds, while others have no foliar activity.

Application rates, historically selected according to label recommendations based on soil texture and organic matter content, nowadays are often (much) reduced. A phrase coined to describe these reduced rates indicates that the goal of "set-up rates" is to provide short-term weed control/suppression before the application of a postemergence herbicide. The presence of waterhemp populations resistant to one or more commonly used postemergence herbicides suggests longer control from a soil-residual herbicide will be needed. Higher application rates generally provide a higher level of weed control longer into the growing season. However, one should not assume that a higher application rate will provide season-long weed control. This level of control is often difficult to achieve with a single soil-applied product, especially when the weed spectrum includes species with prolonged emergence.

For a soil-applied herbicide to be effective, it needs to be available for uptake by the weed seedling (usually before the seedling emerges, but some soil-applied herbicides can control small emerged weeds under certain conditions). Soil-applied herbicides all have the same Achilles heel: when applied to the soil surface, they require either mechanical incorporation or precipitation to move them into the soil solution. Herbicide effectiveness can be significantly reduced when a soil-applied herbicide is sprayed onto a dry soil surface with no incorporation (mechanical or by precipitation) for several days after application.

How much rainfall is required to move herbicide into the soil and how soon precipitation is needed are difficult to define and can vary by product, but surface-applied herbicides generally require 1/2 to 1 inch of precipitation within 7 to 10 days after application for optimal incorporation. Factors such as soil condition, soil moisture content, residue cover, and the chemical properties of the herbicide influence how much precipitation is needed and how soon. If no precipitation is received between application and planting, mechanical incorporation, where feasible, can still help move the herbicide into the soil.

Lastly, crop injury from soil-applied herbicides can be enhanced under certain conditions. Corn hybrids and soybean varieties can vary in their inherent sensitivity to certain herbicides or herbicide families. Herbicide labels often suggest consulting with the seed company to determine if a hybrid or variety is overly sensitive to a particular herbicide. The environment and herbicide application timing have a large influence on crop injury from soil-applied herbicides. Adequate soil moisture levels and low relative humidity can enhance uptake of soil-applied herbicides. Applications made immediately before or after crop planting result in a high concentration of herbicide near the emerging crop plants. Rapid herbicide absorption into the young crop plants may temporarily overwhelm the plant's ability to break down the herbicide, leading to injury symptoms.

Apart from enhancing herbicide uptake, environment-induced crop stress can enhance injury from herbicides. Cool air temperatures and wet soil conditions are good examples of conditions that can induce stress. Why is a crop under stress more likely to be injured from a selective herbicide? In most cases, herbicide selectivity arises from the crop's ability to metabolize (break down) the herbicide to a nonphytotoxic form before it causes much injury. When the crop is growing under favorable conditions, it rapidly metabolizes the herbicide before excessive injury occurs. If, however, the crop is under stress (which can be caused by a variety of factors), its ability to metabolize the herbicide may be slowed enough that injury symptoms develop.--Aaron Hager

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