No. 3 Article 5/April 23, 2010

Improving Performance of Soil-Residual Herbicides

Soil-applied herbicides are an integral part of integrated weed management programs. These herbicides can help foster weed-free conditions during crop establishment and potentially reduce the intensity of selection for herbicide-resistant weed biotypes. Once applied to the majority of corn and soybean acres in Illinois, their use rapidly declined following the commercialization of various herbicide-resistant crop technologies. However, the challenges and consequences that have followed the widespread adoption of total-postemergence weed control programs have prompted many farmers to once again use soil-applied herbicides.

While soil-residual herbicides can provide significant benefits, achieving maximal effectiveness is influenced by many factors, only some of which are under the user's control. For example, the farmer chooses the product, the application rate and timing, and whether the product is mechanically incorporated. But precipitation timing and amounts, soil texture and organic matter content, and weed spectrum and emergence characteristics are largely beyond the farmer's control.

Select a product that provides the greatest efficacy against the weed or weed spectrum of most concern in any particular field. Many current soil-applied products are premixes of one or more herbicide active ingredients, and many can control or suppress grass and broadleaf weed species. However, components and ratios of active ingredients in premixes can vary, so be sure to select the product that contains an active ingredient effective against your most problematic species.

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 prior to the application of a postemergence herbicide. Higher application rates generally can control weeds longer into the growing season, but it's becoming increasingly difficult to achieve satisfactory weed control with a single soil-applied product when the Illinois weed spectrum includes species with prolonged periods of emergence, including waterhemp and giant ragweed.

Early preplant (EPP), preplant incorporated (PPI), and preemergence (PRE) surface are the most common application timings of soil-applied herbicides. Early preplant applications, often made several weeks before planting, largely have been replaced by applications made within several days of planting. PPI applications were once very common but have declined in recent years with the adoption of conservation tillage.

For a soil-applied herbicide to be effective, the herbicide 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. If no precipitation is received between application and planting, mechanical incorporation, where feasible, can still help move the herbicide. Herbicide that remains on the soil surface following application may not provide as much weed control and is subject to dissipation.

In some areas of Illinois, preplant and preemergence herbicides have been on the ground anywhere from a few days to a couple of weeks without having received adequate precipitation to move them into the soil solution. Effectiveness can be significantly reduced when a soil-applied herbicide is sprayed on a dry soil surface with no incorporation (mechanical or by precipitation) for several days after application. How much rainfall is required to move the herbicide into the soil and how soon after application precipitation is needed are difficult to define and can vary by herbicide, but surface-applied herbicides generally require 0.5 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 and how soon after application precipitation is needed.

If weeds have begun to emerge before the herbicide has been moved into the soil solution, it may be time to consider additional management options. Certain soil-applied herbicides may still provide some control of emerged weeds if precipitation occurs soon, but if emerged weeds exceed 1 to 2 inches tall a postemergence herbicide application may be necessary to control them. Don't wait too long to see if the soil-applied herbicide will control emerged weeds, especially if dry soil conditions persist, as the weeds soon become competitive with the corn.

Rotary hoeing can control emerging weeds and give surface-applied herbicides some incorporation (though usually only minor). Rotary hoeing is most effective while weeds are still in the "white stage," following seed germination but prior to emergence. Once weed seedlings have emerged, the effectiveness of rotary hoeing is diminished since the weed's rapidly developing root system helps anchor the plant. Hoeing is generally most effective when done at speeds of 8 to 12 miles per hour. A second rotary hoeing 7 to 10 days after the first might improve weed control. Hoeing may also aid crop emergence by breaking soil crusts that can develop after planting.--Aaron Hager

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