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Effectiveness of Soil-Applied Herbicides

April 13, 2001
Soil-applied herbicides still remain an important part of weed-control programs in corn- and soybean-production systems. Early preplant (EPP), preplant incorporated (PPI), and preemergence (PRE) surface are the most common types of herbicide applications to soil. Regardless of when or how a herbicide is applied to the soil, the effectiveness of soil-applied herbicides is influenced by several factors.

For a soil-applied herbicide to be effective, the herbicide needs to be available for uptake by the germinating weed seedling. The soil-applied herbicide must be absorbed into the germinating weed seedling to provide adequate control. Herbicides do not prevent weed-seed germination; rather, they are first absorbed by the root or shoot of the seedling and then exert their phytotoxic action. This generally happens before the seedling emerges from the soil. For a herbicide to be absorbed by the germinating seedlings, the herbicide must be in the soil solution or vapor phase. How is this achieved? The most common methods for herbicides to become dissolved into the soil solution are by mechanical incorporation or precipitation. Many early preplant applications in no-till systems attempt to increase the likelihood that sufficient precipitation will be received before planting to incorporate the herbicide. If, however, no precipitation is received between application and planting, mechanical incorporation, where feasible, will in most instances adequately move the herbicide into the soil solution.

Many weed species, in particular small-seeded species, germinate from fairly shallow depths in the soil. The top 1 to 2 inches of soil is the primary zone of weed-seed germination and should thus be the target area for herbicide placement. Shallow incorporation can be achieved by mechanical methods or by precipitation. Which of these two methods is more consistent?

Rainfall provides for a fairly uniform incorporation, but mechanical incorporation reduces the absolute dependence on receiving timely precipitation. How much precipitation is needed and how soon after application the precipitation should be received for optimal herbicide performance depends on many factors, but generally 1/2 to 1 inch of precipitation within 7 to 10 days after application is sufficient.

Herbicides remaining on the soil surface or those placed too deeply in the soil may not be intercepted by the emerging weed seedlings. Herbicides on the soil surface are subjected to several processes that reduce their availability. Volatility (the change from a liquid to gaseous state) and photolysis (degradation due to absorption of sunlight) are two common processes that can reduce the availability of herbicides that remain on the soil surface.

Dry soil conditions may be conducive for planting but may also reduce the effectiveness of soil-applied herbicides. If applications are made prior to planting and no precipitation is received between application and planting, a shallow mechanical incorporation may help preserve much of the herbicide's effectiveness.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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