Issue No. 6, Article 4/May 4, 2007
Effectiveness of Soil-Applied Herbicides
Soil-applied herbicides remain an important part of weed management 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. EPP applications are typically made several weeks prior to planting and are more common in corn fields than soybean fields. PPI applications were once very common, but they have declined in recent years with the adoption of conservation tillage systems. PRE applications are generally made within 1 week of crop planting. Regardless of when or how a herbicide is applied to the soil, the effectiveness of soil-applied herbicides is influenced by several factors.
In order 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). Processes such as herbicide adsorption to soil colloids or organic matter can reduce the amount of herbicide available for weed absorption. Soil-applied 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 effect. Generally, this happens before the seedling emerges from the soil. For a herbicide to be absorbed by weed seedlings, the herbicide must be in the soil solution or vapor phase (i.e., an available form).
How is this achieved? The most common methods for herbicides to become dissolved into the soil solution are by mechanical incorporation or precipitation. EPP 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. Herbicide that remains on a dry soil surface following application may not provide much effective weed control and is subject to various dissipation processes, some of which are described in subsequent paragraphs.
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 precipitation. Which method is more consistent? Precipitation 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 it should be received for optimal herbicide performance depends on many factors, but generally ½ 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 remaining on the soil surface. Volatility potential is determined by several soil properties and properties of the herbicide formulation, while photolysis is dependent primarily on herbicide properties.
Dry soil conditions are conducive for planting but may also reduce the effectiveness of soil-applied herbicides. If herbicide applications are made prior to planting and no precipitation is received between application and planting, a shallow mechanical incorporation before planting may help preserve much of the herbicide's effectiveness.--Aaron Hager