Pre-emergence, soil-active herbicides applied around the time of planting are an important part of a good weed management program. However, variability in spring weather leads to concerns about both weed control and crop injury. If you are concerned about the performance of your pre-emergence herbicides, be sure to scout fields to determine your post-emergence weed management program.
Most herbicides that are commonly used to control weeds before they emerge in corn and soybean fields fall into one of five herbicides groups:
- Seedling root growth inhibitors (Group 3)
- Seedling shoot growth inhibitors (Group 15)
- Pigment inhibitors (Group 27)
- Photosynthesis inhibitors (Group 5)
- Cell membrane disruptors (Group 14)
Example products for each group are listed in the discussion below and in Table 1. Herbicides vary in the way they are absorbed by plants and how they are affected by precipitation. Many pre-emergence herbicides require sufficient rainfall for activation. In this context, activation means movement of the herbicide into the soil where the herbicide interacts with germinating weeds. Information from each herbicide label regarding uptake and rainfall after application is also listed in Table1.
Group 3 herbicides, such as Prowl (pendimethalin) and Group 15 herbicides like Dual products (S-metolachlor), Harness products (acetachlor), Outlook (dimethenamid-P), and Zidua (pyroxasulfone) are absorbed by root and shoot material as plants begin germinating. This is why these products only control weeds before they emerge and also why they must be activated.
Group 27 herbicides are absorbed by seeds, roots, and shoots. Some of these herbicides, such as Balance Flexx (isoxaflutole) only control weeds before emergence. Other, like Callisto (mesotrione) have activity both before and after emergence. If pre-emergence activity is desired, rainfall is required for activation. When post-emergence activity is the goal, adequate soil moisture in necessary, because plant roots will take in herbicide with soil water. This is why Group 27 herbicides are sometimes said to be ‘reactivated’ by rainfall later in the growing season.
Herbicides in Group 5, like atrazine and metribuzin, and Group 14, like Spartan (sulfentrazone) and Valor (flumioxazin) are absorbed by plant roots as water is taken in by the plant. Rainfall for activation is less important for Group 5 herbicides, however soil moisture is necessary for activity. In the case of sulfentrazone and flumioxazin, rainfall is needed for activation, however excessive rainfall is associated with soybean injury.
Table 1. Summary of the 5 groups of pre-emergence herbicides for corn and soybean production.
|
Mode of Action |
Grp. |
Site of absorption (When soil applied) |
Herbicide |
Rainfall requirement |
|
|
Amount |
Timing |
||||
|
Seedling root growth inhibition |
3 |
Emerging shoots and roots |
Prowl H2O |
amount not listed |
before weed seedling germination |
|
Photosystem II inhibition |
5 |
Roots |
Atrazine |
none |
|
|
Metribuzin |
0.25 inch |
time not listed |
|||
|
Cell membrane disruption |
14 |
Roots |
Spartan |
0.25 to 1.0 inch |
7 to 10 days after application |
|
Valor |
at least 0.25 inch |
time not listed |
|||
|
Seedling shoot growth inhibition |
15 |
Emerging shoots and roots |
Dual II Magnum |
0.5 to 1 inch |
within 2 days after application |
|
Harness |
0.25 to 0.75 inch. |
within 7 days after application and prior to weed emergence |
|||
|
Outlook |
amount not listed |
before weed seedling emergence from the soil |
|||
|
Zidua |
at least 0.5 inch |
before weed germination and emergence |
|||
|
Pigment inhibition |
27 |
Seed, roots, shoots |
Balance Flexx |
amount not listed |
prior to weed emergence |
|
Callisto |
0.25 inch |
within 7-10 days after pre-emergence application |
|||
Crop injury can be a concern each of the products listed above. Any conditions that increase crop exposure to the herbicide can result in injury. For example, periods of cool weather, especially cool and wet weather, that slow crop growth and limit the plant’s ability to metabolize the herbicide to inactive forms (Figure 1). Planting in marginal conditions that result in poor closure of the seeding slot can is another example of a condition likely to result in crop injury.
Figure 1. This corn seedling emerged during cool, wet conditions and is showing symptoms of mesotrione injury. Photo by Sarah Lancaster, K-State Extension and Research.
Sarah Lancaster, Extension Weed Science Specialist
slancaster@ksu.edu