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Department of Agronomy

Kansas State University

1712 Claflin Rd.

2004 Throckmorton PSC

Manhatan, KS 66506

785-532-6101

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Extension Agronomy

Considerations for pre-emergence herbicides


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.
No.

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