Introduction

48
How Herbicides Work
After applying a herbicide, fields can be scouted to determine the effectiveness of the treatment. The symptoms of different
herbicide groups, and the approximate time it takes to develop these symptoms, are listed in the table below. Weed
patches that are not affected should be noted and checked, as they may be herbicide resistant. Note that symptoms may
take longer to develop when conditions are not conducive to rapid plant growth.
The following table gives a brief description of symptoms that may be exhibited if plants are injured by a herbicide. The
symptoms of each group are addressed for both foliar and soil exposures.
Table 9. The Mode of Action, Site of Uptake and Symptoms of Different Herbicide Groups
Herbicide
Group
Mode of
Action
Site of
Uptake
Grass weeds
1 Systemic Foliar Reduced growth, yellowing of
growing point in 1 to 3 weeks. Newest
leaf of affected plant pulls out easily
in 3 to 5 days.
2 Systemic Foliar/Soil Newest leaves yellowed in 3 to 10
days, dead in 1 to 3 weeks.
3 Systemic Soil Reduced emergence, poor root
development of emerged plants.
Roots often swollen/stunted and root
tips darkened.
4 Systemic Foliar Tolerant to moderate rates. High rates
cause symptoms similar to drought.
Poor timing may cause kernal
abortion.
Weed symptoms/timing
Broadleaf weeds
Tolerant
Discolored (red/yellow/purple) at the
growing point and spreading to the whole
plant in 1 to 3 weeks.
Reduced emergence, poor root
development of emerged plants.
Abnormal growth (twisted stems, cupped
leaves) in 2 to 10 days.
5 Systemic Soil Wilted and yellowed oldest leaves beginning at leaf margins, death in 7 to 10 days.
Contact Foliar Yellowed oldest leaves, death within
days.
6 Contact Foliar Some leaf tip burn or white tissues
possible.
Bleached oldest leaves beginning at leaf
margins, death within days.
Yellowed leaves in 2 to 4 days, death in 1
to 2 weeks.
7 Systemic Soil Yellowed and stunted plants, death in 10 to 14 days.
Contact Foliar Interveinal yellowing of oldest leaves, death within days.
8 Contact Foliar Yellowed leaves in 3 to 7 days, stunted Tolerant
plants.
Systemic Soil Reduced emergence, emerged leaves dark green/blue.
9 Systemic Foliar Wilted, yellowed leaves in 7 to 10 days. Newest growth is impacted first followed
by the rest of the plant.
10 Contact Foliar Wilted, bleached leaves in 3 to 5 days, death in 1 to 2 weeks.
11 Systemic Foliar Plants wilted in 2 to 3 days, bleached
and purpling leaves in 1 to 2 weeks.
Plants wilted in 2 to 3 days, bleached
leaves in 1 to 2 weeks. Perennial plants die
slowly.
13 Systemic Soil Bleached leaves, susceptible seedlings die shortly after emergence.
14 Contact Foliar Some leaf burn at contact points or
leaf edges.
Leaves yellowed and desiccated in 1 to 3
days. (Post applications)
Systemic Soil Bleaching and yellowing, death prior to or shortly following emergence
15 Systemic Soil Reduced emergence, emerged plants stunted.
19 Systemic Foliar Twisting of older leaves, new leaves fail to expand, plant death in 2 to 4 weeks.