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Field Patterns Can Signal Herbicide Carryover
Problems
5/20/2003
Ron
F. Meyer
Area Extension Agent (Agronomy)
Colorado State University
Extension
Golden Plains Area |
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Authored With:
Alan Helm, Area Extension Agent (Weed Science)
Following last year’s dry summer there is some concern about
herbicide carryover damage to crops planted this year. Carryover damage occurs
when herbicide residues persist in the soil at levels that injure a current
crop. Understanding the potential causes of damaging herbicide residues and the
patterns they create in the field can be useful in scouting/diagnosing a field
problem. These patterns may be caused by variations in the environment or in
the original herbicide application.
Normally herbicide carryover damage
is not uniform over a field but rather occurs in a pattern. Sometimes this
pattern appears to be random because we aren’t familiar with the original
cause. One question to answer is why do we have too much herbicide residue in
certain areas and not others? The explanation lies in three broad categories:
1) The original application rate was not uniform; 2) there is increased
herbicide residue remaining in certain areas because of variation in the
herbicide degradation rate across the field, and 3) the activity of the
remaining herbicide residue is greater in certain areas than
others.
Application Rate
Variations in herbicide application
rate can sometimes be observed in a pattern that matches the swath width of the
application equipment. Spray pattern overlap results in carryover strips
reoccurring at intervals that match the sprayer swath. Herbicide carryover at
the ends of fields can result from overlap when the field ends are sprayed
twice, once in each direction. Changes in travel speed with a sprayer not
equipped with a rate controller will result in changes in herbicide application
rate. Over-application can occur at the ends of a field if a sprayer not
equipped with a rate controller slows, resulting in an increased application
rate. Other speed variations such as slowing for obstacles, uphill versus
downhill speed, etc., also will affect the herbicide application rate. Worn
nozzles and an uncleaned sprayer tank can also cause herbicide carryover
problems. Replace worn nozzles and clean sprayer tanks adequately between
jobs.
Herbicide Degradation
Variation in herbicide degradation
rate across the field results from variation in the soil environment. Soil pH
also can influence the degradation rate of certain herbicides. Both soil pH and
organic matter levels can vary within a field and cause carryover patterns
related to these variances.
Herbicide Activity
The
biological activity of most soil active herbicides is affected by soil
properties including organic matter content, clay, and pH. Often more than one
of these factors varies across a field. Soil organic matter, and to a lesser
extent clay, absorb herbicides making them unavailable for plant uptake. More
herbicide is required for equal biological activity in field areas high in
organic matter and clay. This means carryover damage likely will be reduced in
areas of the field high in organic matter and clay or correspondingly,
carryover will be greater in low organic matter high pH areas. Soil pH can
influence the bioavailability of some soil-applied herbicides. With the same
amount of herbicide present, crop injury could vary based on soil
pH.
Summary
The crop response pattern across a field is
useful in diagnosing herbicide carryover problems. The distribution of crop
injury may be due to variations in the environment, variations in the original
herbicide application, or both, and understanding the cause can help avoid crop
injury in future crop years.
Source: Alex Martin, Extension Weeds
Specialist University of Nebraska |
Page Created and Maintained by: Perry D. Brewer, Area
Extension Agent (Technology Education/Youth)
5/21/2003 |
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Washington and Yuma counties cooperating. Extension programs are
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