Transcript Environmental Factors That Influence Herbicide Performance

Environmental Factors That
Influence Herbicide Performance
Dr. Eric P. Prostko
Extension Weed Specialist
University of Georgia
Tifton
For a herbicide to work it must…..
 come in contact
with a plant surface
(root, shoot, leaves)
 remain at site long
enough to penetrate
or be absorbed
 move to its site of
action
Fate of Herbicides in the
Environment
Specific Environmental Factors
 soil
* organic matter, texture, CEC, pH
 climatic
* temperature, moisture, humidity, light
intensity, dew(?)
Soil Factors
Organic Matter and Texture
 most important for soil
applied herbicides
 Indirectly influences
all processes that affect
herbicides!!
 the greater the organic
matter and clay content,
the greater adsorption
of herbicides
Rate (kg/ha)
Amount of atrazine required to
reduce the growth of giant foxtail by
50% at varying OM levels.
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0.8
1.9
3.9
6.4
OM (%)
Parochetti 1973
11
18
OM and texture are used to determine
application rates of soil-applied
herbicides.
Soil Factors
Cation Exchange Capacity (CEC)
 soils ability to
adsorb positively
charged compounds
 fine-textured,
high-organic matter
soils have larger
CEC’s than coarse,
low-organic matter
soils
paraquat
Cation Exchange Capacities of
OM and Clay
Bailey and White 1964
Soil Factors
Cation Exchange Capacity
 influences rate of
application
 not found on many
herbicide labels
Soil Factors
pH
 influences water solubility, adsorption,
and hydrolysis
 triazines
 sulfonylureas
 imidazolinones
Effect of pH on Adsorption of
Atrazine
14
Kd Value
12
10
8
6
4
2
0
3.9
4.7
5.3
pH
McGlamery and Slife, 1966
6
8
The Influence of pH on the
Solubility of Accent
Soil pH may influence rotational
restrictions because of its effects on
adsorption, solubility, and degradation.
Climatic Effects
Temperature
 In showy crotalaria, Blazer absorption
was 4X greater at 810 and 950 than at
640. (Wills and McWhorter, 1981)
 In johnsongrass, Roundup absorption
doubled as temperature was increased
from 750 to 950.(McWhorter et al. 1980)
Climatic Effects
Moisture - PPI/PRE
 activation
* movement
* 0.5” within 7-10 d
 adsorption
* availability
Effect of Rainfall Amount After
Application on Herbicide Performance
Foxtail Control (%)
0
0.1"
0.25"
0.5"
1"
100
80
60
40
20
0
Dual
Frontier
Herbicide
Simmons et al. 1997
Climatic Effects
Moisture - POST
Dry weather causes…..
 plants to develop thicker cuticles
 reductions in absorption, retention,
and translocation
 altered metabolism
Climatic Effects
Relative Humidity
 Liberty treatment at 95% RH resulted
in complete plant death in contrast to
only a 30% inhibition in growth at
40% RH (Anderson et al. 1993)
Climatic Effects
Relative Humidity
 A higher relative humidity level …..
* extends drying period of herbicide
droplets.
* hydrates plant cuticles.
Climatic Effects
Light Intensity
 influences photosynthesis, cuticle
development, stomatal openings, and
photodecomposition
Climatic Effects
What about dew?
 studies have shown that dew can
either increase or decrease foliar
herbicide efficacy (Caseley 1989)
 decrease - runoff and dilution
 increase - cuticle hydration and
uptake
Effect of Volume and Dew on
Roundup Efficacy on Oats
Dew Level
0
50%
100%
Control - %
100
80
60
40
20
0
16
32
Spray Volume (GPA)
Kogan and Zuniga, 2001
48
Summary
 Environmental factors cannot be
controlled (except irrigation).
 Need to understand environmental
effects and make applications when
conditions are favorable for optimum
performance if possible.
University of
Georgia
Extension
Weed Science
(gaweed.com)