Attendees will gain a basic understanding of the processes influencing herbicide availability which is essential to diagnose performance issues in the field.
8. Retention of atrazine on
crop residue
Soil Residue
100
80
% of applied herbicide
60
40
20
0
0 0.2" 0.5" 1.0"
Cumulative rainfall
Ghadiri et al. 1984. Univ. Nebraska
9. Sorption Coefficient
KD = Amount on soil / Amount in water
H H H H
H H
H H H H
H H H
H
H H H H H H
H
H H H H H H
H H
H H H H H
H H H
H H H
H H
H H H
H H H
KD= 7 H2O
Soil Air Soil
KD= 24 2O
H Air
21 molecules bound / 3 free 24 molecules bound / 1 free
11. Herbicide rates by soil type
Metribuzin rate recommendations (lbs)
< 2% OM 2 to 4% OM > 4% OM
Coarse Do not use ½ 2/3
Medium ½ to 2/3 2/3 to 5/6 5/6 to 1
Fine 2/3 to 5/6 5/6 to 1 1 to 1 1/6
Corvus rate recommendations (fl oz)
Coarse soils, 2.0 % OM or less All other soils
3.33 5.6
12. Influence of soil pH on soil
relations of herbicides
• Triazines
– High pH: less adsorption, greater persistence
• Sulfonylureas
– High pH: greater persistence
• Imidazolinones, isoxaflutole
– Low pH: greater persistence
13. Influence of Rain on
PRE Herbicide Performance
• Leaching
– Pro: Move to depth where weed seeds
germinate
– Con: Can dilute herbicide and reach tile
lines/groundwater
• Amount of herbicide in solution
15. Movement of two herbicides
through soil1 (six inches rain)
Depth (in) Alachlor Metribuzin
Koc = 124 Koc = 60
0-3 95% 88%
3-6 5% 10%
6-12 0% 2%
1Sandy
Amount 0.6% OM
loam, remaining
30% 37%
20 DAA
Jones et al. 1990.
Weed Sci. 38:589-597.
16. Soil Moisture Affect on
Herbicide Availability
KD = Amount in soil / Amount in water
Example : KD = 7
H H H H
H H
H H H H H
H H H H H
H H H H
H H H H H H H
H H H
H H H H H H
H H H
H H H
H H H H H
H H H H
H H H H
H H
H H H
H H H
Soil H2O Soil H2O Air
Moist soil Dry soil
19. Enhanced herbicide
degradation
100
1970’s - Eradicane, Sutan+
2000’s - atrazine
80 Non-history soil: ½ life = 17 days
History soil: ½ life = 9 days
60
40 LD95%
20 History soil
0
0 1 2 3 4 5 6 7 8
Weeks after planting
20. Summary
• Only a small fraction of PRE herbicides
is absorbed by plants
• Only herbicide present in soil water
available for absorption by weeds
• Sorption characteristics of a herbicide
critical in determining performance
Notas do Editor
The majority of herbicide within the soil at any given moment is bound to soil colloids, only a small fraction is in solution where it is available to control weeds
The positive charge on triazines increase adsorption, so triazines are more active in high pH soils. Plus, binding to colloids increases degradation rates, thus persistence increases in alkaline soils
Too much rain can dilute herbicide within profile or cause it to reach crop seed (dicamba)
As soils dry, more herbicide moves onto colloids, therefore reducing the performance of herbicides. In this illustration, the moist soil has 3X the ‘killing power’ as the dry soil