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Herbicides, Weeds and Rain
Herbicides and weeds react to rain in several ways. The rains have activated
germination of many annual weeds, which is important for root absorption
of herbicides. However, rain can affect herbicide performance in both
good ways and bad.
Soil-Applied Herbicides:
Generally soil-applied herbicides like Devrinol, Frontier, Dual II Magnum,
Sencor, Lorox and Princep give better weed control when at least 1/2"
of rain falls shortly after application. This moisture dissolves the herbicide
in soil water solution so that developing weed seedlings can absorb the
herbicides as they take up water. For herbicides sensitive to photodegradation,
a good shower will give shallow incorporation.
However, soil-applied herbicides can be leached below the weed-seed germination
zone by excessive rains, especially if heavy rains fall shortly after
application and before herbicides get bound to soil particles.
The effect of leaching depends on the characteristic of the herbicide:
- The layer of herbicide may be washed just below the soil surface,
allowing weed seeds to germinate on the surface.
- The layer of may be diluted from a narrow band (eg. 2 cm at the soil)
to a wide band (eg. 6 cm), which effectively dilutes the concentration
where the weeds are germinating
- The herbicide may be leached below the cropping zone on sandy soils,
allowing both shallow and deep seeds to germinate.
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If the end result of heavy rain is a low or nil concentration of herbicide
on the soil surface or in the incorporation layer, the level of weed control
will be reduced.
This information on soil-applied herbicides for horticultural crops is
from the Herbicide Handbook, Weed Science Society of America:
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Herbicide
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Soil Mobility/Leaching
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Main dissipation route
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Slightly leachable
(Rain required for incorporation)
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Highly photodegradable;
Slowly degraded by soil microbes
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Moderately adsorbed to the soil;
Less leaching potential on higher OM soils; moves < 4" deep
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Microbial breakdown
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Moderately adsorbed to the soil;
Less leaching potential on higher OM soils; moves < 4" deep
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Microbial breakdown
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Leaching may be a concern, especially sandier soils and heavy rainfall.
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Microbial breakdown
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Limited leaching potential
Strongly adsorbed to clay particles
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Microbial breakdown in high pH soils; hydrolysis in low pH soils
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Readily leached in sandy soil; intermediate leaching potential
on medium textured soils; immobile on clay soils, high OM or low
pH; may move with soil run-off
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Microbial breakdown
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Weakly adsorbed to soil particles;
Moderate to high leaching potential
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Microbial breakdown
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Low to negligible leaching potential due to strong adsorption to
the soil.
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Degraded by light: also microbial breakdown (more rapid in flooded
anaerobic conditions)
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Where soil erosion by water is a problem, any herbicides bound to soil
particles will also be moved. Be cautious of higher herbicide residues
where ponding and settling has occurred. Another reason to implement erosion
control measures!
Note: If herbicides have moved down in the soil profile,
crop injury may increase. Repeat applications are not recommended even
if weed escapes appear.
Postemergent Herbicides:
Spring rains have pushed a strong germination of many annual weeds, and
active growth in many perennial weeds. These are good conditions for effective
control with postemergent herbicides.
Rain is a concern for postemergent herbicides if it falls too soon after
application.
Table 4-6 in Publication 75, Guide to Weed Control outlines the time
intervals required after application for absorption of the herbicide into
the plant. This table is also available online.
Table 4-6: Interval Before Rainfall (Postemergence)
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Time Interval
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Herbicides Affected
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0 to 15 minutes
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Dimension, Gramoxone, Reglone Desiccant, Reward
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1 Hour
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Achieve, Assure II, Badge, Buctril M, Caliber 400, Cobutox 600,
Embutox 625, Excel Super, Hoe-Grass, Koril, Mextrol, Pardner, Poast
Ultra, Select, Weedone CB
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2 Hours
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2,4-D ester, Accent, Arsenal, atrazine, Classic, Estaprop, Firstrate,
Garlon 4, MCPA ester, Option 35DF, Prism, Pursuit, Shotgun, Tordon
101, Ultim, Venture L
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3 Hours
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Acclaim Super, Goal 2XL, Sword, Target
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4 Hours
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2,4-D amine, Accent 1-Pass1, Accent Total1,
Banvel II, Battalion1, Clovitox Plus, Distinct, Dycleer,
Dyvel, Ignite, Liberty 200 SN, Liberty Prime1, Lontrel,
Marksman, MCPA amine, Meridian Plus1, Muster, Peakplus1,
Pinnacle, Refine Extra, Reflex, Roundup Transorb, Savage, Summit,
Tordon 101, Transline, Topside, Touchdown iQ, Tropotox Plus, Upbeet,
Vanquish, Vantage Plus, Viper1
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6 Hours
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Amitrol 240, Basagran, Basagran Forte, Betamix, Betanex, Blazer,
Cleansweep1, Credit, Factor, Glyfos, Laddok, MCPA sodium,
Pyramin FL, Roundup, Sencor, Vantage
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8 Hours
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Linuron 480, Lorox
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24 Hours
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Mecoturf plus 2,4-D
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1 - Indicates herbicides sold as a co-pack under this trade
name.
Learn more:
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For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
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