Successful weed control depends on interactions between weeds (growth habit/size), environmental factors (light, temperature, moisture, wind) and characteristics of the spray solution. A good understanding of the dynamic and complex factors involved in these interactions allows for the maximization of glyphosate performance. The environment can influence the physical development of plants:
- the size, shape, and thickness of leaves
- cuticle and wax deposition
- changes in the water and nutrient status within plants
These changes affect glyphosate treatments in several ways:
- interception
- retention
- penetration
- translocation to the site of action.
Also, prevailing environmental conditions before, at and after application affect glyphosate performance.
- good growing conditions are ideal for glyphosate retention and absorption
- rain and high wind at time of application should be avoided.
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Factors
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Effect on Weeds
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Effect on Glyphosate Performance
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Maximizing the Performance
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Light
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· Growth and vigor of plants
· Stomatal opening and hence transpiration rate
· Photosynthesis
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Generally high light intensities lead to better glyphosate absorption and translocation
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Avoid night-time applications
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Temperature
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· Growth and vigour of plants
· Transpiration
· Cuticle properties
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Hot conditions- reduced glyphosate absorption and translocation
Slower glyphosate uptake and translocation – symptoms may be slow to appear
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in hot and dry conditions--apply early in the morning
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Relative Humidity
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· Transpiration
· Cuticle hydration
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High humidity gives slower drying of spray droplets and greater absorption and translocation of glyphosate
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If possible, glyphosate applications should be made under high humidity conditions,e.g. early in the morning.
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Rainfall
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· Spray retention
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Rainfall too soon after application, washes glyphosate off the plant
Roundup Transorb claims to have better rainfastness
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Do not apply if rainfall is forecast for the time of application
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Frost
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· Light to mild frost - weed species vary in their response to frost
· Killing frost -- ice forms inside the plant cell and in the plant plumbing system
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Light frost – increased translocation of glyphosate
Killing frost – little or no translocation
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Avoid application after killing frost
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Dew or Fog
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· Cuticle hydration
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Light dew or fog has no effect on glyphosate performance--heavy dew can washes the chemical off the plant
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If there is a heavy dew on the plants, wait until mid morning before applying glyphosate
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Wind
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· Windy conditions allow glyphosate drift to non-target areas
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Reduced injury to weeds and crop injury to non-target crops
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Avoid high winds during spraying operations--use shielded nozzles under light or medium windy situations
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Surfactant
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· Enhances spray droplet spread on to the treated leaf.
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Surfactant enhances glyphosate penetration and absorption.
Different glyphosate formulations have different types and amount of surfactant
Some formulations (e.g. Roundup Transorb) claim to deliver 50% more glyphosate to the root system
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Use surfactant if recommended on the label, use caution if it is not recommended and you are using below label rates
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Spray Volume
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Spray coverage of the leaf surface
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Lower water volume (5 GPA) generally improves glyphosate performance--it produces smaller droplets containing a higher concentration of glyphosate
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Always use recommended label water volume. Within a range, use lower end of the range. Use higher water volume if coverage or drift is a concern.
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Spray Water
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N/A
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In dirty water most of the glyphosate binds tightly to soil particles, reducing performance
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Always use clean water free from soil sediments or organic matter
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Water Hardness
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N/A
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Dissolved minerals have the potential to reduce the performance of glyphosate
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Adding ammonium sulfate to the spray solution can improve glyphosate effectiveness--use 4-8 kg AMS per 375 liters of water
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Growth Stage of Weeds
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At application time, weeds should be actively growing and have new, healthy and fully expanded leaves
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Actively growing weeds enhance glyphosate absorption and translocation
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Check the label for correct weed/crop staging before glyphosate application
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