Agronomy Library > Soil Conservation

How to Maximize Glyphosate Performance
Author: Dr. Mirza N. Baig, Alberta Reduced Tillage Linkages
Date Created: June 07, 2000
Last Reviewed: April 13, 2007

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.

Effect on Weeds
Effect on Glyphosate Performance
Maximizing the Performance
· Growth and vigor of plants
· Stomatal opening and hence transpiration rate
· Photosynthesis
Generally high light intensities lead to better glyphosate absorption and translocation
Avoid night-time applications
· Growth and vigour of plants
· Transpiration
· Cuticle properties
Hot conditions- reduced glyphosate absorption and translocation
Slower glyphosate uptake and translocation – symptoms may be slow to appear
in hot and dry conditions--apply early in the morning
Relative Humidity
· Transpiration
· Cuticle hydration
High humidity gives slower drying of spray droplets and greater absorption and translocation of glyphosate
If possible, glyphosate applications should be made under high humidity conditions,e.g. early in the morning.
· Spray retention
Rainfall too soon after application, washes glyphosate off the plant
Roundup Transorb claims to have   better rainfastness
Do not apply if rainfall is forecast for the time of application
· 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
Light frost – increased translocation of glyphosate
Killing frost – little or no translocation
Avoid application after killing frost
Dew or Fog
· Cuticle hydration
Light dew or fog has no effect on glyphosate performance--heavy dew can washes the chemical off the plant
If there is a heavy dew on the plants, wait until mid morning before applying glyphosate
· Windy conditions allow glyphosate drift to non-target areas
Reduced injury to weeds and crop injury to non-target crops
Avoid high winds during spraying operations--use shielded nozzles under light or medium windy situations
· Enhances spray droplet spread on to the treated leaf.
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
Use surfactant if recommended on the label, use caution if it is not recommended and you are using below label rates
Spray Volume
Spray coverage of the leaf surface
Lower water volume (5 GPA) generally improves glyphosate performance--it produces smaller droplets containing a higher concentration of glyphosate
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.
Spray Water
In dirty water most of the glyphosate binds tightly to soil particles, reducing performance
Always use clean water free from soil sediments or organic matter
Water Hardness
Dissolved minerals have the potential to reduce the performance of glyphosate
Adding ammonium sulfate to the spray solution can improve glyphosate effectiveness--use 4-8 kg AMS per 375 liters of water
Growth Stage of Weeds
At application time, weeds should be actively growing and have new, healthy and fully expanded leaves
Actively growing weeds enhance glyphosate absorption and translocation
Check the label for correct weed/crop staging before glyphosate application