If you got out ahead of the seeder this spring with a herbicide application, you’re probably feeling like you’re on top of things. But with so many factors that can influence the efficacy of your herbicide, how do you know for sure?
Herbicide efficacy and crop safety are influenced by factors like plant species and size, growth stage, product choice, soil moisture, temperature, and relative humidity. When you get out to scout, these signs will tell you if you got the weed control value you were looking for:
These are patches of weeds that were not controlled with the herbicide. Upon entering a field look at the big picture. Is there a pattern? Are the edges straight? Sprayer misses will follow distinctive patterns.
Check the low, wet spots bordering sloughs. Are the weeds there looking healthy? These areas tend to have higher organic matter, and the herbicide can be absorbed or tied-up. Once you’ve ruled out sprayer misses as the possible culprit, the next step is to check for resistance.
Re-growth of weeds
It’s not uncommon for plants affected by herbicide to recover from symptoms, even when there’s been considerable dieback. This is particularly true with contact herbicides on overwintered perennial weeds. Perennials have an excellent ability to recover from herbicide injury. If you’re looking to save by not using full label rates or by using generic products, you can end up with suppression rather than control of targeted weeds.
Herbicide injury symptoms
Injury symptoms may be visible beginning a few hours to several days after application. Symptoms of herbicide damage vary among products and should not be confused with injury due to environmental conditions. It’s not always easy to differentiate, but some herbicides have distinct injury symptoms. For example, pigment inhibitors such as clomazone (ex. Command® Charge) cause bleaching, while xylem-mobile products such as triallate (ex. Avadex® MicroActive) cause underground leaf-out in grassy weeds, and within a few hours of applying a phenoxy (ex. GoldWing®) you will see twisting of stems and curling of leaves.
Diagnosing injury symptoms for pre-emerge herbicides is not so easy. It’s more a lack of weeds you’re looking for. Products such as flumioxazin (ex. Valtera™) cause root pinching in germinating seeds. Frost can cause damage symptoms like those produced by herbicides containing glyphosate. However, frost damage patterns in the field reflect the movement pattern of cold air flow and where it settles.
Herbicide damage on crops can cause slight to serious injury symptoms and can occasionally cause economic damage as well. This is more often seen with soil-applied residual products, as the conditions that affect herbicide efficacy also affect crop injury. For example, spring applied trifluralin (ex. Treflan® MicroActive™) can cause a short, thickened coleoptile and stunted roots in wheat if it is incorporated so deeply that the herbicide encounters the developing root system. On sandy soils excess moisture from heavy rainfall can cause herbicides to move into the crop germination zone where it can injure the crop. When this happens both crop emergence and seedling vigor are negatively affected.
Stand emergence issues
You could see poorly emerged crop stands when you have herbicide carryover issues, improper sprayer clean-out or improper product usage. Especially after a dry year, check your farm records to see what crops were seeded to what fields and what products were applied in the previous two years against weather conditions over the same period.
In a shortened spring, everyone is pressed for time, but don’t overlook proper spray cleanout. When switching between crops and products, spend those extra minutes to properly clean and flush your sprayers and nozzles.
If you have questions about determining how effective your herbicide application has been, contact your Cargill rep, and when using herbicides, always read and follow label directions.
Command® Charge is a registered trademark of FMC of Canada. Avadex® MicroActive and Treflan® MicroActive™ are registered trademarks of Gowan Canada. Valtera™ and GoldWing® are trademarks of Nufarm.