Herbicide Efficacy and Time of Spraying
Midsouth crop producers must use every available tool to get the most benefit from application of herbicides to control weeds, as well as ensure that applied herbicides are confined to the intended spray site.
One of the oft-overlooked tools is how to time spraying of herbicides to ensure their greatest efficacy against targeted weeds and the least likelihood of their moving off-site.
Click here for an article that summarizes what is known at this time about efficacy of selected herbicides against targeted weeds when the herbicides are applied at varying times-of-day.
There is only speculation regarding the cause(s) of reduced herbicide efficacy in below-optimum spraying periods. Some possible factors are:
● Dew in early morning hours that causes herbicide spray to drip from leaves.
● Diurnal leaf movement or difference in leaf orientation of targeted weeds. Many species have their leaves extended horizontally during the day, then dropping to nearly vertical at night. In an Arkansas study (summarized here), leaves of prickly sida, hemp sesbania, and sicklepod changed leaf angle (degrees from horizontal) from 2 to 32○, from 5 to 90○, and from 15 to 90○, respectively, from 4 PM to 9 PM. A 2 pt/acre rate of Roundup applied at those corresponding times resulted in a drop in control of the respective weeds from 90 to 65%, from 92 to 22%, and from 98 to 50%. These results indicate that leaf orientation may play an important role in reducing the efficacy of herbicides applied at night.
● Higher humidity and lower ambient temperature at night.
● Weed physiological processes and responses at night that may be affected by lack of sunlight, thus causing changes in metabolic activity.
With the coming application of auxin herbicides on a widespread basis, the reasons for low herbicide efficacy during non-optimum spraying times should be investigated so that solutions can be developed that will allow spraying during these times of potentially lower drift potential.
Composed by Larry G. Heatherly, June 2015, larryheatherly@bellsouth.net