Herbicides Timing – Best Hours For Different Products

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 unknown or oft-overlooked tools is how to time spraying of herbicides so they will have the greatest efficacy against targeted weeds, as well as have the least likelihood of moving off-site. The following is the available information about the optimum time of day to spray indicated herbicides to realize their maximum efficacy against targeted weeds.

Time to spray indicated herbicides for greatest efficacy against targeted weeds.

  • SR = sunrise; SS = sunset
  • 12N = 12 noon
  • 12M = 12 midnight
Herbicide or Herbicide mix Targeted Weed Spray TOD* Greatest Efficacy
CULPEPPER and BARENTINE and CULPEPPER
Roundup (Glyphosate) Palmer amaranth <SR–SS> 10AM–7PM
Liberty (Glufosinate) Palmer amaranth <SR–SS> 10AM–7PM
Reflex (Fomesafen) Palmer amaranth <SR–SS> 8AM–7PM
Clarity (Dicamba) Palmer amaranth <SR–SS> 8AM–7PM
2,4-D Palmer amaranth <SR–SS> 8AM–7PM
Gramoxone (Paraquat) Palmer amaranth <SR–SS> 6AM–9PM
MONTGOMERY
Cheetah (Glufosinate) Palmer amaranth SR, 12N, SS 12N
Flexstar (Fomesafen) Palmer amaranth SR, 12N, SS 12N
Ultra Blazer (Acifluorfen) Palmer amaranth SR, 12N, SS 12N
Cobra (Lactofen) Palmer amaranth SR, 12N, SS 12N
Cheetah + Flexstar Palmer amaranth SR, 12N, SS SR to SS
REYNOLDS
Liberty Barnyardgrass 12M, 6AM, 12N, 6PM 12N & 6PM
Select (Clethodim) Barnyardgrass 12M, 6AM, 12N, 6PM All times
Liberty + Select Barnyardgrass 12M, 6AM, 12N, 6PM 12N & 6PM
LOUX AND DOBBELS
FirstRate (Cloransulam) Various broadleafs 6, 9AM; 12N; 3, 6, 9PM; 12M All times
Flexstar Various broadleafs 6, 9AM; 12N; 3, 6, 9PM; 12M 9AM–6PM
Roundup Various broadleafs 6, 9AM; 12N; 3, 6, 9PM; 12M 9AM–6PM
Roundup + 2,4-D Various broadleafs 6AM; 4, 9PM All times

There is only speculation regarding the cause of reduced herbicide efficacy in the below-optimum spraying periods that encompass the dark hours. Some possible factors that may be associated with this low efficacy during this time frame 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 (summarizedHERE), 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 the nighttime hours should be investigated so that solutions can be developed that will allow spraying of herbicides during times of lower drift potential.

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