Herbicides and Hot Weather

Richard K. Zollinger
NDSU Extension Weed Scientist
HTML Editors: Jochum Wiersma & Tracy Allrich

Hot weather can cause some herbicides to become "hotter" and increase the risk of crop injury. Other herbicides act opposite and give better control under cool conditions. The same is true for weeds. Some herbicides are more active and show symptoms quicker in hot temperatures than in cool temperatures. To know how herbicides act on both weeds and crops in extreme weather conditions can be confusing and difficult to remember.

Hot weather may reduce weed control. Plants develop a thicker cuticle in hot weather as a way to reduce water loss. A thicker cuticle acts as an additional barrier and can decrease herbicide penetration. Prolonged hot weather and low soil moisture can cause plants to become drought stressed, resulting in reduced metabolic activity and reducing systemic herbicide translocation and effects in the plant. Most post emergence herbicides require weeds to be "actively growing" for maximum control. If weeds are stressed from a number of environmental or cultural factors, control may be reduced.

Herbicide application can be very effective on weeds of soil moisture is adequate. If soil moisture is depleted weed control may be reduced.

The following are general "Rules of Thumb" for response of weed and crops may be reduced:

  1. Contact herbicides are more phytotoxic on weeds and crop under hot temperatures.
  2. Systemic grass control herbicides in small grains (Hoelon, Dakota, Tiller, Cheyenne) are less phytotoxic on both weeds and small grains in hot weather. However, crop injury may occur in moisture stress.
  3. Systemic herbicides may be less effective on weeds stressed form drought or other factors.
  4. Crops and weeds are more prone to herbicide injury when actively growing and free from stress. Crops under stress from hot and dry conditions may be less prone to injury from most herbicide.
  5. Weeds may be stressed for moisture before the crop because of the shallow root system of weeds compared to the crop. Poor weed control may result under these conditions.
  6. In determining cause of crop injury, remember that oil concentrates (COC's) can be phytotoxic with post emergence herbicides and oil concentrates are excellent tank cleaners for sulfonylurea (SU) herbicides. The oil can cause SU herbicides from previous applications. that have been absorbed onto the plastic matrix of spray tends to go into solution and cause serious crop injury when applying POST to a susceptible crop. Even herbicides applied many tank loads before can be solubilized with oil concentrates.
  7. Scoil is the safest oil additive on crops tested an NDSU. NDSU research has shown no instance of plant phytotoxicity from Scoil, but many oil concentrates have produced significant injury in laboratory tests.
  8. Liquid nitrogen fertilizer may contribute to slight leaf burn in hot temperatures.

The following chart is a summary of response of herbicide in hot weather.

Response of Herbicides to Hot Weather

2,4-D, Banvel, Curtail, Stinger, Tordon

  • Weed Response-- More active on broadleaf weeds.
  • Crop Response--Good crop safety but slight increase in risk of crop injury.

Assert

  • Weed Response -- More active on both wild oat and wild mustard, and wild buckwheat.
  • Crop Response-- Good small grain safety. Extreme risk of sunflower injury. Use lower rates on sunflower.

Avenge

  • Weed Response --Greater wild oat control.
  • Crop Response--Greater risk of crop injury. More contact injury symptoms of speckling, spotted and localized areas of burning.

Atrazine, Bladex

  • Weed Response--Greater grass and broadleaf control.
  • Crop Response--Good corn safety. Oil additives used for post emergence application may cause phytotoxic burn.
  • Do not use liquid Bladex formulation for POST applications.

Basagran

  • Weed Response--Greater broadleaf weed control.
  • Crop Response- Good corn and bean safety. Oil additives may cause phytotoxic burn.

Blazer, Cobra, Resource

  • Weed Response-- Greater broadleaf weed control. More grass leaf burn than in lower temperatures.
  • Crop Response-- Greater risk of soybean injury. More contact injury symptoms of speckling, spotting and localized areas of burning.

Buctril, Bronate

  • Weed Response--Greater broadleaf weed control.
  • Crop Response-- Good small grain safety. Greater risk of contact leaf burn on corn with Buctril. Do note use Bromate on corn.

Sulfonylureas (SU) :(Accent, Amber, Express, Harmony Extra, Permit)

  • Weed Response-- Greater weed control under good soil moisture conditions. Reduced control when weeds are drought stressed. Use of methylated seed oil (MSO) adjuvants may provide enhanced control with Accent.
  • Crop Response-- Normally good crop safety. Small grain leaf burn from Express and Harmony Extra + phenoxy tank mixes have been reported. Good corn safety from Accent.

Basis(SU)

  • Weed Response-- Reduced weed control.
  • Crop Response-- Good crop safety if applied at the recommended window of application.

Pinnacle(SU)

  • Weed Response-- Reduced weed control if drought stressed.
  • Crop Response- Greater risk of soybean injury.

Hoelon, Dakota, Tiller, Cheyenne

  • Weed Response-- Reduced grass control.
  • Crop Response- -Good crop safety.

POST Grass Herbicides (Assure, Fusilade, Fusion, Poast/Plus, Select)

  • Weed Response-- Greater weed control under good soil moisture conditions. Reduced control when weeds are drought stressed.
  • Crop Response- -Good crop safety. Oil additives may cause phytotoxic burn.

Pursuit

  • Weed Response-- Greater weed control under good soil moisture conditions. Reduced control when weeds are drought stressed.
  • Crop Response-- Good crop safety. Oil additives may cause phytotoxic burn. Sun-It II will not cause oil burn on beans.

Stampede

  • Weed Response-- Greater weed control.
  • Crop Response-- Extreme risk of contact leaf burn. No systemic activity - new foliage will be unaffected.



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