Seeding and planting is nearly done on the Canadian prairies and producers are beginning to apply their in-crop herbicide. Many are looking at the weather forecast for the next week and seeing some very high temperatures (30oC+) and wondering how this could impact their herbicide application. There are generally two concerns, how will this impact weed control and can it impact crop safety

Weed Control

Two Factors need to be satisfied for a herbicide to be effective: it needs to enter the plant and it needs to get to its target site.  These readily happen when the plant is actively growing.  If there is a barrier to entry or the plant is not actively growing, this can reduce efficacy of the herbicide.

Hot and dry conditions will drive the plant to conserve moisture. To do this the plant will develop a thick cuticle (the waxy exterior layer on plant), this prevent herbicide entry. In very high heat situations the plants can also shut down, preventing the herbicide to move to its target site.  Fortunately, the Canadian prairies have recently received a widespread precipitation event.  This moisture combined with warm weather has caused many weeds to germinate, stimulated rapid growth and fast stage advancement.  Smaller weeds are easier to control and with a good germination event like this, producers can get in early and get good control while the weeds are still small.  However, we will still need to watch out for extremely high temperatures as this can still cause plants to stop actively growing. Most labels will indicate the specific temperature for products. Many plants will begin to slow down or stop growth once the temperature exceeds +28oC, thus reducing efficacy.

Management Tips for herbicide efficacy this year

  • Apply when weeds are actively growing
  • Utilize the warm temperatures and recent moisture for great early season weed removal, BUT
  • Avoid application in the heat of the day (+28oC)

Crop safety

Crop safety can be the other concern with herbicide applications in high temperatures. Tank mixing herbicides with fertilizers, fungicides, growth regulators increase the overall surfactant/solvent load in the spray mixtures.  This increased solvent load combined with high heat can rapidly dissolve the cuticle and increase herbicide uptake . This may cause crop injury through “leaf burn” or “flashing”.     

The activity of contact herbicides is increased on both crops and weeds at higher temperatures. For photosystem inhibiting herbicides, bright sun and long days also increase activity. This is very positive for weed control but may predispose crops to elevated phytotoxicity.

The most critical time for crop injury following application of a contact herbicide is the first few hours after treatment. Therefore, injury can be minimized by applying the herbicide in the evening after the temperature and solar intensity has decreased. Evening application will allow several hours of moderate temperatures following treatment and is therefore preferred under hot conditions to morning application, which will be followed immediately by increasing temperatures.

Some contact herbicides are labeled for use with various additives.  Most additives also increase the chance for crop injury. Omitting an optional adjuvant or using the lower labeled rate of the recommended adjuvant will reduce injury potential from a contact herbicide applied at high temperatures.

Additionally, crop tolerance to some herbicides is enabled by rapid metabolism before the herbicide can bind to a target site within the plant.  If metabolism is slowed down it can cause a temporary crop injury like stunting or flashing.  Applying while the crop is actively growing will ensure that metabolism is mitigating potential phytotoxicity from the herbicide.

Management Tips for Crop Safety under Hot Conditions - a Summary

  • Apply herbicides according to label - do not add fertilizers, fungicides, or growth regulators which are not recommended by the manufacturer
  • When applying in high temp situations – consider removing or reducing the use of optional adjuvants that are recommended for cooler temperature situations.
  • Ensure the crop is actively growing/metabolizing
  • Avoid application in the heat of the day (> +28oC)