‘Is my crop too stressed for herbicide application?’ has been a common question this week. As mentioned in an earlier post, recent wet weather has coincided with the window for post-emergent herbicide application in many crops.

The ability of a crop to ‘tolerate’ a herbicide application depends on its capability to metabolize or compartmentalize the active ingredient before it causes plant injury. This is the basis for herbicide selectivity.  Excess soil moisture reduces oxygen availability to the crop, which affects physiological processes like metabolism.  As such, water-stressed crops may not be able to effectively metabolize herbicides, resulting in crop injury.

What can you do reduce the risk of crop injury from herbicide application?

  • Check for new growth as an indicator that the crop has resumed physiological processes like photosynthesis and metabolism. If you don’t see any new growth in a stressed crop, wait 24 hours and re-assess.
  • Consider your herbicide choice. Some herbicides are more likely to result in injury to stressed crops than others. Group 2 herbicides, especially the more residual products, are an example. However, the risk of crop injury can also vary among chemistries within a herbicide group. For instance, pinoxaden may be a safer group 1 on stressed wheat than fenoxaprop.
  • What’s in your mix? Increasing the number of different products in your tank can overwhelm a stressed crop’s metabolic capabilities. Tank mixes that cause antagonism generally increase crop safety but also have decreased efficacy on weeds. Avoid using products that ‘heat up’ a tank mix, which can increase the risk of crop injury. Talk to your chem rep if you’re unsure; they may recommend different tank mixes or separate passes.
  • Wait until the end of the day. Applying herbicides in the evening can reduce their impact on a stressed crop, although research has shown that later-in-the-day herbicide applications can also be less efficacious. The trade off may be worth it since daytime temperatures over 27°C can add additional stress to the plants and can increase the activity of certain herbicides.
  • Check the forecast for rain. Trying to get a herbicide application on before a forecasted rain works for healthy crops but may not be the best strategy for stressed crops as addition rain may compound the problem. Besides additional stress, shallow, stressed crops roots can be impacted by herbicides moving into the root zone as well.
  • And finally, compare the risk of potential crop injury (i.e. how stressed is the crop, what proportion of the field is stressed, etc.) to the risk of yield loss due to weed pressure. If the stressed crop is limited to a few low spots in the field, it’s likely worth risking a few acres of injury to protect yield from weed competition. However, if most of the field is water-stressed, it might be worth moving on to a different field and returning after a day or two.

Submitted by Jeanette Gaultier (Weeds Specialist) and Ingrid Kristjanson (FPE Moris), Manitoba Agriculture.

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