Reducing herbicide rates – a good idea?

Does reducing herbicide rates save money or cause more problems than it’s worth?

Jed Colquhoun, UW-Madison

Growers often review their records and past experiences to think about ways to reduce inputs such as labor, fertilizer and pesticides. With that we often hear the limbo question: “how low can you go?”. In terms of herbicide use the risks and potential benefits of reducing application rates should be considered.

• The most obvious and basic risk is poor weed control that plays into all of the other risks listed below.
• Crop yield and quality can be reduced by competition from weeds that survive application.
• Uncontrolled weeds mature to reproduction and form seeds (and/or vegetative tissue for perennials) that make future problems even worse, which might then require more inputs than originally anticipated. Moreover, weeds that survive sublethal injury to produce viable seeds start the selection pressure for herbicide-resistant plants. In short, dead weeds don’t cause problems…
• Additional herbicide applications or control measures may be needed to manage escapes.
• While herbicides can be applied at lower rates than those listed on the label if the label doesn’t prohibit you from doing so, you alone are liable for any problems such as those outlined above from a reduced rate use.

Potential benefits:
• Many growers look at reducing rates as a way to reduce costs, particularly for more expensive herbicides.
• For some herbicides, reducing the rate can also reduce the risk of crop injury and subsequent yield and quality loss. This risk reduction is particularly important when environmental conditions favor crop injury. For example, some soil residual herbicides are known to have more crop injury risk when applications are followed by cool, wet weather.
• In some cases reducing the herbicide rate can also reduce associated environmental risks, such as leaching. However, keep in mind that such risks were considered when the full labeled rates were established, so this benefit may not be as clear-cut as it first seems, particularly if more applications are needed to control escaped weeds.

Several questions should be answered when considering herbicide rate reductions:
• What is the risk of selecting for resistant weeds for the herbicide site of action? Some herbicide sites of action are quite prone to selecting resistant weeds, while others have been used for decades with minimal or no herbicide-resistant weeds. Take a look at to see a list of herbicide-resistant weed cases for the herbicide you’re considering.
• Is there a weed threshold established for the particular target species, and if so what does that look like relative to your weed population? There are some weeds that are more aesthetically displeasing than damaging to the crop. Keep in mind, though, that weeds that reach reproductive maturity can make future problems worse – like fire, they’re easier to “extinguish” when small and limited in area!
• What is your previous experience both with crop tolerance and weed control with the herbicide in question? Good recordkeeping can make the rate decision quite easy – what’s worked for you in the past and were there any negative consequences to the rates you’ve used previously?
• What’s the crop status and has there been any environmental stress that could affect crop response to an herbicide application? Labels often include language like “crop injury risk is greater when air temperatures are above xx degrees” or “avoid applications in cool, wet weather”. These are based on research and experience – pay attention to them!

As always, read and follow the label prior to any pesticide use!