20.4 Herbicide Resistance to Seedling Growth Inhibitors

At the time this chapter was published, 6 weed species have been shown to display herbicide resistance in the US and 12 globally to this herbicide mode of action (3/K1).  In the US these include:

Palmer Amaranth (Amaranthus palmeri), Wild Oat (Avena fatua), Goosegrass (Eleusine indica), Annual Bluegrass (Poa annua), Green Foxtail (Setaria viridis) and Johnsongrass (Sorghum halepense)

For the latest updated information on herbicide resistance, see The International Survey of Herbicide Resistant Weeds, a collaborative effort between weed scientists in over 80 countries, located at www.weedscience.org . 

Final Herbicide Resistance Reminder

At the conclusion of this last section on herbicide modes of action, it is crucial to review  what you can do to prevent the development and spread of herbicide resistant weeds.  This will maximize herbicide effectiveness over a longer period of time.  Earlier chapters discussed the value of incorporating an Integrated Weed Management (IWM) approach which involves several strategies to keeping weeds in check for a longer period of time.  This includes:

• Cultural Practices, such as rotating crops which changes the niche for which the weeds can grow. This also includes adjusting crop row widths and other agronomic practices to help your crop be as healthy as possible and better able to out-compete weed pressures.
• Mechanical Weed Control Practices, including tillage, may provide another tool for you to put into your long-term weed management strategy. For some weeds this helps to provide some effective control and it changes the field’s ecosystem.
• Chemical Weed Control is often a valuable approach to include. You will want to rotate herbicide modes of action and sites of action to help reduce the selection pressures put onto the field’s weed populations.  Failure to do so will be providing a unique opportunity for those weeds which happen to have resistance genes in your fields to really take hold and spread in following years.
• In some cases Biological Weed Control tools can be an effective option.

Essentially, it is important to rotate approaches over a period of several years to maintain low selection pressures which favor resistant weed genotypes present in the populations.