Updated: February 11, 2021
Last fall, I gave a presentation on herbicide resistance management at an Idaho Barley Commission meeting. During the discussion, the issue of herbicide rate came up and one of the commissioners mentioned that his dad taught him to always use full or labeled rate because “dead weeds don’t develop resistance”. This statement has been stuck in my head since then. I believe almost every weed scientist would agree with the statement. Herbicides don’t necessarily “cause” resistance. They select for resistance. Thus, resistance is a numbers game; how many weeds were exposed to the herbicide? How many weeds survived an herbicide application?
While we would all agree that the main cause of herbicide resistance is the repeated use of the same herbicide site of action in a given area, herbicide rate is one of the most important (if not the most important) factor for effective weed control. Research conducted in Australia and published in 2009 demonstrated that the use of sublethal doses of glyphosate for three consecutive seasons selected for rigid ryegrass (closely related to Italian ryegrass) plants that were resistant to glyphosate. A similar thing happened when wild oat (a weed that is gradually becoming a problem in the pacific northwest) was sprayed with glyphosate. A related experiment in the United States found that application of sublethal doses of 2,4-D for three consecutive seasons resulted in Palmer amaranth (one of the most troublesome pigweeds) that were 2-fold less susceptible to 2,4-D than the original population.
But why would anyone apply an herbicide at a rate less than what is recommended? Well, agriculture is a business, and therefore, there will always be the temptation to save on weed control costs by using the cheapest herbicide or cutting the herbicide rate. History has taught us that the long-term cost of resistance management would outweigh the short-term gains of cutting herbicide rates. It is also important to note that the lethal dose of an herbicide depends on weed size. Most herbicide labels would normally state the weed size for optimum control. For example, the optimum size or growth stage of Italian ryegrass for optimum control by Axial Star herbicide at 16.4 fl. oz/A is 1 to 5-leaf on the main stem, before the emergence of the 3rd tiller. Research conducted in North Carolina and published in 2013 found that the application of glyphosate 8 days after the optimum size of Italian ryegrass resulted in a 14% reduction in control. As we plan our herbicide programs for the year, let’s keep this in mind; dead weeds don’t develop resistance. Let’s apply herbicides at labeled rates and optimum weed sizes, whether we are applying one herbicide or a combination of herbicides in a tank-mix or sequence.