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30 POTATO GROWER | MAY 2018 Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In HERBICIDES | By Pamela Hutchinson, University of Idaho Targeted herbicide tank mixtures for weed control in potatoes The Right Mix The number of weed species and densities in potato fields can vary greatly from one field to the next even if fields are in close proximity and/or cultural practices such as the weed management program and rotational crops have been similar for a number of years. Scouting and keeping a weed history record is a must for success. Although some feel that it can be simpler to use the same herbicide combinations regardless of what's going on in a given field, recommendations are for growers to target the weed species in each field with the appropriate herbicide tank mixture and not to use the same herbicide program on all their potato fields. The latter thought process can be detrimental, especially if herbicide-resistant weed populations are developing because the same mechanism of action herbicides is used time and time again. For example, herbicide resistance is suspect for decline in weed control by metribuzin in potato production areas. Metribuzin-resistant common lambsquarters populations in Washington and redroot pigweed in Idaho and Washington have been confirmed. This phenomenon is most likely happening because metribuzin has been and still is a staple for potato herbicide programs. The old adage that "they" will come out with a new herbicide is not true. In fact, it's been almost 30 years since a herbicide with a new mechanism of action has been introduced. Currently, there are 10 herbicides labeled for use in Pacific Northwest potatoes to control broadleaf weeds. No labeled potato herbicides were initially developed for use in potato; rather, they come from other crop herbicide markets. Since potato is considered by many crop protection manufacturers to be a minor crop, the reason could be that manufacturers consider the potato market too small to invest development dollars in. The result is a diminishing number of herbicide options for potato growers for controlling resistant weed populations already present, and preventing or delaying the development of resistance. Enter the targeted tank-mix program for best weed control possible and also managing herbicide resistance. First of all, what to do about metribuzin? With careful planning, this herbicide can still be used to control many troublesome weeds in potatoes. Growers are encouraged to target those weeds, however. Research in a recent multi-location tank-mix study funded by the Northwest Potato Research Consortium showed that there are herbicides besides metribuzin that can provide effective, season-long weed control. Don't use metribuzin—or any potato herbicide for that matter—always and everywhere, regardless of what's out there. The key is to use a tank mix or sequence of herbicides that 1) control the same weed or weeds, and 2) have different mechanisms of action. Economic feasibility can be a factor; however, keep in mind that poor weed control can result in a reduction in potato yield and tuber quality of 25 percent or more and weed seeds that can germinate in the following years. The following are two scenarios for targeting and managing. Not all herbicide possibilities are being mentioned, and of course, growers should always read and follow the full and special-use labels. Be cognizant of which rotation crops can follow use of a given potato herbicid,e and rotate herbicide mechanisms of action. Carefully formulating tank mixtures will help growers better control problem weeds such as hairy nightshade.