This article appears in the May 2018 issue of Potato Grower.
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.
Scenario 1: Hairy nightshade is the only weed present in the field. It would be easy to use only one herbicide. On the other end of the stick, use the same two- or three-way tank mix that is planned for other fields, which have more than just hairy nightshade. Absolutely apply more than one herbicide in this scenario. Remember the basics, though: different mechanisms of action, all of which have activity on hairy nightshade. A few of the possible pre-emergence-applied combinations could be Outlook (dimethenamid-P) with Eptam (EPTC), Chateau (flumioxazin) plus Dual Magnum (S-metolachlor), or Matrix (rimsulfuron) with Reflex (fomesafen), both applied pre-emergence or Reflex pre- followed by Matrix post-emergence.
Scenario 2: Hairy nightshade and common lambsquarters exist in the same field. This situation is a little trickier because many of the herbicides that control hairy nightshade well do not provide effective common lambsquarters control. Therefore, it can be challenging to find herbicides with the needed characteristics, and a two-way mix might not be the best. University of Idaho research has shown that pre-emergence-applied Linex (linuron) has activity on hairy nightshade and controls common lambsquarters in potatoes. Prowl H2O (pendimethalin) controls common lambsquarters. A three-way tank mixture of Outlook + Linex + Prowl H2O would provide two herbicides that control the nightshade and two that are effective on common lambsquarters. Here, metribuzin could be substituted for Prowl H2O, unless that common lambsquarters population is metribuzin-resistant.
Remember, this three-way mix would not be the right mixture for Scenario 1, where only hairy nightshade was present.
In summary, target the weeds in each potato field; don’t use a shotgun approach. Weed populations resistant to metribuzin or other herbicides are out there, and the potential for an increase is also available, unless proactive measures are taken by using the most appropriate herbicide tank mix.