May 3, 2018Targeted herbicide tank mixtures for weed control in potatoes
In the latest issue of Potato Progress, editor Andrew Jensen looks at how to find the right tank mix to control the unique weed problems in each field. He also makes the case for not just using the same mix in all fields and the disadvantages that a “one size fits all” approach can bring.
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 the weed management program, rotational crops, etc. 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. In other words, do not to use the same herbicide program on all potato fields. The using-the-same-combo thought process can be detrimental, especially if herbicide resistant weed populations are developing due to the same mechanism of action herbicides being used time and time again.
Example: Herbicide resistance is suspected for the decline in weed control by metribuzin in PNW and other potato production areas. Metribuzin-resistant common lambsquarters populations in WA and redroot pigweed in ID and WA have been confirmed. This phenomenon is most likely happening because metribuzin has been for many years and still is a staple for potato herbicide programs. That in itself is not enough…just think, though, when growers were still planting potatoes in a two-year rotation, metribuzin was used in a field every other year. The development of resistance in a weed population can be described as a slow-burning forest fire. Two-year rotations with metribuzin throughout the 80s and most of the 90s, then finally a switch to three-year rotations has resulted in suspected metribuzin-resistant redroot pigweed populations in 2002 and confirmed in 2005.
The old addage that “they” will come out with a new herbicide is not true. In fact, it has been almost 30 years since a new mechanism of action has been introduced. Currently, there are 10 herbicides labeled for use in PNW 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 a minor crop, the reason could be that crop protection manufacturers consider the potato market too small to invest development dollars. 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. Target those weeds, however. Research in a recent multi-location tank-mix study funded by the Northwest Potato Research Consortium showed that there are other 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 with these two fundamental characteristics 1) the herbicides control the same weed or weeds, and 2) have different mechanisms of action. Economic feasibility can be a concern, but keep in mind that poor weed control can result in a reduction in potato yield and quality of 25% or more and adds weed seeds that can germinate in following years.
The following are two scenarios for targeting and managing. They are relatively simple since there’s only one weed in the first and two in the second. The idea is to get the idea! Not all herbicide possibilities are being mentioned, and of course, always read and follow the full and special-use labels. In addition, be cognizant of which rotation crops can follow use of a given potato herbicide and rotate herbicide mechanisms of action, too.
Scenario one: Hairy nightshade is the only weed present in the field. It’d be easy to use only one herbicide, or on the other end of the stick, just 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. Remember the basics, though…different mechanisms of action all of which have activity on hairy nightshade. A few of the possible preemergence-applied combinations could be Outlook (dimethenamid-p) + Eptam (EPTC); Chateau (flumioxazin) + Dual Magnum (smetolachlor), or Matrix (rimsulfuron) + Reflex (fomesafen) both applied preemergence or Reflex pre- followed by Matrix postemergence. All of these herbicides have activity on hairy nightshade.
Scenario two: Hairy nightshade and common lambsquarters are the weeds in the 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 sometimes a two-way mix might not be enough. University of Idaho research has shown that preemergence-applied Linex (linuron) has activity on hairy nightshade and also 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 weed 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 would not be the right mixture for Scenario one where only hairy nightshade was present.
In summary, target the weeds in each potato field, don’t use a same-old-same-old approach. Besides controlling the weeds in that field, time and money can be saved as a result. Weed populations resistant to metribuzin, or other herbicides, are out there and the potential for an increase in resistance is also out there unless proactive measures are taken by using the most appropriate herbicide tank-mix.
Andrew Jensen, editor, research and extension for the potato industry of
Idaho, Oregon and Washington