From potato planting to row closure is a busy time for growers. It’s also a busy time for weeds. Getting the herbicides on at the right time and linking up with tillage operations between planting and row closure is the key for weed control from start to finish. Time from planting to emergence can be three to four weeks. From emergence to row closure, when the potato crop finally starts to help control the weeds with shading and competition, can be another four to five weeks.
Here’s why timing matters: University of Idaho research shows that when hairy nightshade is at a density of 2 per meter-row, it must be controlled, or a yield loss of U.S. No. 1s of 5 percent or great will occur. This time is from a few days after potato emergence to 22 days later. If only 1 per meter-row competes all season long, as much as 21 percent U.S. No. 1 yield is lost.
Using an integrated weed management strategy will provide the best weed control in potatoes. Such an approach includes cultural, mechanical, chemical and biological tools available for weed control, rather than relying on any single tool.
There are different approaches to tillage-timing out there. This article will concentrate on two:
Method 1:
“Drag-off” before potato emergence and a hilling-reservoir tillage operation (often referred to by the brand name Dammer Diker) after potato emergence. Post-emergence herbicides are applied after that tillage. Drag-off occurs with a harrow or similar equipment to knock down the moderate hill that was built at planting so that the seed piece is closer to surface. If the field is leveled, however, the final hilling-reservoir tillage cannot occur until after potato emergence when the rows can be seen—unless a precision planter with GPS features is used
Method 2:
Hilling-reservoir tillage conducted after planting but before potato emergence. Herbicides are applied after that tillage but before potato emergence.
For both scenarios, the reservoir tillage equipment is set up to hill at the same time, and that is the last tillage operation to be performed until harvest. In both scenarios, another herbicide application after potatoes emerge may be needed.
Tank-mix herbicides with different mechanisms of action in order to achieve broad-spectrum weed control and prevent or delay the development of herbicide-resistant weed populations. Design herbicide tank mixes to target the weeds present in a given field. Rotate herbicide mechanisms of action from year to year.
Studies in Idaho have shown that when weed populations are low and a competitive potato variety is grown, cultivating when weeds are small and potato plants are 4 to 6 inches tall can provide economical weed control. However, hilling potatoes larger than 8 to 10 inches tall can cause root pruning and reduced tuber yields and quality. Multiple cultivations may cause soil compaction, reducing aeration and potato growth and producing clods that bruise potatoes at harvest. Tillage after herbicides already have been applied will disrupt the herbicide barrier and bring up untreated soil, and perhaps additional weed seed.
Properly timed cultivations can control early germinating of annual weeds not larger than two- to three-leaf (usually less than 1 inch tall) at the time of cultivation. Three- to 4-inch weeds can sometimes still be eliminated; however, once they surpass 4 inches, cultivation is no longer effective. Even though large weeds are initially covered with soil during cultivation, they can survive and re-emerge.
If soil is wet, cultivation should be delayed so that compaction does not occur and weeds can’t re-root in the wet soil. In-row weeds will most likely not be controlled with the hilling-reservoir.
Method 1
Though post-emergence herbicides are applied after drag-off and hilling, the cultivation can control weeds that have already emerged. Some herbicides can even be incorporated with a shallow tillage operation with care taken not to damage the seed piece or potato shoots close to the surface. However, as mentioned, tillage after herbicides already have been applied will disrupt the herbicide barrier.
The most effective means of herbicide application after reservoir tillage include chemigation; attaching a spray boom to the back of spray equipment, then sprinkler-incorporating; use of sprayers designed to maintain a level spray boom for even distribution of the herbicide.
Only the following herbicides are labeled for application after potato emergence:
- Rimsulfuron (Matrix and others)
- Metribuzin (multiple trade names)
- EPTC (Eptam)
- Pendimethalin (Prowl H2O and others)
- S-metolachlor (Dual Magnum)
- Boundary, a formulated mix of s-metolachlor and metribuzin.
- Metolachlor (Stalwart and others)
Method 2
The hilling-reservoir tillage in this scenario can be performed any time after planting and before potato emergence. Regardless of timing, a clean bed is created because emerged weeds can be controlled with this operation. Herbicide application should occur as soon after the tillage as possible to be effective.
Now herbicides that can only be applied before potato emergence are an option. All of these pre-emergence herbicides can be applied by ground or via chemigation. If applied by ground, rain or sprinkler irrigation must occur for incorporation and activation in the top 2-inch layer of soil where most weed seeds germinate. Incorporate as soon after application as possible to activate the herbicide.
In addition to herbicides in Method 1, the following can be used when utilizing this second method:
- Dimethenamid-p (Outlook)
- Ethalfluralin (Sonalan)
- Flumioxazin (Chateau).
- Chateau—to be applied as early as possible after planting
- Fomesafen (Reflex).
- Trifluralin (Treflan and others)
- Linuron (Linex or Lorox and other trade names)
- Linex (has some activity on emerged weeds but must be applied before potato emergence to avoid injury)
- Sulfentrazone (Willowood Sulfentrazone 4SC and others).
- A formulated pre-mix of sulfentrazone and metribuzin (Willowood MTZ 4SC)
Post-emergence Herbicides
In addition to the pre-emergence and early-post-emergence herbicides listed above, another post-emergence application may be needed when a crop is under heavy weed pressure, sees late-germinating weeds, or the initial application did not have activity on certain weed species. Matrix and/or metribuzin are commonly used in this instance, since they can be applied post-emergence to the potatoes and have activity on emerged weeds. Sethoxydim (Poast or Poast Plus and others) and clethodim (Select and others) can control grasses but not broadleaf weeds post-emergence. Timing in relation to weed size is extremely important.
Glyphosate, paraquat, and/or carfentrazone (Aim) are labeled in potatoes for burndown of weeds emerged before potato emergence and before or after drag-off and/or hilling-reservoir tillage. These must be applied before potatoes emerge—even before cracking. Tillage after application is okay in this situation; however, an appropriate amount of time for the herbicides to work must occur before that tillage.
Summary
Timing matters. Coordination of properly timed tillage operations and herbicide applications is critical for successful, season-long weed control. Weed size matters. Effective weed control measures should be implemented before potato row closure. Allowing weeds to remain for as long as three weeks after potato emergence can result in yield loss.
Pamela J.S Hutchinson is an associate professor and potato cropping systems weed scientist with the University of Idaho. She is based at the Aberdeen Research & Extension Center and can be contacted at phutch@uidaho.edu.