Next year’s weed management plan starts now!
By Douglas J. Jardine, Professor Emeritus, Kansas State University
Most areas of the country are seeing the 2020 soybean crop rapidly approaching the last stages of crop development. Harvest in the south has already started and many fields are rapidly moving toward the R6 growth stage and leaves will soon begin to yellow and eventually drop. All of the inputs have been made to this year’s crop and growers are waiting to begin harvest and hope for good weather and minimal harvest issues. One harvest issue that can greatly slow progress down is weed pressure. In recent years, we have developed weed populations that are resistant to multiple herbicide modes of action. Much of the current problems stem from overuse of glyphosate in both the corn and soybean crops. Palmer amaranth, water hemp, kochia, marestail (horseweed) and other weeds have created significant harvest headaches.
Now is a good time to walk your fields and take inventory of what weeds may be present late in the season. If your fields are weed free, congratulate yourself for having a good weed management program. If not, you need to begin planning now for the next soybean crop to go into that field.
Weed problems can be of two types. The biggest headache will be if you have resistant populations in your field. This will take more thought in developing an integrated management plan. The other problem is that your chemical did not hold long enough and you had later season germination and weed development. This often occurs in fields that did not completely canopy over, allowing light to the soil surface to aid in seed germination and weed development.
The most effective way to manage weeds in soybeans is to use an integrated weed-management program that includes crop rotation, good crop production practices, cultural weed-control practices like narrow rows or cover crops, and a diversified herbicide program.
Marestail is a winter annual weed. Excellent control can occur with fall applications (October through early December) or early spring applications (March and April). Herbicide combinations with different modes of action provide the broadest spectrum of control and help prevent the development of herbicide-resistant weeds. Glyphosate (Group 9), 2,4-D (Group 4), dicamba (Group 4), metribuzin (Group 5), Canopy EX (Group 2), and Autumn Super (Group 2) are some of the more commonly used fall and early-spring burndown treatments in soybeans. Consult herbicide labels for required preplant intervals ahead of soybeans and any guidelines regarding soil texture, soil pH, precipitation requirements, or geographical use restrictions.
Summer annual species especially resistant populations of Palmer amaranth, waterhemp and kochia are best managed with preplant and preemergence herbicides. Preplant and preemergence herbicides with multiple effective sites of action and good residual pigweed activity are critical for their management. The best approach for pigweed control is to use overlapping residual herbicides as preplant, preemergence and/or postemergence treatments. Relying solely on preplant herbicides too early in the spring creates a situation where herbicides will not persist long enough to control later-germinating pigweeds. There is a wide range of postemergence products that can be used in conventional soybeans. State Extension weed and herbicide specialists can provide information on what products have worked best in your state.
Where extensive populations of glyphosate-resistant weeds have developed, alternative herbicide-resistant soybean traits may be warranted. These technologies should be used as part of a diversified weed management program with effective residual herbicides to achieve acceptable weed control and sustain the technologies. These products include Liberty-Link, Roundup Ready 2 Xtend and Enlist E3. Others may be introduced in the future.
Regardless of the herbicide-resistant trait, the most effective weed control programs are integrated programs using multiple weed control tactics and a diversity of herbicide sites of action.