All Michigan dry bean growers strive to have weed-free fields at harvest. However, due to limited herbicide options for use in dry beans and many other constraints, this is often a challenge. While we cannot always guarantee weed-free fields, there are several key points that should be considered before planting that will make this goal closer to reality.

Weed management considerations

It is important to start clean! As with all weed control strategies, starting with a weed-free seedbed is essential. Removing weeds with tillage or an effective burndown herbicide treatment prior to crop emergence can give the crop the competitive advantage over new emerging weeds.

Consider using preplant, preplant incorporated or preemergence herbicides with residual activity as a foundation program for hard to control weeds. This is particularly important in dry beans since there are very few herbicide options for POST weed control. Relying on a total POST weed control program can be difficult. Weeds can become too large, be affected by the environment (hardened off by hot-day weather) or may just not be effectively controlled with postemergence herbicide applications. Getting a start on weed control early in the season will insure greater overall success for your weed control program.

Also, using a weed control program with a soil-applied herbicide that has residual activity limits the use of “rescue” herbicide treatments late in the season that can further stress dry bean growth. Since we are getting closer to planting, I have provided a short review of soil-applied herbicide options following the weed management considerations portion of this article.

Make timely POST applications or cultivation, target POST herbicide applications to weed between 2 and 4 inches tall. Many of the herbicides labeled for dry beans are more effective when weeds are small. Additionally, with some of these herbicides, lower use rates can be used to control smaller weeds ultimately reducing overall weed control costs that can help the bottom line. Applying a soil-applied herbicide will also help extend this window by providing some early season control of these weeds.

Beware of herbicide resistant weeds! Throughout Michigan, we are seeing more and more populations of herbicide-resistant weeds showing up in dry bean fields. Weeds resistant to ALS-inhibitors (Group 2), PSII inhibitors (Group 5), glyphosate (Group 9) and more recently PPO-inhibitors (Group 14) are becoming more common. If herbicide resistance has been a problem on your farm in the past, take this into account when planning your weed control strategy.

For example, ALS-resistant common ragweed can be a problem in dry beans. Permit, an ALS-inhibitor, has good activity on common ragweed, but since Permit is an ALS-inhibitor it will not control ALS-resistant common ragweed populations and a herbicide like Reflex (Group 14) should be used to control common ragweed. However, keep in mind we now have populations of common ragweed in a few fields in Clinton, Huron, and Saginaw counties that are resistant to both Group 2 and Group 14 herbicides.

Consider herbicide rotation crop and pH restrictions. Several herbicides have rotational crop and pH restrictions. For example, all postemergence broadleaf herbicides, except Basagran, have rotation crop restrictions. Major rotation restrictions for each of the soil-applied herbicides will be listed below. Also, refer to Table 12: Herbicide Crop Rotation Restrictions in the Michigan State University Extension “Weed Control Guide for Field Crops” (E-434) or the individual herbicide labels.

Review of soil-applied herbicide options

Preplant incorporated ONLY

Triflualin (many trade names), pendimethalin (Prowl H₂O, Prowl) and Sonalan: These are Group 3 herbicides with no known cases of resistant weeds in Michigan. Annual grass control is excellent with all three herbicides. Trifluralin (1 pt/A) and Sonalan (2 pt/A) provide better pigweed control than Prowl H₂O (1.6 pt/A). Prowl and Sonalan provide better common lambsquarters control than trifluralin. None of these herbicides will control eastern black nightshade or common ragweed. Eptam should be tank-mixed with each of these herbicides for additional annual grass and broadleaf weed control.

Eptam is a Group 8 herbicide with no known cases of resistant weeds in Michigan. Eptam provides excellent control of several annual grasses and good control of common lambsquarters. Eptam also will suppress common ragweed, wild mustard and nightshade species. Thus tank-mixing Eptam with trifluralin, Prowl, or Sonalan will improve control of these weeds. Eptam is applied at 1.25 qt/A. It is important to weigh the cost of this product against the weed control benefits, because we do have Reflex POST to control common ragweed. However, it is important to remember that there are specific crop rotation restrictions that need to be followed when applying Reflex.

Preemergence or preplant incorporated

s-metolachlor (Dual Magnum, Dual II Magnum, Cinch, or others) and Outlook (dimethenamid-P): These herbicides are all Group 15 herbicides and currently in Michigan there are no known cases of resistant weeds to these herbicides. However, in other states in the Midwest, there have been reports of waterhemp populations resistant to the active ingredient s-metolachlor. Applying either Dual Magnum (or other s-metolachlor products) or Outlook preplant incorporated minimizes crop injury compared with preemergence applications. Navy and black beans are more sensitive to Outlook than Dual Magnum. Dual Magnum and Outlook both provide good control of pigweed, including waterhemp, and excellent control of annual grass weeds. However, Outlook provides better pigweed and eastern black nightshade control compared with Dual Magnum.

Dual Magnum and Outlook can be tank-mixed with Eptam, Prowl, Sonalan or trifluralin for common lambsquarters control. Postemergence applications of a broadleaf herbicide may be needed for additional broadleaf weed control. The typical use rate for Dual Magnum is 1.33 pt/A and Outlook is 14 fl oz/A.

Sequence is a pre-mixture of Dual Magnum and glyphosate that can also be used prior to dry bean emergence (preplant or preemergence). The application rate is 3 pt/A. This may be a good choice for growers that are planting into a reduced tillage or stale-seed bed situations that may need glyphosate to control existing vegetation. There are also several generic metolachlor products that are also labeled for use in dry beans, consult the labels of these products for registration and application rates.

Permit is a Group 2 herbicide and in Michigan we have several Group 2 resistant weeds, including common ragweed, common lambsquarters, eastern black nightshade, waterhemp, and Palmer amaranth just to name a few. Permit provides excellent control of pigweeds and wild mustard, and good control of common ragweed and velvetleaf. Remember Permit will not control Group 2 (ALS) resistant common ragweed. Permit can be tank-mixed with Dual Magnum or Outlook for grass control or Eptam (PPI) for control of grasses and additional common lambsquarters control. Application rates of Permit range from 0.5 oz/A to 0.67 oz/A. The lower use rate should be used on sandier soils. Dry bean varieties vary in their sensitivity to Permit. From MSU trials kidney and black beans were slightly more sensitive to Permit applications than the other classes of dry beans. The rotation restriction for Permit to sugar beets is 21 months.

Pursuit is another Group 2 herbicide. Pursuit may be applied preemergence or preplant incorporated. Pursuit 2L at 2 fl oz/A can be tank-mixed with other soil-applied herbicides for additional broadleaf weed control and to control annual grasses. Remember, Pursuit will not control common ragweed. Crop injury can occur if conditions are cold or wet within one week of application, so precautions need to be taken to avoid injury. These precautions include: do not apply on sands or loamy sand soils; do not apply if cold or cold-wet conditions are predicted one week after planting; and dry bean varieties vary in their sensitivity to Pursuit applications. Rotation restrictions are critical with Pursuit. These restrictions include 40 months and a bioassay for sugar beets, cucumbers and tomatoes; 18 months for oats; and 26 months for potatoes.

Preplant or preemergence

Reflex is a Group 14 herbicide. Currently in Michigan, we have some areas in the state where Group 14 resistant common ragweed, waterhemp and Palmer amaranth have been identified. Reflex can be applied preplant or preemergence. This application can provide potentially four to five weeks of residual weed control/suppression of pigweeds, common ragweed and eastern black nightshade. However, since we are only limited to using a maximum of 1 pt/A of Reflex every other season, we recommend you save your Reflex applications for postemergence control of these weeds. Remember the rotation restrictions for Reflex are four months for wheat, 10 months for corn and 18 months for alfalfa, sugar beets, potatoes, cucumbers, canola and tomatoes.

For weed control in dry beans, I strongly recommend a two-pass program that starts with one of these soil-applied herbicide programs. Using a two-pass program may add to the overall cost of your weed management program, but this program will result in overall more consistent control of several weed species and help protect your dry bean yields.

For more information on weed control in dry beans, consult the “Weed Control Guide for Field Crops” (E-434).