Dennis Lange, Manitoba Agriculture
Dry bean weed control, or weed management might be a better word to use, can be a challenging adventure some years. Edible beans are not competitive with weeds, especially when the plants are small. A management strategy should be implemented before the crop is in the ground. Growers should look at the previous crop rotation and weed spectrum, to identify any problem weeds that may have caused past quality issues. This article highlights a few points that growers should consider when planning their dry bean weed management and will not discuss products that can be used as spring burn-off, which can be another management tool.
It is important to take notes on weed problems that occur during the growing season. This will help you plan future crops and avoid problems before they arise. One weed that can cause issues in dry beans is volunteer soybeans. A weed is defined as a plant that is undesirable in the crop being grown and soybeans fit this bill. The reason is twofold. One, there are no products that can take out the soybeans and leave the edibles untouched; and two, companies who buy edible beans state that it is very difficult to clean the beans and ensure that all the soybeans have been removed. End-users of edible beans consider soybeans a food allergen and could potentially reject loads if they are present.
So how do you control them? The only way is to avoid planting edibles on fields with a history of soybeans. Reports this past season saw volunteer soybeans in harvest samples from fields that have not seen soybeans in the past two and even three years. Always check with your buyer for their specifications and let them know if there are potential problems.
PRE-PLANT INCORPORATED HERBICIDES
Dry beans should be kept weed-free until the sixth trifoliate, which is approximately six weeks after planting.
- Eptam will also give you control of hairy nightshade, lamb’s-quarters, green and yellow foxtail and pigweed.
- Frontier Max is another PPI that also gives control of annual grasses and certain broadleaf weeds, including nut sedge, foxtail, crabgrass and nightshade, according to the label.
- Permit broadleaf herbicide must be applied after seeding, but before soil cracking and needs moisture for activation. It is registered to be applied post-emergent between the second to
fourth trifoliate but prior to flowering, and has planting restrictions of three years for canola and sunflowers. (Source: Nebraska Dry Bean Production Guide).
Weeds that emerge with the crop cause greater yield loss than weeds that germinate once the crop is up. A good foundation for weed management in dry beans is a pre-plant incorporated (PPI) herbicide. These products include Trifuralin, Edge, Eptam, and Frontier Max. These products generally require the first incorporation within 24 hours and the second within three days. The weeds that these products control are listed in the 2015 Manitoba Guide to Crop Protection (page 50). A few weed control highlights for PPI herbicides are as follows:
- Trifuralins control lamb’s-quarters, green foxtail, wild oats, pigweed and wild buckwheat.
- Edge offers control of some of the same weeds but also includes suppression of nightshade, cleavers and kochia, as well as volunteer wheat and barley
Please respect restrictions based on soil type and organic matter content, which some of these products may have. With any of the PPI herbicides the amount of control will vary with weather, soil moisture and soil temperature conditions so scouting to evaluate level of control and to determine which product would be best to follow up with in crop.
RATING OF LEVELS OF WEED CONTROL
The Manitoba Guide to Field Crop Protection does not rate the level of control of various registered products. The Ontario Weed Control Guide has weed control ratings in their guide – available here. These ratings are listed as 0–9 where 0 indicates no control and 9 indicates 90–100% control under ideal conditions. Please note that not all products listed in the Ontario guide are registered in Manitoba and should only be used for reference.
POST-EMERGENT WEED CONTROL
Once the beans are up and you begin your field scouting, you may notice a few escaped broadleaf weeds. Depending on weeds present and the market class of beans grown, there are a number of products you can use. Scouting should be done just prior to the beans reaching first trifoliate.
Registered products have some restrictions on when the product can be used (Table 1). You must also pay attention to the stage of the weeds. Smaller weeds are easier to control than large ones so it is best to scout early to ensure you are on top of the weed problems in the field. With any of the post emergent herbicides, it is a wise decision to read the label to be aware of rotation restrictions that could affect future crops. Table 1 lists the post emergent herbicides and weed stages for best control.
WEATHER CAN ALSO AFFECT HOW SOME OF THE PRODUCTS WORK.
A few general cautions are that you should not spray if temperatures are forecasted to be 5°C or less within three days of application and do not apply if temperatures are above 28°C after application. It is also not recommended to apply products if crops are under stress from flooding, drought, heat, etc. as crop injury may occur. Lastly, under cool or dry conditions, control of some weeds may be reduced.
Dry bean weed control takes good planning. It starts with selecting a field with low weed pressure, and considers rotational crops that come prior to growing edible beans (volunteer issues) and crops that follow edible beans (herbicides that may have certain crop rotation restrictions). The use of a PPI herbicide will help control weeds early in the season allowing the beans to get off to a clean start. Scouting the field early after emergence to evaluate control will aid in making decisions on what post emergent product will work best for you. The smaller the weeds are, the easier they are to control so pay close attention to crop stage and weed stage in order to maximize your weed control effort.