Features Agronomy Soybeans
Weed management in IP soybeans

Many factors must be kept in mind to achieve cost-effective control.

Weed management in IP soy-beans can be very challenging. Historical records and detailed information on each individual field will help producers develop a strategy that is both economical and effective. Knowledge of a particular field’s weed spectrum, soil type and soil pH, and using a flexible, but specific strategy for each individual field, are the keys, says Dr. Peter Sikkema, associate professor of field crop weed management at the University of Guelph Ridgetown Campus. “Weed management is variable field to field and year to year,” he says.  John Waters, a Certified Crop Advisor at Lakeside Grain & Feed Ltd. in Forest, Ontario, agrees. “You can’t use the same program over the entire farm,” he says. “You have to change it for every field.”

Understanding the factors that affect weed management can help IP soybean growers avoid potentially overwhelming weed populations in their fields. Advertisement

Sikkema notes there many strategies for weed management in IP soybeans. He recommends starting each season with the appropriate soil applied herbicide for each field, and then applying a post-emergence herbicide if needed. “Early timing is critical for post-emergence herbicides,” he says. “The later you apply, the more you risk soybean yield losses due to weed interference. In addition, applying herbicides to younger weeds gives improved control.” However, growers must know which weed species are emerging in order to avoid applying the wrong herbicide. “Annual grasses are always difficult to identify,” notes Sikkema. “You just plain have to be good at it. Also, some of the broadleaf weeds look similar.”

Soil applied herbicides
There are five important points to consider when selecting a soil applied herbicide, says Sikkema. First, he says “You must match the weed spectrum controlled by a given herbicide to the weeds you have in each individual field.” Second, application must be carefully planned with regard to expected precipitation. “With soil applied herbicides,” explains Sikkema, “you want rain shortly after application to dissolve the herbicide into soil water solution so it can be taken up by the developing weed seedlings.
 
Resistance is the third factor that must be considered when selecting a pre-emergence herbicide in IP soybeans. “If you have Group II (ALS) and/or Group V (triazine) resistant weeds, certain herbicides will not be effective,” Sikkema says. These are Pursuit, Conquest and Broadstrike for Group II and Boundary and Conquest for Group V.

Soil characteristics, including pH and texture, are also important factors in herbicide choice, and make-up the fourth point on the list. Some herbicides do not work as well or may cause crop injury in acidic or basic soils. In terms of texture, Sikkema says some herbicides become bound by the soil clay and organic matter after application, making them unavailable for weed control. Soil texture may also influence the rate required for commercially acceptable weed control.
  
Lastly, the farm’s crop rotation schedule will affect herbicide choice. Some herbicides such as Pursuit, Conquest and Broadstike are persistent in the soil and will affect crop rotation options in the future, says Sikkema.

With respect to herbicide cost, Sikkema says in many situations it is cheaper for the grower to purchase a co-pack or premix that controls both grass and broadleaf weeds such as Boundary or Conquest than to buy the individual components and tankmix them.

Post-emergence weed control
Some of the same factors must be considered when selecting a post-emergence herbicide, including weed spectrum, soil pH and precipitation. With regards to application timing, growers do not want to apply post-emergence herbicides when rain is expected, as it will wash the herbicide off the weeds, however rain is ideally wanted leading up to application day. This will mean that the herbicide will land on succulent and rapidly growing weeds and will be absorbed effectively. Dry conditions leading up to application results in tough weeds with thick waxy coatings that prevent good herbicide absorption.

Residual activity is also a critical consideration, notes Sikkema. “The residual activity affects the length of weed control but also may affect your crop rotation options. There are severe re-cropping restrictions with some products.  For example, you can’t plant sugar beets and quite a few other vegetables the year after applying some post-emergence herbicides. Figure out your planned rotation for the next two, three or five years, and plan your herbicide use accordingly.” Sikkema says Clean Sweep provides good post-emergence control with pre-emergence residual.  Meridian Plus provides less residual and timing is more critical, but the cost is lower.

Weed size and weed density must also be considered. Escapes must be monitored every season on a field-by-field basis. Sikkema’s research has found for most grass escapes, Assure, Venture, Poast, Excel or Select provide comparable control. He adds “For quack grass and volunteer corn and cereals, choose Assure or Venture. For sandbur, choose Assure, and for wirestem muhly, choose Venture.”

For broadleaf escapes, Sikkema recommends Pinnacle, Classic, Blazer or Reflex for pigweed, Pinnacle or Basagran for lamb’s-quarters, and Pinnacle, Classic or FirstRate for velvetleaf. For common ragweed, use FirstRate, Classic, Blazer or Reflex, and for giant ragweed and common cocklebur, use FirstRate.

As a last note, Sikkema stresses that applying knowledge on a field-by-field basis will increase the likelihood of success in controlling weeds in IP soybeans.