Kris Folland, an MSR&PC director from Kittson County, contacted me last growing season with a few problem weeds found in soybean fields in his native northwest Minnesota.

The common trait, glyphosate (Roundup) was not controlling the problem. For each, the number of post emergent (POST) control options is limited.

So the question becomes: what are your options?

Volunteer Roundup Ready Canola: Canola is both a crop and weed. It fits the old definition of weed; a plant out of place. Canola, like soybeans, is grown for oil and meal production, and is an annual broadleaf crop and has similar herbicide sensitivities as soybeans.

“Volunteer RR canola is in many fields spread from fertilizer, wildlife, wind, etc.,” Kris says. “Most of it is in fields where canola has never been grown. Post spray includes flexstar, raptor, and similar chemicals. Low rates of each prior to soybeans flowering. Crop oil seems to help. Research on chemical control and affect on soybean yield are needed. The beans always turn yellow after spraying rates of flexstar as low as 3-6oz. It does not appear to affect yield though.”

Recently, canola has been found invading soybean fields that have not included canola in the rotation.  It is thought canola was introduced as a contaminate of bulk fertilizer shipped from Canada. Most of the over-the-top material that will kill canola will damage/kill soybeans.  It is far easier to practice prevention (crop rotation, pre-emergent herbicides, etc.) than to rescue the soybean crop.  This is an instance when alternative GMO crops (Liberty Link, Dicamba tolerant or 2, 4-D tolerant soybeans) may shine.

According to this website, the best management practices for volunteer Canola are to reduce canola harvest loss, use no-till or delayed tillage after canola harvest, plant competitive rotational crops (wheat, corn, etc.) where diverse herbicide sites of action can be used, remove volunteer canola early with pre-plant burndown herbicides, PRE residual herbicides and early POST herbicides, control canola before it reaches the 4 leaf growth stage.

If you are following canola with conventional or Roundup Ready soybeans, use 2,4D LVE or saflufenacil as a burndown, use a burndown with a PRE (2,4-D LVE + flumioxazin or saflufenacil + imazethapyr)  Imazamox is the POST option, but is only effective for volunteer canola at less than the 4 leaf growth stage.  Once volunteer canola is past the 4 leaf stage, mechanical control and hand weeding become the primary option for conventional or Roundup Ready soybeans.

Glyphosate Resistant Marestail: Marestail (Conyza Canadensis), aka horseweed or Canada fleabane, is an annual broadleaf plant, native to much of North America, frequently found in waste areas, fence rows and fallow cropland.  Like waterhemp, this weed can germinate over a long window, is a prolific seed producer and biotypes have become resistant to a number of herbicides.  Marestail population increase is associated with increased no-till acreage over time.

As an annual broadleaf, once established, marestail is difficult to control in soybeans.  If glyphosate resistant marestail is known to exist, rotating to a grass crop (wheat, corn, etc.) will allow more control options.  Also, as with volunteer canola, use of a glufosinate, 2,4-D or dicamba tolerant soybean variety may show immediate results.

Similar to control of volunteer canola, tillage can be helpful. BMT’s for marestail are used as a herbicide application for emerged plants in the fall, or early spring, to reduce heavy populations (additional applications may be required closer to planting).  Use a spring burndown, including a residual herbicide, to control <4 inch tall marestail to prepare a clean seedbed.  Always use full labeled rates of herbicides for all (PRE and POST) herbicide applications to control herbicide resistant weeds such as marestail.  Use multiple modes of action in all areas, especially in areas with known herbicide resistant marestail.  Include a POST program to control escapes.  Rotate crops and include tillage in your program to reduce heavy marestail infestations.  Consult product labels for precautions and cropping restrictions to manage carryover crop damage.

Scouring rush (Equisetum hyemale): is a perennial weed commonly found in wetter areas of a no-till field.  This plant is more closely related to ferns than it is to a rush.  It can spread by roots (rhizomes) and spores.  Once established, it forms a dense, extremely competitive sod that tolerates high moisture levels and limits herbicide effectiveness.  The absence of leaves (reducing surface area), the thick cuticle and the accumulation of high concentrations of silica on its surface makes it nearly completely tolerant to herbicides.  Once established, this weed can be tough to control.

In noncropland area, repeated mowing and/or tillage has been used to control Scouring rush.  However, no scientific literature has been found to document the intensity required for effective control.  It appears the area should be mowed or tilled whenever new growth is evident to deplete the carbohydrate reserves in the roots.  This process should be repeated until no growth is evident.

Herbicide options are available, but knowledge of efficacy is limited.  MCPA has been reported to suppress scouring rush, but the plant will not be found on the label.  Chlorsulfuron, halosulfuron dicholbenil and triclopyr, all labeled for non-cropland use, have also been reported to be active on it.  Foliar applied glyphosate has been found to be ineffective; however a research project has shown injection of concentrated (41 percent) glyphosate directly into the root was very effective in controlling scouring rush, albeit extremely labor intensive.

In cropland, tillage will prevent establishment of scouring rush, however, light tillage, much like with nutsedge, will actually spread the rhizomes creating a bigger problem.  Flumetsulam has been shown to have fair to good control.  Be sure to check the label as not all products containing flumetsulam are labeled for use in soybeans.

Scouring rush, while not an overly competitive weed, is resistant to control. Persistence is required.

For all three weed species, prevention is by far and away the most profitable option.