A weed free start is the most critical aspect of a weed management program for double-crop soybeans. This can be challenging to achieve where glyphosate-resistant marestail are present after wheat harvest. Problems with marestail include the following:

1. Most populations are now glyphosate-resistant and many of these are also ALS-resistant
2. It’s usually not possible to use 2,4–D ester and wait 7 days until double-crop soybean planting.
3. Marestail that were tall enough to be cut off by harvesting equipment will be even more difficult to control.

Where the marestail are not resistant to glyphosate, application of glyphosate at 1.5 or more lbs ae/A should be an economical and effective approach, or a mixture of glyphosate and Sharpen.

The addition of FirstRate or a chlorimuron-containing product can also improve control if the population is not ALS-resistant, and provide some residual broadleaf weed control. However, results of a trial conducted in 2010 suggest that a switch away from a glyphosate-based burndown may be the best strategy where the marestail are resistant to glyphosate.

The population in this trial was resistant to glyphosate and ALS inhibitors, and had survived early-May application of glyphosate. We applied burndown treatments on June 1 when most of the plants were about 4 to 15 inches tall.

The upper part of the taller marestail plants had branched out in response to the early glyphosate treatment, and were generally bushier than marestail left undisturbed through early June.

We applied a variety of treatments, including combinations of glyphosate with 2,4–D and/or Sharpen, and combinations of Ignite or Gramoxone with 2,4–D and/or Sharpen and/or metribuzin.

None of the treatments provided more than 83% control of the marestail 4 weeks after treatment, although several were in the 80–83% range. The better treatments included (all treatments included AMS also):

We considered this to be a worst-case situation with regard to marestail burndown, and it’s likely that these treatments would be considerably more effective in some populations.

We applied these same treatments on June 4 in another field where marestail had been growing undisturbed up until that point. Plants were up to 30 inches tall in this field, but were mostly unbranched.

We did not have prior information about the herbicide resistance characteristics in this population, but our results indicated a relatively low level of glyphosate resistance and apparently no resistance to ALS inhibitors. At the 14-day evaluation, almost all of the treatments resulted in 100% control. The primary exception was glyphosate applied alone, which resulted in about 60% control.

Regardless of the type of soybean planted (RR vs. LL vs. nonGMO), it’s essential to control marestail and other weeds present at the time of planting. The treatments shown above should control the other broadleaf weeds present after wheat harvest, although Ignite rates of 22 to 32 oz should be used where grasses are present.

Other considerations include cost of the seed and POST herbicides, potential for POST soybean injury, and need for POST control of marestail that emerges late or regrows following a burndown treatment. Soybeans planted at this time of the summer should not be subjected to injurious POST treatments that might result in a cessation of growth if possible, since there is limited time for regrowth to reach maximum yield potential.

Assuming use of an effective burndown treatment, some of the herbicide/seed type options are:

• Plant any type of soybean, and include a residual herbicide with the burndown treatment so that POST herbicides are not needed. A good strategy in Roundup Ready or non-GMO soybeans even where POST treatment is needed, since POST marestail control might be impossible in these systems. Residual herbicides used at this time of the year should be restricted to those that have little or no carryover risk - such as metribuzin, Valor, or low rates of chlorimuron or cloransulam products.
• Plant a LibertyLink soybean, and apply Ignite POST as needed. Probably the best option for control of later-emerging marestail or plants that regrow after the burndown.
• Plant a Roundup Ready soybean and apply glyphosate POST. Should work for most weeds, but not a good choice if the POST application needs to control marestail.
• Plant a non-GMO soybean and apply conventional POST herbicides (Flexstar, Fusion, Select, etc) as needed. This system has the most potential for soybean injury, but seed may be cheaper than the other systems. Not a good choice if the POST application needs to control marestail.