This is essentially a repeat of an article that ran last year at this time. Little has changed since then with the exception that Liberty has been even harder to procure this year, so using a glyphosate+Sharpen burndown may be more realistic.
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; and 3) marestail that were tall enough to be cut off by harvesting equipment will be even more difficult to control.
Our research indicates that there are no herbicide treatments that consistently control glyphosate-resistant marestail populations that have regrown following mechanical disturbance or prior herbicide treatment.
Certainly one of the best options is to plant LibertyLink soybeans, which allows for a POST application of Liberty to help control plants that survive a preplant burndown. The following are the most effective burndown options for control of marestail prior to double-crop soybean emergence:
Liberty (32 to 36 oz) + Sharpen (1 oz) + MSO + AMS (can also add metribuzin)
Liberty (32 to 36 oz) + metribuzin (4 to 8 oz of 75DF) + AMS
Glyphosate (1.5 lb ae/A) + Sharpen (1 oz) + MSO + AMS
We suggest using a spray volume of 20 gpa for any of these treatments, and avoiding nozzles that produce large droplets. Results with a combination of glyphosate and 2,4-D may be more variable then the treatments listed.
With regard to the control of weeds that can emerge after double-crop soybean planting, and the entire weed control system, the following approaches can be considered.
1. 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.
2. Plant a LibertyLink soybean, and apply Liberty POST as needed. Probably the best option for control of later-emerging marestail or plants that regrow after the burndown, assuming that there is any Liberty available.
3. 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.
4. 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.