Marestail is still the most problematic weed for soybean producers.
In our fall weed surveys, it has been the number one weed to survive herbicide programs for the past five years. To combat this weed, farmers have to change their reliance on glyphosate and herbicide programs applied after soybeans have emerged (called post programs).
Marestail may emerge in the fall and survive as a winter annual or it may emerge in the spring as a summer annual. Farmers must have it controlled by planting time since post programs will not kill it. The new dicamba products will work post programs for Extend soybeans, but farmers have to be concerned about dicamba moving to other susceptible crops.
Ideally, farmers would have applied herbicides last fall to get any fall-germinating marestail. However, farmers may not have applied fall herbicides because of the extra input cost, the potential injury to certain cover crops, and limited time for field operations.
Farmers have often added 2,4-D to their spring burndown programs for marestail since glyphosate is ineffective on large plants. A burndown program is the use of herbicides to kill weeds prior to or at planting.
Farmers may not want to use 2,4-D because the herbicide label requires a seven-day waiting period between application and planting.
However, by May, overwintered marestail may be large enough that even a mixture of glyphosate and 2,4-D is ineffective and farmers have to add additional herbicides, such as Sharpen and metribuzin. A farmer’s burndown program still has to control other weeds in addition to marestail.
Many farmers have already been able to plant soybean fields. However, as rains delay planting, farmers may have to make adjustments to get larger weeds, especially marestail.
Dr. Mark Loux, Ohio State Extension weed specialist, has the following recommendations for farmers with later plantings and large weeds:
1. Increase glyphosate rates to at least 1.5 pounds active equivalent per acre. This will not improve marestail control, but should help with most other weeds.
2. Where at all possible, keep 2,4-D ester in the mix, even if it means waiting another seven days to plant soybeans. Apply 2,4-D to soybean fields and plant cornfields while waiting the seven days. If labor or time is short, have the burndown applied by a commercial operator.
3. To improve control with glyphosate/2,4-D, add Sharpen. Deciding to include Sharpen can result in a need to alter the residual herbicide program.
Labels still allow mixtures of Sharpen with residual herbicides that contain Valor, Authority, or Reflex, but only if applied two or more weeks before planting. Farmers use residual herbicides in the burndown to control germinating weeds after planting.
Metribuzin-containing herbicides can be used in place of the above residual herbicides. For these products, such as Canopy-metribuzin mix or Zidua Pro/metribuzin, the metribuzin fraction needs to be at a rate equivalent of 8 to 12 ounces per acre of metribuzin 75DF.
4. Instead of glyphosate, a farmer may consider Gramoxone or glufosinate for burndown. Gramoxone is less effective than glufosinate on marestail, but the mix of Gramoxone/metribuzin/2,4-D controls marestail well unless weeds are big. Farmers that drop 2,4-D because of the seven-day wait will only get fair weed control.
Glufosinate is less effective against dense, large weeds in no-till burndown situations, and should be applied with metribuzin and 2,4-D. Higher-labeled rates should be used and a spray volume of 15 to 20 gallons per acre.
A consideration here is that in large no-till weed situations, high rates of glyphosate typically have more value than high rates of Gramoxone or glufosinate, with the exception of glyphosate-resistant weeds.
5. Among all of the residual herbicides, chlorimuron contributes the most activity on emerged annual weeds and dandelions. This is probably most evident when the chlorimuron is applied as a premix with metribuzin, such as Canopy/Cloak DF.
The chlorimuron may not control marestail, since many populations are resistant to chlorimuron. This is also true for FirstRate. However, First Rate has activity on emerged ragweeds and non-resistant marestail. Some residual herbicides will reduce the activity of chlorimuron and First Rate.
6. Tillage may be used in place of burndown herbicides. Tillage must be deep to uproot weeds. Weeds that regrow after being “beat up” by tillage are often impossible to control for the rest of the season. Tillage tools must also uniformly disturb the upper few inches of the soil surface to control shallow emerging weeds such as marestail.
Farmers have already planted many soybean fields. Periods of rain may cause some fields to be planted later this month. Weeds may become large because of this delay. Farmers may adjust their herbicide strategy to control these weeds, particularly marestail.
Lentz is extension educator for agriculture and natural resources for The Ohio State University Extension Service in Hancock County. He can be reached at 419-422-3851 or via email at firstname.lastname@example.org.
Lentz can be heard with Vaun Wickerham on weekdays at 6:35 a.m. on WFIN, at 5:43 a.m. on WKXA-FM, and at 5:28 a.m. at 106.3 The Fox.