Wet weather and delayed planting have several effects on no-till burndown programs, especially in soybeans. First, the weeds get bigger and what is a relatively tame burndown situation in April can become pretty hairy by the first two weeks of May. With this and the price of glyphosate in mind, the rate of glyphosate in any glyphosate-based burndown treatments should be increased, preferably to 1.5 lbs. acid equivalent (ae)/acre. Second, in the rush to plant when it finally dries out, it can be difficult to keep 2,4-D in the burndown mix and wait yet another seven days to plant soybeans. Here’s a review of soybean burndown options for this situation.
One option is to just omit the 2,4-D ester and rely on glyphosate for the burndown. This is probably the worst choice on this list, but it can work in some fields. This is not an option in fields with marestail or glyphosate-resistant ragweeds. Use a glyphosate rate of 1.5-2.2 lbs. ae/acre. We recommend applying with a residual herbicide that contains chlorimuron (Canopy/Cloak, Valor XLT, Envive, Authority XL) to improve control of dandelions and many summer and winter annual broadleaf weeds (but not ALS-resistant marestail or ragweeds). The mixture of glyphosate and chlorimuron or cloransulam is likely to be variable for control of even ALS-sensitive marestail by the time we can get into fields.
Keep 2,4-D ester in the mix with glyphosate and wait another seven days to plant. Plant the corn acres first and come back to soybeans to allow time for this. Have the burndown custom-applied if labor or time is short.
Apply a combination of glyphosate, Sharpen (or other saflufenacil product) and MSO. Major drawback to this is that saflufenacil cannot be used in combination with residual products that contain flumioxazin (Valor) or sulfentrazone (Authority/Spartan). Best options for residual herbicide in this mixture, especially where marestail are present: metribuzin or Canopy/Cloak DF + metribuzin, making sure that the metribuzin rate is at least 0.38 lbs ai/acre. A spray volume of at least 15 gpa should be used and it is essential to use MSO as the adjuvant.
Use Ignite (32-36 oz/acre) for burndown. One benefit of this compared with option 3 is that Ignite can be applied with any residual herbicide. The addition of metribuzin can improve control, especially under cloudy, cool conditions. A spray volume of at least 15 gpa should be used. Avoid use of nozzles that produce primarily large droplets.
Substitute tillage for burndown herbicides. Make sure that the tillage is deep and thorough enough to completely uproot weeds. Weeds that re-grow after being “beat up” by tillage are often impossible to control for the rest of the season.
Some things that probably will not work
We have not had much success with combinations of glyphosate and 2,4-DB, although high rates of 2,4-DB can help control certain weeds. Gramoxone usually works well only when combined with both 2,4-D ester and metribuzin, especially when weeds have much size. Valor and Authority do not have enough foliar activity to help control emerged weeds, although they do cause contact herbicide symptomology on weed leaves. Aim and Cadet also do not have enough activity on most weeds typically found in burndown situations, with the possible exception of small lambsquarters. Metribuzin does help control emerged weeds, although this is expressed best in mixtures with Ignite or Gramoxone.
The cost of several of these options can be about twice the cost of a typical glyphosate/2,4-D mixture. This is not really the situation to balk at spending a few extra dollars. Failing to effectively control weeds at the time of no-till soybean planting creates problems for the rest of the growing season.