Over the last year, I’ve noted an increase in questions on controlling weeds in conventional soybeans. It appears that we are seeing at least an uptick in acres that are planted to conventional varieties. In the spring of 2014, the driver was federal refuge land where GMOs could not be planted (see Farmers slowly pushed from refuge).
More recently, the driver has been lower cost of conventional soybean seed when compared to Roundup Ready and LibertyLink varieties. In talking to some local seed sales folks last month, this trend appears to be gaining steam. They told me they were sold out of conventional varieties due to unexpected demand.
The thing that is most notable in this trend from a weed control perspective is that the weed spectrum the growers were struggling to control has changed. It is not Palmer amaranth they ask about. These growers are running programs designed for pigweed control by utilizing overlapping residuals. Large-seeded broadleaf weeds like cocklebur and sicklepod gave them the most fits to control.
Everyone is pretty much tuned in on how to manage Palmer pigweed in soybeans. It does not always work due to weather or application timing issues, but overlapping residuals with good pigweed activity has, for the most part, been successful. Herbicides that are leaned on very heavily for pigweed control in that system — like Valor, Dual and Zidua — do not provide much help on large-seeded broadleaf weeds. This is not a problem in Roundup Ready or LibertyLink soybeans because glyphosate or Liberty will readily control weeds like cocklebur and sicklepod.
However, in a conventional program they are not an option, and once cocklebur and sicklepod get some size, they are very difficult to control. Everyone knew that 15 years ago and in conventional soybeans we are relearning it.
This points out two things. First, unlike pigweed, cocklebur and sicklepod have long-lived seed that can lay dormant in the soil for over a decade to later become a problem. Second, the Roundup Ready and LibertyLink technologies offer some value to growers on the large-seeded broadleaf weeds that we all have tended to take for granted.
Cocklebur and sicklepod
It was not easy to control sicklepod and cocklebur prior to Roundup Ready, but it can be done. As for cocklebur, most of it is ALS-resistant, so metribuzin pre- followed by Storm or Reflex early-post is the best herbicide options. Sicklepod is not ALS-resistant, but Scepter and Pursuit were never very effective on it.
According to research by Bob Hayes, Classic and FirstRate are both good options for sicklepod applied either pre- or post. Classic or FirstRate is often added in pre-mixes with our main Palmer herbicides like Valor and Authority.
If you go the pre-mix route, be sure to follow the label and go with the higher use rates for the given soil type to be sure to have enough Classic or FirstRate to provide consistent control. As with all pre- applied herbicides, control may not be adequate and an early post application of Classic or FirstRate may be needed.
I believe going back to conventional soybeans on some acres is a viable option. Herbicide costs for the conventional system will be about as expensive as for the Roundup Ready or LibertyLink systems. Seed costs should be anywhere from $10- to $40-per-acre less expensive, depending upon whether you are pulling them out of your bin or buying from a seed company. The biggest limitation to planting conventional soybeans is obtaining good-yielding varieties, particularly Group IVs.
From an herbicide-resistance management perspective, it makes sense to rotate technologies.
Finally, non-herbicide options like narrow soybean rows, higher soybean planting populations, tillage and cover crops can help manage weeds regardless of the herbicide technology in the soybean.
email Larry Steckel at firstname.lastname@example.org