As commodity prices have declined in the last few years, growers have shown interest in planting conventional soybeans as a way of cutting input costs. While conventional soybean seed is less expensive than herbicide-resistant seed, planting conventional soybeans also means changing up your herbicide program. Last year, Amit Jhala, Nebraska Extension weed management specialist started a project to explore the effectiveness of different herbicide programs in conventional soybeans, which he's continuing this year at the South Central Ag Lab near Clay Center.
"There has been a lot of interest in the last couple years. I think No. 1 because of the increasing number of glyphosate-resistant weeds we have. If a weed is glyphosate-resistant and is predominant in the field, there is no utility of glyphosate-resistant corn or soybean in that field, because you aren't going to be able to control that weed with glyphosate," Jhala says. "Another reason is with low commodity prices, growers don't need to spend much on seeds or pay a tech fee. They have to invest a little bit more in the herbicide program, because now you don't have the option to apply glyphosate."
Because postemergence herbicide options are limited in soybeans, the most important component of conventional soybean herbicide program is a good preemergence herbicide with multiple effective modes of action.
"A preemergence herbicide must be applied as soon as soybeans are planted. That will provide 25 to 35 days of residual activity, depending on which herbicide is used and weed pressure in the field," Jhala says.
For example, there are several premixed herbicides that provide residual control up front, including Zidua Pro, Trivence and Fierce XLT, all of which have three active ingredients. "There are seven or eight different options for herbicides, with at least three different active ingredients," Jhala says. "They provide broad-spectrum weed control, and in my opinion, can provide at least a month of residual activity."
Postemergence option needed, too
The next step is finding a good postemergence option, and those options are limited when it comes to soybeans. So, the best option, Jhala says, is ALS chemistries like Pursuit, Classic, etc., or PPO inhibitors like Cobra, Ultra Blazer or Flexstar.
Tank-mixing a postemergence herbicide with a residual herbicide provides overlapping residual activity — for example, tank-mixing Cobra for foliar control with Warrant for residual control.
"The idea is to overlap residual activity, and then the foliar activity will be provided by the postemergence herbicide you are applying," Jhala adds. "Using this approach, with an effective pre and following up with post, plus a residual herbicide for overlapping residual activity, you can easily cover the first eight to nine weeks of soybeans. By that time, the soybeans will close the canopy."
Of course, using a PPO inhibitor means you can expect some herbicide injury, although in most cases, this injury is temporary and won't be noticeable after a couple weeks. In addition, it's best to apply PPO inhibitors when weeds are 5 inches tall or less to ensure optimum control.
Jhala notes overlapping residuals may not be necessary for every field. It's a good idea to scout a few weeks after the preemergence application to see if there is any need for additional control, or double-check the weed history of that particular field. "Tank-mixing a postemergence herbicide with a residual product is not a rule of thumb, but can be used as needed if the field has a lot of glyphosate-resistant waterhemp or Palmer amaranth," he says. This is because both of them have a longer emergence period and can be extremely competitive if not controlled.
Growers should also consider the cost-benefit ratio of using conventional soybeans, and the change in their herbicide program. "Growers should consider the cost of herbicide-resistant seed and the tech fees they're paying compared to conventional seed," Jhala adds. "The cost of seed will be much less, and growers may be able to make up the added cost of the herbicide program required for conventional soybeans."