Once again, horseweed is rampant in a lot of row crop fields across the Delta this spring. In some situations, horseweed survived low doses of 2,4-D.
There are many formulations of 2-4-D and active ingredient concentrations range from 1.74-pound 2,4-D acid per gallon (Unison) to 5-pound 2,4-D acid per gallon (Salvo). It is important to adjust the amount of product applied based on active ingredient concentration.
The cold weather this spring has also reduced overall burndown control in some situations. Not utilizing a residual product like Valor, Canopy EX, or another residual product in the burndown program allows a spring flush of horseweed to emerge between the burndown and planting.
It has essentially become imperative to use residual herbicides either in the fall or in the spring with the burndown program to control horseweed.
There are few good options to control horseweed once we get too close to soybean planting to use 2,4-D. For most 2,4-D formulations, 15 to 30 days between application and planting soybean is required.
For the 2,4-D rates we should be using in our burndown programs, we should apply 2,4-D at least 30 days prior to planting soybean.
Even though dicamba (Clarity or Banvel) is a little better on horseweed than 2,4-D, dicamba is even more injurious to soybeans than 2,4-D.
Our most consistent option for controlling horseweed if it is too close to soybean planting to use 2,4-D is Ignite 280 SL herbicide.
Firstrate herbicide is also a good option, but control of large horseweed close to planting can be inconsistent, depending on the size of horseweed and when the horseweed emerged (fall or spring). Firstrate is an excellent, and in a lot of cases about our only, in-season option to control horseweed, morningglories, and other broadleaf weeds in the crop.
Firstrate also provides in-season residual control of several broadleaf weeds in soybean.
Things to keep in mind when using Ignite 280 SL to control horseweed close to planting.
Plantback interval for corn, cotton, soybean, and rice: 0 days.
Warm weather after application is important: Day and nighttime temperatures for several days after application should be in the 70s and 50s. Cooler temperatures can reduce control.
Rate range: 29 to 40 fl ounces per acre. The 32-ounce per acre rate has worked well on 4- to 8-inch tall horseweed. Higher rates should be used on larger horseweed. We have been applying 36 to 40 ounces per acre on horseweed larger than 18 inches tall. It is expensive, but it has been consistent on really large horseweed.
Spray coverage is critical. Applying 15 to 20 gallons of spray solution per acre using flat fan, hollow cone, or twin-jet nozzles works extremely well. Using 10 gallons of spray solution per acre is the absolute bare minimum and control can be inconsistent. Keep in mind, we often get only one shot at controlling horseweed this late before planting and Ignite 280 SL is not cheap. Water is cheap, the more the better.
If 10 gallons is applied, be sure to use one of the above-mentioned spray nozzles. Air induction nozzles are not recommended, but if they are used be sure to use one of the above-mentioned nozzles and spray at pressures of at least 70 psi so that spray droplets are busted up and as small as possible.
Ammonium sulfate should be added at 8.5 pounds of ammonium sulfate per 100 gallons of spray solution.