With herbicides, it’s no rain, no gain

COLUMBUS – Pre-emergence herbicides are on the ground, but not a drop of rain has fallen in some corn-growing areas of Ohio and Indiana.

Weed specialist Mark Loux, Ohio State University, said generally, herbicides applied at planting need a half inch to 1 inch of rainfall within seven to 10 days, and more rain is needed in overly dry soils.

“Lack of rain is likely to result in some pre-emergence herbicide failures,” Loux said. “While we recognize the importance and economics of pre-emergence corn herbicides, we question the wisdom of applying herbicides when no rain is in the forecast.”

Herbicides needing the least moisture for activation, in order, are: Balance; Harness, Topnotch; Degree, Outlook; Dual II, Axiom, atrazine, Python, Hornet; simazine; Prowl.

If dryness continues, farmers should manage their applications to get the best results possible, Loux said.

Guidelines to go by.

The following guidelines generally pertain to atrazine premix products, with some exceptions:

* Incorporating a herbicide can reduce its dependence on rainfall. The best incorporation implement has S-tine shanks; the poorest is the disk.

The effective depth of incorporation should be no more than 1-2 inches, or about half the depth of soil movement from the implement.

* Delay application at planting. Wait for a forecast calling for rain, then apply a day or two before rain. Atrazine will control most small weeds that have emerged, while rain will activate the herbicide to control not-yet emerged weeds.

* Try an early post-emergence treatment with corn emergence and the first flush of weeds when grasses are no more than 1 inch tall.

The atrazine rate should be 1.5-2 pounds of active ingredient per acre for grass control. Add Accent or a similar product if grass is more than 1 inch tall.

Do not apply Balance, Epic, Axiom or simazine after corn has emerged.

* Switch to a post-emergence program, such as Basis gold, Accent Gold or Celebrity Plus, applying the product before grasses are 3-4 inches tall.

Products with some residual activity, or adding atrazine, will help control weeds that emerge after the application.

* When dry weather continues 7-10 days after herbicide application, pull out the rotary hoe, Loux said.

“The rotary hoe is most effective on weeds that are just about to emerge from the soil, rather than on emerged weeds,” he said. “This is sometimes called the ‘white stage.'”

* Keep track of emerging weeds when rainfall is insufficient to activate herbicides. Applying post-emergence herbicides when weeds are small can reduce both rates and costs, Loux said.