URBANDALE, Iowa - Heavy rainfall and flooding have delayed planting for some farmers, and delayed replanting for others. Now it appears it may hinder the proper application of glyphosate, Iowa’s number one soybean herbicide.

Weeds will always grow, even in adverse, environmental conditions, and they are very competitive with the soybean crop. Once weeds become competitive, they can quickly rob producers of 1 percent of their yield per day. The Iowa Soybean Association (ISA) recognizes the need for glyphosate stewardship, including managing weed populations, not just killing weeds.  It also includes knowing when to apply glyphosate for best results.

According to Mike Owen, Iowa State University professor of agronomy and weed specialist, there are two reasons why the timing of the application is so important.

“First, the larger the weed is, the more difficult it will be to control,” Owen says. “If you wait to spray and plant, the weeds that exist in the fields will become inordinately large and extremely difficult to control. Second, the longer you wait to control weeds after the crop emerges, the less yield potential you’ll have, and losses build pretty quickly at this time of the growing season.”

However, he says it is still very important to observe appropriate environmental stewardship.

“Despite the problems we are experiencing getting the crop planted and appropriate weed control tactics established, spraying during conditions that are environmentally questionable is not appropriate and will potentially cause more problems than waiting for better conditions,” he says.

When applying the glyphosate, Owen says it is important to understand what weeds you have to determine how much glyphosate to use for best results.

“Identify the weed species that occurs in the field and adjust the glyphosate rate for the species that is largest and most difficult to control with glyphosate (refer to the label),” Owen says. “Now is not the time to try and save money on glyphosate by cutting the rates. Penny wise, pound foolish! Poor control now on larger weeds will cost more money in lost yield and expenditure of more time than the anticipated savings by using low rates of glyphosate.”

Owen also says combining glyphosate with insecticides will not give producers much of an advantage with soybeans.

“Consider carefully whether other pesticides should be included with glyphosate,” Owen says. “The tendency is to wait with the herbicide until insect or disease problems are established and then spray. You should apply the pesticides at the optimum time for the respective pesticide; in the case of herbicides, spray earlier rather than waiting for the insects of diseases and then apply a tank mixture with the herbicide.”

He also suggests not applying glyphosate in the evening hours.

“The evening is not the best time to apply, even if you increase the rate of glyphosate,” Owen says. “Plants are more sensitive when they are in the sun.”

To learn more about glyphosate application and stewardship, check out to ISA’s Production Research Web page:
to listen to a podcast entitled, “Glyphosate Stewardship.”