Shoulda, coulda, woulda may be the theme if you didn’t use a pre-emerge herbicide, and rain has kept you from returning to spray. That could get to be expensive.

In soybeans, you have from 9 to 19 days after soybean emergence before losing yield. Expect about a 2% yield loss for every leaf-stage delay past the critical stage of weed control, according to research by Stevan Knezevic, University of Nebraska 
professor of integrated weed management.

After 9-19 days (depending upon row width), you lose about $3.50 per day in lost yields, Knezevic says.

That loss number can be much higher, depending upon weed species, says Iowa State University Weed Scientist Mike Owen. “This yield hit from weeds more than pays for the cost of using soil-applied residual herbicide,” Owen adds. “The yield hit could be higher if you have a weed like Palmer amaranth that can grow 3-4 inches a day. Farmers tell me ‘it’s been wet, I’ll come back after things dry out and spray,’ but Mother Nature is fickle.

“This Nebraska research is very pertinent because weed development has not changed, but what has changed is weed resistance.

“Adopt a field-specific weed control plan the same way you tailor hybrids and varieties to different fields,” Owen adds. “Closely scrutinize each field differently according to its weed species and soils. A number of fields should be treated the same way, but a number need extra care and diverse weed-control tactics. Truthfully, all fields need more weed-management diversity; and that will probably be in the form of different herbicides.

“You would never risk planting all of your acres to the same hybrid, so why treat them all the same to control weeds? Rotate your chemistries based on group number and mode of action.

“Contrary to the old saying, if your weed-control program is not broken, DO change it anyhow because it will be broken,” Owen says. “Invest more time and thought in managing weeds. Farmers have to recognize it will take more time, and anything simple and convenient is ordained to fail.

“We have 60-70% fields with multiple resistances in Iowa, so there are fewer available effective herbicides. Approximately half of our crop fields have waterhemp that no longer responds to glyphosate, so you then rely on different postemerge products,” Owen says.

See the table for Knezevic’s recommendations across other row widths – they hold true up to the R3/beginning pod soybean stage. Yield losses accelerate after R3.

row spacing and weed control