When it comes to post-emergent weed control, timing is everything.

"If you let weeds stay in the field for too long, you lose yield, but if you remove the weeds early you may have to do it again later," said Stevan Knezevic, University of Nebraska weed scientist at the Haskell Agricultural Lab near Concord. "If you can get by with one herbicide application or one cultivation, it saves time and money."

Knezevic and graduate student Sean Evans researched the critical weed control periods for two years at Mead and Concord. For most of the season, crops can successfully compete with weeds for nutrients and moisture. They just need a little help for part of the season, this research found.

"Each crop has a critical period during which weeds must be controlled to prevent yield losses," he said. "Weeds that emerge before or after this period may not present a threat to crop yields."

In corn, the amount of nitrogen fertilizer applied significantly influences the timing and duration of the critical period. Less nitrogen means longer critical periods and less crop tolerance for weeds. The research showed that when no nitrogen is applied, weed control should start early in the season and continue until canopy closure, or from eight to 45 days after crop emergence. The critical period was 10 to 42 days after emergence with 55 pounds of nitrogen applied per acre and 15 to 39 days after emergence when 110 pounds of nitrogen was applied. With 210 pounds of nitrogen per acre, the period was 20 to 39 days after crop emergence.

"I compare this relationship to a bunch of corn and weeds sitting around a table," Knezevic said. "If there is plenty of food, the corn and weeds won't fight, but if there is not enough, they will fight. Corn can tolerate weed presence longer with more nitrogen."

In soybeans, row spacing influences the critical control period. The wider the row spacing, the earlier weed control is needed. Knezevic found the critical period for soybeans spaced 30 inches apart begins when plants develop their first trifoliate leaves. In 15-inch rows, the period starts at the second trifoliate stage, four to six days later than wider-row soybeans. Soybeans in 7.5-inch rows have the latest critical period onset, at the third trifoliate stage. Farmers would get eight to 12 more days to control weeds by planting in 7.5-inch rows instead of 30-inch rows.

"Crops and weeds compete for light, water and nutrients," Knezevic said. "By adjusting row spacing, you control how much light weeds get. If the leaf canopy closes faster, the weeds can't get enough light, and they won't compete with the crop as much."

The first step in applying this research to an individual farmer's fields is determining how much yield that farmer is willing to sacrifice. Knezevic based his studies on a 5 percent acceptable yield loss, but farmers can adjust up or down to fit their needs, he said. Once corn or soybeans reach the critical period, every additional leaf stage that passes before weeds are controlled costs an extra 2 percent yield. Corn leaf stages last three to five days; soybean leaf stages last four to six days. That means waiting an extra week to control weeds can sacrifice another 4 percent or 5 percent yield, Knezevic said.

Weed size at control time also is a concern.

"If the weeds are going to be taller than the crop, you need to initiate weed control four to five days before the critical period to prevent the weeds from shading the crop," Knezevic said. "It's important for farmers to scout fields for weed emergence and pressure so they can make adjustments if necessary."

– Heather Corley

Weed Scientist Stevan Knezevic cuts up corn plants and checks ears to measure growth and production for his research on weed control timing. Removing weeds too soon may require a repeat treatment later; leaving them too long can cut yields. This IANR research identified critical periods of weed control for corn and soybeans under different growing conditions.

Graduate student Sean Evans counts weeds and compares their size to the soybeans in a test plot. This IANR research shows that row spacing influences the timing of the critical weed control period in soybeans. In corn, the amount of nitrogen available to the crop is a major influence on timing.