THE BEST time to apply glyphosate to control early season weeds is when weeds are only 4 in. tall. Unfortunately, many producers wait too long to spray and end up losing important crop yield.

“If any weed population is present, it is critical to remove them when they reach 4 in. of height. For corn, that syncs well with the V4 growth stage,” says Jeff Gunsolus, extension weed science specialist at the University of Minnesota.

After that time, weeds can grow rapidly — as much as 1 in./day. “So in the course of a week, weeds can be up to 12 in. tall, and they are already starting to use quite a bit of nitrogen. In that time period, weeds can use 30 to 60 lbs. of nitrogen per acre that is not available to the corn plant,” Gunsolus adds.

Studies indicate that, after weeds reach 4 in. and are not controlled, corn yield drops about 3 bu./acre/day. “There is some potential to bring those nitrogen levels back up through sidedressing, but it is often not cost-effective,” Gunsolus says. “It's best to protect the nitrogen that is out there by controlling the weeds.” He recommends using a preemergent weed-control program with glyphosate. — Mark Moore