Corn growers who used a pre-emergence herbicide to manage grass pressure this year are encouraged to watch their yield monitor during harvest as a way to evaluate the effectiveness of their herbicide choice. While combining their fields, growers can determine if their herbicide program worked effectively, and if not, how that affects yield and the bottom line.

“Growers need to know if their pre-emergence grass herbicide has provided the season-long weed control that was promised last spring,” says David Saunders, product development manager for DuPont Agriculture & Nutrition. “If you have a yield monitor, survey your field to look for the weedy patches and note them.”

If growers are seeing heavy grass pressure at harvest, they may need to re-evaluate their herbicide programs.

“We suggest that a grower move toward using a reduced rate of a pre-emergence grass herbicide and following up with a post treatment that controls both grass and broadleaf weeds,” says Saunders. “This program offers weather flexibility, is long-lasting and offers the broad-spectrum weed control that is needed in their fields. It is important to not only watch your yield monitor, but to also note what grass species you are seeing. This will help you make better herbicide selections next year.”

Saunders points out that once grasses reach 4 inches or taller, they can begin to compete with the corn for moisture and nutrients which has an impact on yield. A two-pass program consisting of a reduced-rate pre-emergence herbicide and followed with a postemergence application gives the most consistent and effective grass control.

“Pre-emerge herbicides are not always dependable,” says Saunders. “Rain is needed for activation, and in dry conditions, growers will find that they do not get the reach-back activity to take down later-emerging weeds. In these conditions, harvest can be an eye-opening experience.”

A yield monitor is an effective tool for determining the true cost of weed infestation in a field. Yield monitors offer a feature called field markers, which allows growers to note anything they see that might have an impact on yield, whether that be grass pressure, wet spots or thin soil.

“When a grower comes upon a grassy patch they can simply switch on the field marker and note the patch,” says Tom Doerge, agronomy research manager for Pioneer Hi-Bred International Inc. “After harvest the grower can go back and compare the field marker map with the yield map and identify how grass pressure has impacted yield. Until we had yield monitors, no one understood the normal variations of yield in fields. We now have the tool to allow us to accurately identify where grass and weed pressure is impacting yield.”