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Corn+Soybean Digest

Weed Size Matters

Size does make a difference, at least in the relationship between weed size and herbicide rates.

For years, weed scientists have advised that scouting fields early to identify weeds gives a couple of herbicide rate options. You can hit the small and easier-to-kill weeds with the full labeled rate, stacking the deck in favor of excellent weed control. Or you can trim herbicide rates, assuming more weed risk and perhaps product performance while striving for a more cost-effective match between dollars and dead weeds.

The choice depends on your management style, scouting ability and available time to walk fields.

Most agronomists define small weeds as those in the 2-3" range. Identifying weeds that small means scouting early and often.

"You need to monitor weeds on a weekly basis from planting," says Bill Johnson, Missouri extension weed scientist.

If you're faced with tough weeds, consider checking emerging weeds and the crop every three or four days, he adds.

Johnson advises frequent early scouting whether you're using post-only programs or those with a mix of pre- and post materials. Herbicide prices for 2000 provide a "lot of value in the market" and offer several weed control choices, he says.

A good example of how it's possible to cut rates on smaller weeds is Johnson's work with Roundup. By hitting weeds early, he's been able to cut Roundup rates in half, down to 1/2 pint/acre in each of two post applications.

However, he quickly cautions that this is a "grower risk" treatment. It's perfectly legal to apply less than the labeled rate of a herbicide, but you'll assume performance liability. And with Roundup you won't be eligible for the company's marketing guarantees.

Agronomists point out that you need to know your weed species and herbicide label recommendations before trimming rates. You may get by with lower rates on annual grasses and sensitive broadleaves, but stay with higher rates for tougher broadleaves.

"Don't let the weed size approach the crop size," warns Jeff Rawlinson, a University of Nebraska assistant weed specialist. "Once weeds have caught up to the crop, you've had yield loss. That's easy to happen in soybeans.

"In a 6" corn crop, weeds 2-3" tall may not be an immediate threat to yield," says Rawlinson. "But you don't want to miss the opportunity to kill weeds when they're small." ?

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