April 13, 2004

Soybean farmers should tackle weeds to protect yields

by K.C. Jaehnig

CARBONDALE, Ill. -- You can kill those giant weeds in your bean fields, but the bigger they get, the harder your fall.

"We've found at least a four bushel-per-acre yield difference (in fields with tall weeds)," says Southern Illinois University Carbondale weed expert Bryan G. Young.

"If you get $7 a bushel at harvest for your beans -- and that's fairly conservative -- that yield difference is costing you $28 per acre."

Hit those weeds while they're small, he advises, and don't be disappointed if you have to hit them again later.

"I don't want to sound like a herbicide pusher, but the best way to manage weeds is a two-pass application program," he says. "Over the last five years we saw a 10-percent yield increase with a two-pass program compared to a one-pass total-post program."

While farm chemicals haven't spawned a new race of King Kong weeds, the way they're managed -- especially in soybeans -- accounts for why farmers are tackling bigger weeds than they once did. Soybean farmers no longer spray before the weeds pop up and again shortly afterward to get the weeds they missed the first time. Instead, they often wait until the weeds are nearly as big as the beans before heading out to the fields.

"Sure, you can kill a 3-foot-tall weed with something like Roundup, and if you can get away with one application instead of two, it's cheaper, but that's short-term economics," Young argues.

"Gains in weed control help protect the yield. It's an investment to protect productivity, not just an expense. It's a way of gaining money, not just spending money."

And weed killer is cheap, Young notes, coming to about what a farmer gets for a bushel of beans. "With the cost of herbicide right now, there's no sense in risking that 10 percent yield difference," he says.

Timing is everything when it comes to spraying because weeds, like children, grow dramatically.

"As you get past that 4- to 5-inch stage, you can have an inch of weed growth in a single day," Young says.

Yields can drop steadily after weeds pass the 8-inch mark.

"On one research trial in 2003, soybean yields were reduced by 0.7 of a bushel per acre for every inch of weed growth," Young says.

"That's a potential profit loss of at least $5 per inch of weed. This may not happen in every grower's field, but it does demonstrate the risk we take when weeds interfere with crop productivity."