The popularity of Roundup Ready soybeans has made neighboring cornfields susceptible. The chemical drift can be deadly when it lands on non-tolerant corn hybrids.
Surprisingly, off-target drift problems have been minimal, maybe because farmers know what Roundup can do when it drifts where it does not belong.
Nevertheless, scientists caution that violating prudent precautions can get you in trouble - perhaps with your neighbor.
"There's no question that if any amount of Roundup drifts into cornfields there's going to be damage," says Southern Illinois University weed specialist George Kapusta. He has three years of data to prove the point.
"Our data show that the earlier the application, the worse the damage," says Kapusta.
"We applied Roundup Ultra to corn at the V3 stage - that's approximately 10 days after emergence - and at the R1 stage, about 20 days after emergence," he says. "We used a 3/4 pint (12-oz)/acre rate and varied the percentage of herbicide that contacted the crop. When 20% or more of the 3/4 pint/acre rate was applied to the corn, we started to get substantial damage. That's the same as 10% of a 11/2 pint/acre rate that most farmers used. The earlier the herbicide was applied, the more severe the injury."
In other tests, however, larger corn was damaged more than the small stages. University of Missouri agronomist Don Null applied Roundup Ultra to 4-, 12- and 36"-tall corn at 2-, 4-, 8- and 16-oz/acre rates in 1997.
All corn treated with the 16-oz rate performed poorly in Null's tests. The 8-oz rate dropped the 36" corn yields to 15.5 bu, while the 4" corn yielded about the same as the field check, 82.3 bu. The 4-oz application rate affected only the 36" corn, and the 2-oz rate didn't affect yields at all.
"Normally, you would expect Roundup to be more injurious on the smaller plants," says Monsanto's Roundup specialist Bill Parker. Neither of the scientists can explain the differences, either.
It's obvious from the results that, under different environmental conditions, Roundup drift can damage corn at a number of growth stages. The best plan is to avoid drift in the first place.
"To minimize drift, we recommend farmers apply Roundup in 10 to 20 gallons per acre of carrier and run their pressure from 25 to 35 psi," says Parker.
"You want to use spray nozzles that keep droplet size in the 400 to 800 micron range. Some of the low-drift nozzles available are Driftgard, Turbo TeeJet and Turbo FloodJet."
"The best recommendation for minimizing drift is also the least popular," says Kapusta. "That's don't spray if it's windy. If there's a 5- to 10-mph wind, I doubt if you're going to get much more than cosmetic damage to corn. The particles that will drift then are too small to carry enough herbicide to do significant damage. It's the gusts of wind that carry larger droplets that will cause yield damage."