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Reap weed control benefits with a two-pass program

Single-pass weed control programs in corn initially save time and money. However, odds are a two-pass weed control program will net you more money at season's end.

That's what University of Wisconsin scientists found as part of 2002-2003 on-farm trials they conducted called the "Two-Pass Challenge." They worked with Wisconsin farmers in 33 trials in comparing one-pass weed control to two-pass weed control in corn. In 20 of the 33 trials, farmers were dollars and yields ahead by going with the two-pass program.

Single-pass drawbacks
"In Wisconsin, the majority of herbicide programs rely on a single pass, whether it is preemergence or postemergence," says Richard Proost, a University of Wisconsin regional agronomist who reported on the findings at the recent Integrated Crop Management Conference at Iowa State University. "The reason is the time it saves and it's also cheaper than a two-pass program."

However, single-pass programs have serious drawbacks. One-pass preemergence concerns include:

  • Effectiveneness only on emerging weeds
  • Poor control on most perennials
  • Limited herbicide options on large-seeded broadleaf weeds like giant ragweed
  • Need for a burndown herbicide in no-till systems
  • Reduced efficacy from a lack of activating rainfall

Drawbacks of single-pass postemergence programs include:

  • Need for a burndown in no-till systems
  • Weather impacts on weed efficacy
  • Crop injury
  • Issues with weed flush timing

"Weeds don't come up all at the same time," says Proost. "In Wisconsin, at least, farmers tend to wait (to apply herbicide) until most of the weeds are up. When that happens, you have lots of competition between weeds and the crop going on. This results in yield loss."

Two-pass perks
Advantages of using a two-pass program include:

  • Offsetting the weaknesses of one program with the strength of another
  • Lengthening the application window for postemergence herbicides
  • Rotating herbicide modes of action is easier
  • Lowering crop injury risk
  • Improving weed control

However, two-pass control also increases time, labor, and cost. "There's also a greater risk if management is lacking," says Proost.

In the UW's Two-Pass Challenge, farmers could use a number of combinations in their two-pass programs, including a preemergence application followed by a postemergence treatment, or a preemergence or postemergence application followed by a cultivation. As expected, average two-pass program costs were higher than single-pass program costs, $40.25 per acre and $33.05 per acre, respectively.

However, 20 out of those 33 trials had yield increases large enough to outweigh the increase in costs. Of those 20 trials, 12 tallied returns more than $20 per acre. Of the 13 trials with no yield increase, four had losses of $20 per acre. On average, the net benefit of the two-pass program across all trials was $13.50 per acre, as it increased corn yields on average by 8.5 bushels per acre.

Although they're gathering rust in many tree lines and farm sheds, cultivation can still play a role in weed control. In the 20 yield-increase trials, growers cultivated in 11 of them. Of the 13 that did not have a yield increase, just one grower cultivated.

"From a weed management standpoint, I don't know of any weed that's resistant to steel," says Proost. "There's also a cultivation effect. Research indicates than when cultivation is done properly, cultivated plots perform better. You are aerating that soil, breaking that crust, if you do it right. If you don't do it right, though, you can get lots of root pruning."

When two-pass pays to use
In Wisconsin, Proost says single pass programs can still work if targeted on low-risk corn fields. "Make sure the weed pressure is light, and you have no problem weeds like Canada thistle, giant ragweed or crabgrass," he says.

But on fields with moderate to heavy weed pressure, two-pass programs have plenty of perks.

"The increased costs of a two-pass program can be offset by increased crop safety, weed control, and yield," says Proost. "It can provide consistent long-term weed management, and can help control herbicide-resistant weeds."

Single-pass weed control programs in corn initially save time and money. However, odds are a two-pass weed control program will net you more money at season's end.

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