Knowing the location and the amount of weeds present could someday affect how you buy and apply herbicides.

Ever wish you could stump one of those smooth-talking chemical salesmen at those Styrofoam cup and paper plate winter meetings?

Next time, ask him how field topography affects his herbicide's performance.

Hmmmmm. That's enough to confuse even the sharpest Florida presidential voter.

Location, location, location. Unless you farm in flatiron areas like the Red River Valley of Minnesota or North Dakota, your fields likely consist of level areas interspersed with gently rolling hills and valleys. Researchers term these spatial features the field's topography.

While spraying, you probably assume that the herbicide will kill weeds throughout the field. Yet weed populations and types vary widely according to field topography. Knowing the location and the amount of weeds present could someday affect how you buy and apply herbicides.

“It's especially critical for non-residual herbicides like Roundup and Liberty,” says Gregg Johnson, a weed scientist at the University of Minnesota Southern Research and Outreach Center at Waseca. “What we see is that the efficacy of these herbicides depends upon application timing and weed density in different areas of a field.”

Variation in weed growth, weed species. Johnson and fellow researchers Nick Schneider and Bev Durgan launched a study last year to assess how and where weeds grew in 12 low, high and flat field sites.

Certain weeds, such as common ragweed, thrived in low areas. In low areas, common ragweed density tallied 80 plants/sq. m. On hilltops, ragweed numbers dipped to 5 plants/sq. m.

This wasn't surprising, because common ragweed loves lush, wet low-lying areas.

“What did surprise us is that the growth rate at the bottom of a hill is much less than it is on the top of the hill,” Johnson says.

“Although the germination rate of those weeds [at the bottom of a hill] is high, they are not growing very fast. At the top of a hill, where density isn't as high, weeds are growing at a very fast rate.”

Not only does weed growth vary according to topography, but so do weed species. Although ragweed ruled the low areas, common lambs-quarters growth was more dense in high areas.

Field topography also affects crop competition, which is a natural weed-control tool. South Dakota State University researchers found that in wet years excess water in low areas stunted crop growth. As a result, weeds in those areas became more competitive. Meanwhile, crops in high field areas thrived and overtook weeds because soils dried sooner.

The opposite occurred in dry years. Weeds proliferated in high areas, due to lack of crop competition. However, crops stymied weeds in low areas that retained moisture.

Broadcast isn't best. If weeds and crops grow at different rates and in different areas of the field, a one-size-fits-all herbicide application won't give you the best control.

For example, postemergence herbicides work best when foxtail reaches a 4-in. height, Johnson says. However, foxtail reaches this height at different times in high and low field areas. U of M researchers found that by the time foxtail reaches a 4-in. height in low areas, it tallies 11 in. in high areas.

At this height, herbicide effectiveness decreases. In addition, significant yield loss may have already occurred, due to decreased crop competition keyed by the tall foxtail.

SDSU researchers considered differences like these in a 1998 study in which they used a weed-control program developed by the University of Nebraska called WeedSoft. First, scientists selected high, low and flat field areas. They then selected the most prevalent weed in each location and the best herbicide to treat it under preplant, postplant and pre-post applications. The researchers then compared these treatments against a broadcast herbicide applied by a farmer across the field outside of the targeted areas.

Yields were similar between the computerized treatments and the farmer's broadcast application. However, the targeted topographical applications had lower herbicide costs and better net returns in most cases. Where common ragweed ran rampant, the topographical applications boosted both yield and return.

These findings may not be in synch with “real world” spraying conditions, says Sharon Clay, an SDSU weed scientist who coordinated the study. Unlike researchers, farmers won't treat each field's dip or rise with a different herbicide via multiple timing. In today's era of 90-ft. spray booms and increasing acreages, the last thing farmers want to do is slow down and reload chemicals, Clay says.

Herbicide selection also hinges on the myriad of marketing programs offered by chemical manufacturers. “It's difficult for those of us in research to come out and say this is the bottom line, because you don't know all the marketing programs that farmers and co-ops use,” Clay says.

Multiple treatments? However, differing weed growth patterns may prompt growers to consider multiple treatments, Johnson says. For example, a total postemergence program could enable common ragweed in low-lying areas to gain a foothold prior to herbicide application.

“What I'd do is lay down a preemergence herbicide to keep those weed populations on the bottom in check. Then, I'd come back with a postemergence application to clean up those taller weeds on the hillsides,” Johnson states.

Knowing relationships between field topography and weed growth may also enable researchers to design postemergence spot spraying programs.

“Eventually, we'll be better able to understand the drivers that give rise to spatial weed growth,” Johnson says. “Once we understand what those drivers are, we can use them as prediction tools for weed control.”