Source: Syngenta Crop Protection

Like the fashion industry, trends in agriculture come and go. And, just like black is the classic choice, residual pre- emergence herbicides are coming back on the scene as a classic, valuable option for weed control. Growers are steering away from total post-emergence, glyphosate programs and looking for better early season weed control.

Eric Wright, Country Pride Services in Bingham Lake, Minnesota is seeing changes in management programs for growers in his area. "Producers realize that total post- emergence glyphosate systems are not controlling their weeds," says Wright. "Often they need twice the rate of glyphosate to get satisfactory control, causing them to look towards pre-emergence herbicides."

Pre-emergence is "In"

Residual pre-emergence herbicides provide critical early season weed control. Early season competition in corn ravages yields and a soil-applied herbicide protects the
crop through multiple modes of action.

"Early season weed competition has to be managed, and the 2005 drought plus weed competition made corn suffer," says Chris Boerboom, University of Wisconsin weed scientist. "The benefit of pre-emergence herbicides in corn was evident in several fields in the dry areas of Wisconsin. In plots where weeds grew 4 to 5 inches before post-emergence treatment, corn growth was really stunted."

Pre-emergence residual herbicides also make choosing between a one-pass and a two-pass system during the season a realistic possibility. Growers can gauge weed pressure early and make a planned second pass, or scout to see if a second application is needed.

And, pre-emergence residual herbicides provide good control of weeds well into the critical weed-free period,
4 to 7 weeks after planting when corn is most susceptible to weeds. They open the application window wider so growers have more time to make a planned second pass in management programs, if needed. Plus, today's high-performance pre-emergence herbicides offer greater crop safety.

Bob Hurst, Bowie Fertilizer in Overton, Neb., is seeing excellent control with a one-pass Expert® program, which contains both a residual and glyphosate. "We consistently see a 7 to 8 bushel per acre advantage with a residual," says Hurst. "2005 was a fantastic year; we had the cleanest fields I have ever seen with a one-pass program."

Non-Residual Total-Post is "Out"

The popularity of total post-emergence systems can be attributed to the ease of glyphosate-tolerant (GT) cropping systems. However, with the sole repeated use of glyphosate growers are seeing holes in their weed control.

"Growers using total post with two passes of glyphosate say it isn't doing the job," Hurst says. "They see a lot of pigweed and velvetleaf escapes later in the season."

Success of a total-post program depends on the types of weeds present, weed pressure and application timing.
According to the 2005 Weed Control Guide for Ohio and Indiana, total post-emergence programs should not be used in fields with moderate to high levels of annual grasses and high levels of broadleaves.

"Fields with giant ragweed pose a real problem for a total-post program", says Bob Hartzler, Iowa State University weed scientist. "Giant ragweed emerges early and grows very rapidly. Farmers tend to delay the post application to allow foxtails and other weeds to emerge, by then the ragweed has reached a size that is difficult to kill and may have already reduced corn yields."

Non-residual herbicides provide a narrow window of opportunity for growers to get into the fields and rainy weather, equipment breakdowns or custom applicator scheduling challenges could delay the timing of the application. Allowing weeds to grow 2 to 5 inches could result in 6 to 12 percent yield loss in corn, according to the University of Minnesota.

"In southwest Minnesota, spring 2005 was particularly rainy around the time that our growers needed to get into the fields," says Wright. "It was so wet growers couldn't get in when they needed to, and they delayed application for as many as 10 days, which really hurt yields. The growers with a residual down had fewer weed control issues and saw better yields."

Sensible Solutions

In years past, growers backed away from pre- emergence, residual herbicides because the economics and ease of a total post-emergence non-residual program. But, as Boerboom explains, there are other cost-effective options for corn.

"In corn, we have price-competitive, non-glyphosate herbicides," says Boerboom. "Glyphosate in glyphosate- resistant corn isn't as simple as glyphoate-resistant soybeans. Early weed competition makes a greater difference in corn than in soybeans."

In Guideline to the Management of Herbicide Resistance, the Herbicide Resistance Action Committee (HRAC) advises growers to use multiple modes of action in weed management programs. Many pre-emergence herbicides provide multiple modes of action, allowing growers to follow these recommendations with ease.

"If history has taught us anything, over-using a technology is basically throwing it out the window," Wright says. "Glyphosate only offers one mode of action, while our pre-emergence recommendation, LUMAX®, brings three different modes of action into the mix."

It is easy to see why pre-emergence herbicides are making a comeback. They provide growers more weed control options. Residual herbicides bring early-season weed control and multiple modes of action to the table, picking up areas where glyphosate-only programs leave holes.

Pre-emergence herbicides are quickly becoming today's classic choice.

Expert® and LUMAX® are trademarks of a Syngenta Group Company.