If you think a post herbicide program allows weeds to steal more yield than a soil approach does, think again. Research says it's all in the timing.

As summer field day season winds to a close, it's clear that last winter's hottest topic has been a focal point of plot tours and field days.

Much ink was devoted to the question of whether early weed presence steals soybean yields. Major marketers squared off in ads and news columns to debate the issue of weed competition.

Research conducted at university experiment stations this summer should provide answers to that question soon.

"The issue of early weed competition really intensified with the introduction of Roundup Ready and Liberty Link crops," says Dr. Jim Kells, a weed scientist at Michigan State University. "There is a lot of research going on right now on early weed competition."

Age-old topic. In addition to initiating more research, scientists also have taken a closer look at existing science. This weed control timing issue goes back to the advent of postemergence herbicides.

Last spring Kells and a colleague, Dr. Karen Renner, reviewed existing research on weed competition. Their conclusion: In a majority of cases, soybean yields won't be hurt if weeds are controlled by the third week after crop emergence or before soybeans reach the V4 stage. At that point, the third trifoliate is fully developed and weeds are usually 6 in. tall or less. In corn, yields generally are not hampered if weeds are removed by the time weeds reach 4 in.

Kells says these conclusions don't tell a grower whether to use a soil residual herbicide, a total over-the-top program or a combination of both. That's an individual decision based on economics, time and equipment management, and perennial and annual weed pressures. But the decision should not be driven by yield-loss worries, provided the grower is prepared to control weeds in a timely manner and the weather allows.

"There is a fair amount of data on weed duration and crop yield," Kells says. "We are confident that the general conclusions we reached will cover most situations."

"We don't have as much understanding of the exceptional situation - if it is wet or dry, or if there are extremes in weed density, for instance," Kells says. "With very high weed densities, we may have to remove weeds earlier. If the weeds emerge before the crop, they must be removed sooner. If they emerge later, they probably can be removed later. Tillage and row spacing are other factors that may affect the competitiveness of weeds with a crop."

Research reveals risks. Nine studies conducted at seven universities across the United States in 1997 showed that soybean yield isn't affected until weeds are taller than 9 in. A summary of the research, commissioned by Monsanto Company, is shown in the table.

Field conditions at test sites ranged from dry to relatively wet. Trials were on Roundup Ready soybeans treated with Roundup Ultra herbicide at various weed heights. Plots were initiated in weed-free soil and were kept clean once the initial treatment was made, although follow-up weed-control measures were rarely needed. Yields were compared to yields of Roundup Ready soybeans grown with Prowl or Treflan applied preemergence or incorporated before planting, followed by Pursuit. Yield was statistically identical at 3-, 6- and 9-in. weed heights, as well as with the residual herbicide program.

Monsanto's recommendations that weeds be sprayed at 4 to 8 in. fit within the maximized-yield zone identified in the research.

In most cases, weed pressure in test plots was extreme - "a worst-case scenario from a farmer's perspective," says Dr. Steve Hart of the University of Illinois.

This year Hart and other weed scientists advised growers to control weeds early to assure that spraying was completed before weeds began hurting yield.

"At 4-in. weeds, 99 percent of the time there will be no yield impact on soybeans," Hart says. "The risk starts to increase at 8 in. In between is a gray area. We've got a ton of work out on this subject, both in corn and beans. We'll know more after this year."

Variables affect timing. Dr. Tom Bauman of Purdue University agrees that scientists have a general picture of when weed competition begins to reduce yields. But more research is needed, especially in narrow-row and no-till systems, he says.

"Most all the data on weed competition have been from rowed beans," he points out. "In general, I think that if you spray beans within 3 to 3 1/2 weeks, I would be comfortable that you are not losing yield. In drilled beans you may need to spray earlier to get good coverage. In no-till, you may not need to remove weeds as early on a calendar basis because the crop and weeds grow slower."

Weed species also can dictate optimum spray timing.

Tom Hoverstad and Dr. William Lueschen at the University of Minnesota point out that in their research, control of common lambsquarters is improved if sprayed with Roundup Ultra at 4 to 6 in. instead of 2 to 4 in.

"If lambsquarters is one of your pivotal weeds, I would spray at 4 in., and not earlier," Lueschen says.

Corn studies. The research base on timing of weed control in corn is narrower than the research on soybeans, so there's more to learn, Kells explains.

At this point, a 4-in. weed height seems to be the magic number.

"Looking through the literature, I have only seen a yield loss one time at 4-in. weeds," he says. "I would say that if you remove weeds in the 2- to 4-in. range, you have very minimal risk of yield loss."

When weeds exceed 4 in., yield trails off rapidly, Lueschen says.

"Where we have heavy grass pressure, if we don't get it out by the time it is 3 to 4 in. tall, we see serious yield loss," he says. "Even if we get 100 percent control, we see as much as a 20 percent loss with 6-in. grass."

Scientists are uncertain why it's more critical to control weeds earlier in corn than in soybeans. Some say it is because reproductive structures develop earlier in corn than in soybeans. Lueschen speculates that grasses may sap nitrogen from the upper soil profile, leaving corn starved until roots reach deeper nitrogen supplies.

Stay tuned. Over the next couple of years, more detailed recommendations will emerge on the timing of weed removal. For now, research indicates that early weed presence does not impact yield if weeds are removed in a timely manner.