COLUMBIA, Mo. - At the height of summer, many weeds such as foxtail, common cocklebur and sunflower can be seen poking through the soybean canopy, but late-season applications of herbicide often have little or no impact on yields, according to a University of Missouri weed scientist.

"Unfortunately, many late-season weed control practices are driven by aesthetic reasons rather than the bottom line," MU agronomist Bill Johnson says. "Our research indicates that light infestations of late-emerging weeds do not impact yield if there was a weed-free period earlier in the season."

Typically, growers use a combination of soil-applied herbicides and post-emergent weed management practices to control weeds three to six weeks after planting. They rely on residual herbicide activity or the crop canopy to suppress weeds for the rest of the growing season.

"The question to be addressed is: ‘How much yield loss do I suffer from either not controlling the weeds or applying herbicides to soybeans in the reproductive stages of growth?’" Johnson says. He cited "several factors to consider."

First, most late-season weeds are at least 3 feet tall, while most herbicides are labeled for use on weeds no higher than 1 foot. Second, high temperatures and limited soil moisture reduce absorption of the herbicide by the weeds.

"Third, weeds that have emerged above the soybean canopy have already exerted their competitive effect on the soybean," Johnson said, pointing to research by MU weed scientist Andy Kendig at MU Delta Research Center in Portageville, Mo.

Kendig’s study, partially funded by the Missouri Soybean Merchandising Council, indicated that when no weed control operations took place until eight to 12 weeks after planting, yield reductions ranged from 32 percent to 100 percent. "And no yield was recovered by making late-season herbicide application," Johnson said.

Using common sunflowers, Johnson conducted a weed-interference study at two Missouri locations in 1998 and 1999 to determine the optimal "weed-free window" for Roundup Ready soybean fields. Sunflower densities of three per square meter were established a week or two after planting. Then, the sunflowers were removed at various intervals and subsequently re-established at the same density two weeks after removal. After the two-week weed-free interval, the sunflowers remained in the plots until harvest.

"Our results indicated that the earlier the sunflowers were allowed to reestablish, the greater the soybean yield reductions," Johnson said. "Yields were significantly less in plots where sunflowers were reestablished by four weeks after planting. Sunflowers that were reestablished at six, eight or 10 weeks after planting did not cause significant yield reductions in three out of four sites."

Johnson also noted that many of the post-emergence herbicides used in soybeans have at least a 45-day pre-harvest interval. "Late-season herbicide applications are not labeled, and the grower is responsible for yield loss due to bloom and pod abortion and excess herbicide residues that may be present in the harvested grain," he said.