Biologists focus on weed/crop competition

At one time, weeding was a tedious, backbreaking part of farm work. Today it’s a tedious, backbreaking part of research. Just ask Jed Colquhoun, Shawn Conley or Mike Moechnig.

These UW–Madison graduate students spent hundreds of hours the last few summers on their hands and knees planting, thinning and measuring weeds in corn and soybean plots. Conley estimates that his field work on giant foxtail and common lambsquarters in soybeans took him 900 hours each in 1998 and 1999. The research plots ranged from those that were perfectly free of weeds to others clogged with just over 75 weeds to the square yard — a grower’s nightmare.

The potential impact of the research is enormous. These initial studies are the groundwork of a College of Agricultural and Life Sciences effort. One day it will help growers and crop scouts decide when weeds are a threat and what options growers should use to manage them successfully. The research could also help growers predict yields, so they can consider marketing options such as futures contracts.

Working with CALS weed scientists Larry Binning, Chris Boerboom, and David Stoltenberg, the graduate researchers are beginning to fill in the gaps in our understanding of weed biology.

“There’s some information out there on the relationship between the density of weeds early in the season and the eventual crop yield loss,” says Moechnig. “But few people have followed weed growth in crops as closely as we have.”

Most researchers ask how weeds affect crop yields. Colquhoun also wants to know how crops affect weed seed yield. That information will help scientists predict future weed problems based on seed numbers in the soil. For three years, Colquhoun has studied lambsquarters in both corn and soybeans; he measures how the competition between the weed and crop affects the seed yield of each.

Hoping for straightforward, repeatable patterns in the growth of lambsquarters from one year to the next, Colquhoun has found complexity and variability instead. Some lambsquarters plants in the plots remain small, while others grow tall. And the height they achieve differs from one year to the next.

“The growth of crop plants is very similar from one plant to the next within a year and from one year to the next,” Colquhoun says. “But lambsquarters is all over the place.”

In 1997 and 1998, for example, lambsquarters grew about three feet tall, half the height of the corn in Colquhoun’s plots. But in 1999, the lambsquarters’ seeds he had planted with the corn emerged three days earlier than the two previous years and before the corn emerged. That year the weeds ended up 6 1/2 feet tall. In 1997 and 1998, the weed produced about 100 seeds per plant, but in 1999 the weed produced 4,000 seeds per plant.

Like Colquhoun’s work, most research on weed-crop interactions has focused on a single weed. But farmers face problems with several weeds at the same time. As a result, Conley and Moechnig have been studying how different ratios and densities of giant foxtail and common lambsquarters — two dominant weeds in Wisconsin — affect soybeans and corn.

Moechnig’s plots without weeds averaged 200 bushels of corn per acre in 1998 and 180 bushels per acre in 1999. However, even weed densities of 4 to 5 plants per square yard, produced a measurable reduction in corn yield. That yield loss increased to 40 percent in 1999 at weed densities of just over 75 per square yard.

Farm fields typically have few weeds after herbicide applications, according to Moechnig, a former crop scout. “When weed densities are high, it’s easy for farmers to decide they need to control them. The tough part is deciding what to do at low densities,” he says.

Although giant foxtail grew more rapidly than lambsquarters, the two had about the same impact on corn yields, according to Moechnig. Foxtail was a slightly better competitor than lambsquarters in 1998. The reverse was true in 1999 when lambsquarters emerged early, just as it did in Colquohoun’s plots. In 1999, foxtail never caught up to the lambsquarters.

Moechnig believes that the height of the weeds relative to each other and corn may be as important as the density of weeds. Initially, corn grows more rapidly than either weed. If the weeds were half the size of corn early, they generally maintained that ratio throughout the growing season, he says.

Weeds have a great opportunity in corn fields because the crop is planted in 30-inch rows and is unlikely to cast enough shade to suppress weeds. With soybeans, the story is different. Shawn Conley has been studying the two weeds in soybeans drilled in 7-inch rows.

“Previous studies suggested that lambsquarters would reduce soybean yields more than foxtail,” Conley says. “But we found that the effect of foxtail was almost twice that of lambsquarters. I wasn’t expecting that.”

Where Conley removed all weeds, the crop produced almost 100 bushels per acre in 1998 and more than 80 bushels per acre in 1999. That’s well above the state average of 47 and 46 bushels per ace for those years. At the other end of the spectrum — just over 75 weeds per square yard — lambsquarters reduced yield by about 50 percent but foxtail decreased it by 95 percent.

In a study started in 1999, Conley has begun to examine how the time of weed emergence relative to the crop affects competition between weeds and soybean plants. During the first year, Conley found maximum yield reduction when the weeds emerged with the crop and decreasing yield loss as the weeds sprouted later. There was no yield loss associated with weeds that emerged at the third trifoliate stage.

Several of the studies show that early season weed competition is critical in affecting yield. Future studies are sure to concentrate on that period. One of the things the weed scientists want to know is critical times and how long growers can wait before making decisions on managing their weed problems.

The research was supported by: state funding to the UW–Madison College of Agricultural and Life Sciences, Hatch grants from the College of Agricultural and Life Sciences, and a grant from the Wisconsin Corn Promotion Board.