Early season weed control in corn has long been recognized as a critical management factor in obtaining optimum yields.

Speaking at a recent Syngenta Media Summit, Canadian weed scientist Clarence Swanton put a new twist on the effect of these early season pests.

Though it is commonly accepted that early season weeds do cause yield loss, exactly when and how yields are affected is not so clear. Swanton says, “yield losses to weeds are fast and forever.” Weeds can cost a grower three bushels per day, and waiting a week to control weeds, he says, routinely costs growers 10-15 bushels per acre, regardless where corn is grown.

A series of research by the Canadian scientist has caught the eye of weed scientists across the U.S. A basis for his research is that the well-established rule of thumb — that weeds compete with plants for light, water and nutrients may not be the real deciding factors in losing yield. He doesn't' doubt this yield loss happens, but believes it is more of an effect than a cause.

“If you think about it, corn is usually planted with adequate moisture, which is one of the basic principles of choosing time of planting. There are plenty of nutrients in the soil, because typically, we fertilize our corn to get a good start. And, if you look at a row of corn and weeds that germinate at the same time, corn has a clear advantage,” Swanton explains.

“We wanted to know how it is possible to lose yield at the rate of 3-7 bushels per day, at a time in the crop growth process in which there is limited competition for light, water and nutrients. That yield losses occur is well documented, but why this happens is not well explained, if at all, in the literature, he contends.

To prove his theory, Swanton planted corn in a greenhouse, then planted weeds adjacent to the corn. The weeds did not compete with the corn for light, nutrients or water, because each group of plants had their own environment.

His question was: will weeds that are physically near corn, but not competing for water, light and nutrients still cause yield losses. It appears from results so far that corn plants, and Swanton says likely any other crop, sense the presence of weeds and adjusts how the plant grows.

The difference in quality of light has been documented, but never incorporated into studies of early season yield losses to weeds, he says. “We believe the changes in light quality cause a physiological loss in plants. Understanding the impact on the plant will help us understand how these light quality issues affect yield and cost of production,” Swanton contends.

In his tests, corn plants with green from weeds nearby, but not competing for light, water or nutrients; were 17 percent higher than corn plants with no weeds nearby. There was a 45 percent increase in leaf area and leaf dry weight on weed-free plants increased by 40 percent. All these factors were influenced solely by being close to weeds — no competition he says.

All these growth factors were accomplished at the cost to the root system, Swanton says. Corn plants made a shift in carbon allocation in a reaction to the weeds. Above ground, there is no apparent difference, in fact, the crop may look excellent, Swanton says.

The Canadian scientist says corn does not read, or sense, the presence of other corn plants because these plants are adjusted to the same species. When a different species, as with weeds, comes in, the sensory process is different, Swanton explains.

As soon as corn comes up, it can detect physiological changes. Receptor genes inside the corn plant begin to change within hours of germination, based on changes in light rate and quality.

In his greenhouse studies, Swanton found that corn plants with no weeds around them had 63 percent of their leaves growing perpendicular to the row, compared to 49 percent in plants with adjacent weeds. By growing more leaves perpendicular to the row, he points out, the corn plant more quickly produces a canopy that more quickly shades out weeds. He believes the 13 percent increase in positive leaf-growth direction will correlate directly to increased yields.

“Our research strongly indicates that corn plants sense the presence of weeds and adjust to the changing quality of light creating by this sensory process. These morphological changes come at a cost to the corn plant, and to the yield and bottom line to the grower,” Swanton concludes.

When he took his greenhouse findings to the field, though still in a somewhat unnatural environment, the greenhouse findings held up. The crops were grown out and harvested in October 2006 and combined with 2005 data from similar treatments.

Two year's data indicate corn plants in weedy plots, though corn and weeds did not compete for light, water or nutrients, were about half a leaf behind non-weedy plots. By the 10th, 12th and 15th weed stage, the height of the weedy and non-weedy plants were the same, but the weed-free plants were heavier. Root dryweight in non-weedy plants was likewise heavier. These are all factors that will affect yield, but cannot be seen above the ground.

Though the weedy and non-weedy plants look the same, physiologically they are different. The weed free plants have heavier leaves and a bigger root system. The root to shoot ratio was consistently better for weed-free plants. Swanton is convinced that these changes in light quality will cost the grower yields and will ultimately change the way growers look at weed management.