Why are early-season weeds so competitive?

Published on: 17:16PM Jul 07, 2009
Every year, growers face weed competition in their crops. There’s no doubt that weeds can negatively affect yield, especially when they compete early in the season. But why?
I learned more about why when I participated in a panel with Dr. Clarence Swanton of the University of Guelph, Ontario, this winter. Dr. Swanton shared revolutionary new research on early season weed control, which shows how irreversible yield loss occurs if plants emerge in the presence of any weed competition. His data from the past several years suggests that plants can lose between 0.3 to 3.5 bushels per day and that even after weeds were controlled, yield loss was never ever regained.  According to Dr. Swanton, corn can “see” weeds, and that light quality affects plant root and stem development.
Another member of that panel, Syngenta agronomist Bob Kacvinsky, has been working on trials similar to Dr. Swanton’s in Nebraska research plots. Bob shared his work, as well.  In this photo from Bob’s study, the top plants were competing with weeds that were controlled when they were 3 to 3 ½ inches tall. The bottom plants were treated with a pre-emergence herbicide, and never had weed competition. Notice that the leaves are orientated differently. Ideally leaves should be perpendicular to the row. When they are lined up with the row, the plants are actually competing with themselves. There’s a lot better light-gathering capability in the bottom photo than in the top.

All this means that the weeds in fields now are more of a threat for adding to the weed seedbank and causing harvestability issues than actually cutting crop yields.