Researchers have proven it in labs – if corn plants sense light reflected off a green surface, they assume the green surface are weed leaves, affecting how the plant develops over the first few weeks. Even if the weeds are removed with postemergence herbicides after a few weeks, some of the changes are permanent. The possibility of yield reductions of 9 to 10 bushel per acre is realistic.
Syngenta has made this point as it has urged growers to use residual herbicides with their postemergence program to control weeds early. More farmers are going that route, although some of them are doing it to try to get ahead of resistant weeds, not so much to knock out early competition.
Mark Lawson, Danville, a Syngenta agronomist but also a farmer, heard about the research and decided to try it for himself. He did it last year and again this year in two entirely different seasons. This year he did it with sweet corn.
"The corn last year wasn't much taller than sweet corn anyway," he quips.
Lawson laid green carpet between two rows of plants, and repeated it between another pair of rows. Then he let the area between the plants blank so that bare soil, signifying that weeds had been controlled, was visible.
In both years the plants growing next to the green carpet grew taller, he says. They also oriented their leaves differently. All the changes in growth were not conducive to top yield potential.
The plants sensed competition based of the reflectance of green light," he says. "They really do think it is weed competition."
When they sense it they make adjustments like growing taller. That's not really what you want plants to be doing at that point in the season.
Lawson is convinced that starting with weed-free fields is a much better proposition.