Weeds rob nitrogen from crops
An MSU timing study on post-emergent weed control in corn showed that 95 percent of weed control or better can be achieved when weeds are 9 inches tall, but yields were reduced 25 bushels per acre.
Post-emergent herbicides provide a greater window of opportunity to control weeds, but allowing weeds to grow too long can be “robbing” nitrogen, and other nutrients, from the intended crop.
In a two-year study in Wisconsin, there was no yield loss in corn when weeds were controlled at the 4-inch stage, but delaying application on 12-inch weeds resulted in an average 9 % yield loss. Looking at the data from another view point, 2006 and 2007 data showed the economic optimum nitrogen rate was 96 lbs. per acre when weeds were controlled at 4 inches, compared with an economic optimum nitrogen rate of 200 when weeds were controlled at 12 inches. Timely weed control will ensure valuable nutrients are used for crop production rather than weed production.
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Source: Michigan State University