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The critical period of competition is the point of time when weeds that emerge with the crop begin to impact yields. This information is essential when implementing total postemergence weed management programs. Most studies have found that crop yields are protected if weeds are controlled before they reach a height of 4 to 5 inches in corn or a height of 6 to 8 inches in soybeans. When making management decisions, it is important to remember that the critical period varies widely, depending upon weed species and densities, environmental conditions, and cultural practices.