Plain Language SummaryDiversified crop rotations can provide many benefits that include greater yield stability, better nutrient cycling, and improved pest management. In a long‐term experiment, we observed weed differences in oat that was the result of whether the previous crop was corn or grain sorghum. Horseweed, a common weed in North America, was more prevalent in oat when the previous crop was corn. We speculate that grain sorghum, considered to have allelopathic compounds, suppressed horseweed the following year better than corn. We show that crop sequence within diversified can result in distinct differences in weed presence and production.