Long term weed management programs for annual weeds should take into account seed production as well as yield losses from weeds. In this research economic optimum thresholds, determined with a simulation model, were compared to the economic thresholds derived from field studies with common sunflower and velvetleaf in soybean. An economic optimum threshold model, including a biological submodel and a submodel evaluating the financial benefit from controlling each weed species in soybean, was constructed to estimate the biological events and processes and economic costs of each weed. Long-term weed management programs were simulated in continuous soybean over a 15-yr period. The model optimized a POST treatment of bentazon by computing costs associated with weed interference and changes in estimated seedbank size. The economic optimum threshold populations were estimated to be 0.3 to 0.4 velvetleaf plants per 10 m2 and 0.4 common sunflower plants per 10 m2. The economic thresholds calculated to be 2.6 velvetleaf plants per 10 m2 and 1.5 common sunflower plants per 10 m2. The economic optimum threshold was calculated to be 7.5-fold and 3.6-fold lower than the economic thresholds for velvetleaf and common sunflower, respectively. The larger ratio for velvetleaf reflected the large quantity and longevity of seeds produced. The smaller ratio for common sunflower reflected the shorter persistence of the seeds and higher seed predation rates.

Itchgrass [Rottboellia cochinchinensis (Lour.) Clayton ♯ ROOEX] is a large aggressive annual in the Poaceae. This weedy grass is native to Southeast Asia. It now occurs also in tropical and subtropical areas of Australia and Africa and has been introduced into the West Indies and North, Central, and South America. It apparently was introduced into the United States at Miami, Fla. and Lafayette, La. in the early 1900s (1, 6, 7).

Common tansy [Tanacetum vulgare (L.) Bernh. ♯CHYVU], a native of Europe and Asia, is a perennial herbaceous composite growing erect to 1 m. It bears finely divided leaves and, in summer, umbels of small, button-shaped yellow flowers. It has had many traditional uses as medicine, preservative, and repellent from Medieval times to modern—uses which have been validated by contemporary research.