Group IV Glycine max cultivars, chlorimuron postemergence, and paraquat + sodium chlorate applied as preharvest desiccants were evaluated for their effect on weed seed production and viability. Early-maturing G. max cultivars reduced seed weight, seed production, and seedling growth in Ipomoea lacunosa in most instances and usually reduced seed weight, germination, emergence, and growth in Sesbania exaltata by allowing harvest prior to physiological maturity of these weeds. Tall, late-maturing G. max decreased weed seed production and seed weight of both species, presumably through increased competitiveness of G. max. The seeds produced by weeds that emerged after chlorimuron application were usually lower in weight, germination, and emergence, and growth of seedlings was reduced because most weeds present at the time of application were controlled. Thus, only late-emerging or stunted plants were present to produce seed. Preharvest desiccation of late-maturing S. exaltata parent plants with paraquat + sodium chlorate reduced height and fresh weight of the resulting S. exaltata seedlings 35 to 50% by reducing the amount of time for the S. exaltata seeds to mature.