Common management alternatives were compared in a factorial arrangement for 2 yr to determine their effects on green foxtail and yellow foxtail seed production in spring wheat in the Northern Great Plains of the United States. Seed production was measured twice, at wheat harvest (in August) and postharvest (after first lethal frost in autumn). Management alternatives were early, middle, and late crop-sowing dates; no-till, chisel, and moldboard plow tillage systems; and broadleaf herbicide only and broadleaf herbicide plus fenoxaprop applications. Fenoxaprop reduced foxtail seed production at wheat harvest but not at postharvest. Early sowing also decreased seed production at wheat harvest but increased postharvest seed production. Tillage system had no consistent effects on foxtail seed production. Postharvest seed production often was greater than or equal to that at wheat harvest regardless of management system. These results indicate that in-crop management alternatives, such as postemergence grass herbicide and early crop sowing, may lower the number of foxtail seeds at harvest substantially, but they must be accompanied by postharvest weed control to reduce overall seed production.

Use of composted swine manure produced in deep-bedded hoop structures is a promising approach for recycling farm waste products and improving soil fertility, but little is known about its effects on crop–weed interactions. A 2-yr study was conducted to evaluate the effect of compost amendments and tillage on soybean–common waterhemp competition. Experiments were conducted in no-tillage and chisel plow main plots with compost applied to one of two types of subplots. Common waterhemp and soybean growth was measured in sub-subplots accommodating weed-free soybean and soybean with common waterhemp sown at soybean planting, soybean emergence (VE), soybean second-node stage (V2), and soybean sixth-node stage (V6). Soybean heights were not influenced by compost or common waterhemp sowing time. Soybean stem diameters were influenced by year, tillage regime, and an interaction between compost and common waterhemp sowing time. In contrast, common waterhemp heights and basal diameters were greater when sown at planting and VE in compost-amended subplots than in compost-free subplots. Overall, there was a negative quadratic relationship between common waterhemp biomass and soybean yield (r 2 = 0.746). The extremely low common waterhemp emergence in V2 and V6 treatments suggested that early-season weed suppression was sufficient to protect soybean from common waterhemp competition. The sex determination of 2,557 common waterhemp plants showed a marginally higher male to female ratio in compost-amended treatments than in compost-free treatments (P = 0.0611). A linear-slope regression indicated that common waterhemp fecundity was positively related to individual plant biomass, with a change in slope occurring at 118.7 g. Under the conditions present in this experiment, compost did not enhance soybean yield but increased the competitive ability of waterhemp. Because composted swine manure can have a major influence on competition of common waterhemp with soybean, effective weed management practices should be in place when this soil amendment is used.