A study was conducted near Garden City, KS with irrigated corn to determine how the integration of a terminated winter wheat cover crop with various atrazine rates would affect Palmer amaranth control and corn water use efficiency (WUE). Without atrazine, the presence of a winter wheat cover crop, killed in the boot stage, resulted in a threefold weed biomass reduction in irrigated corn. The highest rate of atrazine completely masked the weed control effect of the cover crop, producing a greater than 15-fold reduction regardless of the presence or absence of the cover crop. A terminated winter wheat cover crop without atrazine elevated corn yield in only two of nine location-yr, and in one instance, depressed yield. However, a terminated wheat cover crop elevated corn yield in six of nine location-yr combinations when used in conjunction with 1.6 kg ha−1 atrazine. Although increases in WUE associated with reductions in soil water evaporation produced by the cover crop seemed to be responsible for some of the increase in corn grain yield and stored soil water at the end of the summer growing season, end of season Palmer amaranth biomass had a more profound impact.

Palmer amaranth (Amaranthus palmeri) is a major weed in corn (Zea mays) fields in the southern Great Plains of the United States. Field studies were conducted in 1996, 1997, and 1998 near Garden City, KS, to evaluate the effects of Palmer amaranth density and time of emergence on grain yield of irrigated corn and on seed production of Palmer amaranth. Palmer amaranth was established at densities of 0.5, 1, 2, 4, and 8 plants m−1 of corn row both concurrently at corn planting and when corn was at the three- to six-leaf stage. The control plots were weed free. The Palmer amaranth planted with corn emerged with corn, whereas that planted later emerged at the four-, six-, and seven-leaf stages of corn. The Palmer amaranth emerging with corn reduced yield from 11 to 91% as density increased from 0.5 to 8 plants m−1 of row. In contrast, yield loss from Palmer amaranth emerging later than corn was observed only when the emergence occurred at the four- and six-leaf stages. The corn leaf area index (LAI) decreased as Palmer amaranth density increased. Reduction in corn LAI from Palmer amaranth interference was smaller for the second emergence date than for the first emergence date. Seed production per Palmer amaranth plant decreased with greater density, but seed per unit area increased from 140,000 to 514,000 seeds m−2 at densities of 0.5 and 8 plants m−1 of row, respectively, when Palmer amaranth emerged with corn and from 1,800 to 91,000 seeds m−2 at the same densities for later emergence dates. Although Palmer amaranth is highly competitive in corn, this study shows that yield loss is affected more by time of emergence than by density.