This research explored the use of downy brome (BROTE) as a cover crop in irrigated corn. Although BROTE is a difficult weed to control, it could not be maintained as a cover crop in no-till irrigated corn for more than one season. A 10-fold reduction in BROTE occurred in the second year of corn. By the fourth year, only one BROTE plant could be found at the two locations. Because BROTE did not persist across years, soil coverage decreased 5 to 18% in the later location-years. At one location, normal herbicide rates decreased Johnsongrass biomass more than 22-fold both years it was applied. Increasing herbicide input decreased Palmer amaranth density more than 3-fold, but only in a single location-year. In three of six location-years, level of herbicide input had no significant effect on evapotranspiration (ET). Increased BROTE biomass decreased ET 0.033 to 0.083 cm/d during the first season at both locations. Increased irrigation increased corn yield by 240 to 1,900 kg/ha in five of six location-year combinations. Half rates of in-season herbicides reduced yield only in one of six location-years. High BROTE density reduced ET but did not translate into increased crop yield. In three of six location-year combinations, high BROTE density decreased yield by 300 to 1,000 kg/ha. In a single location-year, increased surface residues provided by BROTE increased yield by 560 kg/ha. Increased irrigation inputs decreased water use efficiency (WUE) by 6.3 kg/ha-cm in a single location-year and increased WUE by 10.8 to 121.6 kg/ha-cm in four of six location-years. Increased herbicide inputs increased WUE by 10.3 kg/ha-cm in one location-year. BROTE density had no significant effect on WUE at location 1. At location 2 in the first 2 yr, WUE was increased 9.4 to 22.2 kg/ha-cm.

Nomenclature: Downy brome, Bromus tectorum L. BROTE; Johnsongrass, Sorghum halepense (L.) Pers.; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; corn, Zea mays L.