INFLUENCE OF NITROGEN RATES AND WHEAT (TRITICUM-AESTIVUM) CULTIVARS ON WEED-CONTROL

Experiments were conducted to determine the influence of nitrogen (N) fertility and winter wheat cultivars on weed infestations in a winter wheat-ecofallow sorghum-fallow rotation near North Platte, NE. Centurk 78 and Lancota winter wheat suppressed density and growth of barnyardgrass and green foxtail significantly more than Eagle winter wheat before and after wheat harvest. Increasing N rates applied to winter wheat decreased annual grass weed population and weed yields. However, 67 and 101 kg N ha-1 reduced winter wheat grain yields compared to 34 kg N ha-1. Plots treated at 2.8 plus 0.3 kg ai ha-1 of atrazine plus paraquat 31 d after wheat harvest had more barnyardgrass before grain sorghum planting in 1983 than plots treated 17 d after wheat harvest but the reverse was true for green foxtail after grain sorghum emergence in 1984. Increasing N rates from 34 kg ha-1 to 67 and 101 kg ha-1 in the previous wheat crop decreased weed density before and after grain sorghum planting. There was no advantage in weed control in the grain sorghum from applying N to winter wheat in the fall vs. spring. Nomenclature: Atrazine, 6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine; paraquat, 1,1'-dimethyl-4,4'-bipyridinium ion; barnyardgrass, Echinochloa crus-galli (L.) Beauv #3 ECHGG; green foxtail, Setaria virdis (L.) Beauv # SETVI; winter wheat, Triticum aestivum L., 'Centurk 78', 'Lancota', 'Eagle'; grain sorghum, Sorghum bicolor (L.) Moench, 'Funks G1460'.