Influence of nitrogen rates and wheat (Triticum aestivum) cultivars on weed control.
Trials were carried out to determine the influence of N fertility and winter wheat cultivars on weed infestations in a winter wheat-ecofallow-sorghum-fallow rotation on silt loam soil near North Platte. Winter wheat cv. Centurk 78 and Lancota suppressed the density and growth of barnyardgrass (Echinochloa crus-galli) and green foxtail (Setaria viridis) 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 reduced winter wheat grain yields compared to 34 kg N/ha. Plots treated with atrazine at 2.8 kg/ha + paraquat at 0.3 kg 31 d after wheat harvest had more barnyardgrass before grain sorghum was sown in 1983 than plots treated 17 d after wheat harvest, while the reverse was true for green foxtail after grain sorghum emergence in 1984. Increasing N rates from 34 kg/ha to 67 and 101 kg in the previous wheat crop decreased weed density before and after grain sorghum was sown. No advantage was gained in terms of weed control in the grain sorghum from applying N to winter wheat in the autumn vs. spring.