The effect of Russian thistle density and duration of interference on spring wheat was investigated in two 3-yr field studies. In the density study, the best fit regression equation predicted a loss in spring wheat yield of 0.5, 0.5, and 0.6% in 1983, 1984, and 1985, respectively, for each percent of the total biomass contributed by Russian thistle. In 1985, when rainfall was 46% below normal, the highest weed density produced greater than 70% of the total plant biomass and reduced yields more than 50%. In contrast, in 1984 when rainfall was 65% above normal, the highest weed density produced less than 20% of the total plant biomass and reduced yields 11%. In the duration study, the predicted yield loss was 2.3 and 0.7% for 1983 and 1984, respectively, for each week of interference. In 1985, yield was not significantly reduced until after 6 weeks of interference. In both studies, spikes/m of row had the highest correlation with yield and were affected the most by weed interference.
