In eight of nine trials spanning 5 yr, relative yield of semidwarf hard red spring wheat (yield expressed as a percent of estimated weed-free yield) decreased linearly as Canada thistle shoot density increased when measured in late July to early August in the northern Great Plains. Differences between yield loss assessment (YLA) equations could not be distinguished statistically between no-tillage and chisel-plowed production systems. Multiple linear regression equations of relative wheat yield versus wheat density plus Canada thistle shoot density accounted for more variability in YLA equations than simple linear regression equations of wheat yield versus Canada thistle shoot density alone. Estimated weed-free wheat yield and negative slope (b) for yield loss assessment equations increased as cumulative growing-season (April to August) rainfall increased. Thus, relative wheat yield was decreased more by increasing Canada thistle density (slope b became more negative) in years of greater growing-season rainfall.
