In three replacement series experiments, winter wheat (Triticum aestivum L.), jointed goatgrass (Aegilops cylindrica Host. #3 AEGCY), and downy brome (Bromus tectorum L. # BROTE) were paired in all possible combinations to determine competitive relationships during vegetative growth. Under growth chamber conditions of ample fertility and soil moisture and day/night temperatures of 18/10 C, relative yield totals for the three species were similar, indicating that they compete for the same resources. Both winter wheat and jointed goatgrass had greater plant growth and higher relative crowding coefficients than downy brome, which indicated a hierarchy of relative competitiveness of winter wheat > jointed goatgrass >> downy brome. In other growth chamber studies, winter wheat was slightly more competitive than jointed goatgrass regardless of fertility levels. Winter wheat was the superior competitor at 18/10 C and −33 kPa (soil moisture), whereas jointed goatgrass was superior at 27/10 C and −300 kPa, conditions that are frequently encountered in the Pacific Northwest.
