Addition series field experiments were conducted near Moscow, ID, in 1987 and 1988 to determine the relative aggressiveness of spring barley and wild oat and to determine the effect of barley and wild oat density and proportion on barley grain yield and wild oat seed rain. Regression analysis was used to describe the relationship of the aboveground biomass and grain yield to species density. Barley was more aggressive than wild oat. Barley biomass was affected most by intraspecific competition, while wild oat biomass was affected most by interspecific competition. Barley aggressiveness changed little throughout the growing season. Wild oat aggressiveness varied but was always less than barley aggressiveness. Increasing wild oat density had a negative, asymptotic-type effect on barley grain yield at all barley densities. However, the effect of wild oat was greatest at the lower density of barley. Increasing barley density decreased wild oat seed rain.

Differing life histories contribute to difficulties establishing perennial grasses on lands dominated by exotic annual grasses. In a greenhouse study, we investigated to what extent allowing the perennial grass bluebunch wheatgrass to emerge before the exotic annual grass downy brome would increase its competitive ability and whether modifying nitrogen (N) would affect competition. We conducted an addition-series factorial experiment. In three cohort treatments, the two species were seeded concurrently or bluebunch wheatgrass was at the two- or four-leaf stage when downy brome was planted. N treatments were low (ambient) or high (N added to maintain 10 mg kg−1 [0.1286 oz lb−1]). Larger bluebunch wheatgrass avoided suppression by downy brome regardless of N. Under concurrent sowing, doubling downy brome density decreased bluebunch wheatgrass biomass by 22.6% ± 2.38 SE. In contrast, when bluebunch wheatgrass had a four-leaf size advantage, the same increase in downy brome density decreased bluebunch wheatgrass biomass by 4.14% ± 2.31. Larger bluebunch wheatgrass also suppressed downy brome more effectively, but N enrichment decreased the suppressive ability of bluebunch wheatgrass.