Of the grasses studied over a 3-year period, Alta tall fescue (Festuca arundinacea) was most competitive and Troy bluegrass (Poa pratensis L.) least competitive with Canada thistle (Cirsium arvense). Ladino white clover was more competitive than birdsfoot trefoil (Lotus corniculatus) when present in full stands. Winter-killing of Ladino clover (Trifolium repens L.) resulted in an increase in Canada thistle, particularly at low irrigation levels. Irrigation favored an increase in Canada thistle numbers in the year the grass stands were established. In the next two years thistle numbers declined more rapidly on irrigated than on non-irrigated plots. High rates of nitrogen increased the competitive ability of the grasses.