A survey of the occurrence of seedling weeds in spring annual crops in Manitoba.
A floristic survey of weeds in spring annual crops throughout Manitoba, conducted in June 1975-78, revealed a total of 83 species in 654 fields. The 28 annual dicotyledonous weeds were the most abundant group of the 50 species that occurred in >1% of fields. Polygonum convolvulus [Fallopia convolvulus], Sinapis arvensis, Amaranthus retroflexus and Chenopodium album were the most widespread and abundant species in this group, with mean densities of 9.6-16.2 plants/m2. The combined abundance of Setaria viridis, Avena fatua and Echinochloa crus-galli, the 3 major annual monocotyledonous weeds, was nearly equal to that of the annual dicotyledonous weeds. S. viridis was the most abundant weed, occurring in 89% of fields at mean densities of 270.5 plants. Cirsium arvense and Sonchus arvensis were the most abundant species in the biennial and perennial weed group and had mean densities of 4.4-4.8 plant in infested fields.