WEED SURVEYS IN DIFFERENT TILLAGE SYSTEMS IN SOUTHWESTERN ONTARIO FIELD CROPS

A weed survey of 593 corn, soybean and winter wheat fields in southwestern Ontario was conducted during 1988 and 1989 to determine the abundance and distribution of weeds under a variety of tillage systems. The survey was conducted after all weed-control measures had been carried out. A total of 82 weed species and groups of species were recorded. Many weeds were found infrequently. The most abundant weeds were green foxtail (Setaria viridis (L.) Beauv.), lamb's-quarters (Chenopodium album L.), quack grass (Agropyron repens (L.) Beauv.), redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.) and dandelion (Taraxacum officinale Weber). These weeds accounted for 54% of the total relative abundance. Weed communities in individual fields were highly variable. Most fields had fewer than 10 species, and nearly half of the fields had fewer than 6 weeds m-2. Six weeds, yellow foxtail (Setaria glauca (L.) Beauv.), quack grass, crab grass spp. (Digitaria spp.), lamb's-quarters, green foxtail and chickweed (Stellaria media (L.) Vill.), occurred at densities greater than 100 plants or shoots m-2 in at least one field. Similar weed communities were found in fields that had different tillage histories. Annual weeds were found at higher densities and perennial weeds at lower densities in reduced tillage than in conventional tillage. Weeds were found at highest total densities in fields with no tillage. The six most abundant weeds in the current study were present in Ontario prior to the 1900s.