COMPETITIVE ABILITY AND TOLERANCE OF ORGANICALLY GROWN WHEAT CULTIVARS TO NATURAL WEED INFESTATIONS

Competitive ability of a traditional winter wheat cultivar (Maris Widg eon) was compared with two modern cultivars (Hereward and Genesis) whe n grown organically in each of four seasons in Gloucestershire, UK. In two seasons, cultivars were compared at two sowing dates (September a nd October or November). Cv. Maris Widgeon was the tallest cultivar an d intercepted most photosynthetically active radiation (PAR), particul arly if sown early. Cv. Hereward was taller than cv. Genesis when sown early. Cv. Maris Widgeon accumulated most nitrogen and dry matter ear ly in the season (until the end of March) when sown early, but not whe n sown late. Relative nitrogen and dry matter accumulation by cultivar s later in the season depended on season and sowing date. In one seaso n cv. Maris Widgeon had significantly more early season ground cover. Cultivars did not differ significantly in early tiller production. Alt hough most of these indirect measures of competitive ability were grea ter for the older variety cv. Maris Widgeon, infestation of Veronica s pp. was greater in plots of this cultivar than in plots of either one or both of the shorter cultivars in two successive seasons. In the 199 3/94 season, the soil seedbank from plots previously cropped with cv. Maris Widgeon produced more seedlings of Sinapis arvensis. In the 1994 /95 season, cv. Maris Widgeon plots contained less weed in terms of gr ound cover, numbers, dry matter and nitrogen than the other cultivars, and soil sampled from plots previously cropped with cv. Hereward sown late gave rise to larger numbers of weeds than cv. Maris Widgeon. Sow ing date affected weed burdens and emergence to a much greater extent than wheat cultivar. In years and sowing date treatments with relative ly low weed presence the shorter cultivars tended to yield more than c v. Maris Widgeon. When weed burdens were severe cv. Maris Widgeon yiel ds were equal to or greater than those of either cvs Hereward or Genes is. The results suggest that the traditional tall cv. Maris Widgeon co uld be used beneficially to tolerate weeds in organic systems when hig h weed infestations were anticipated, but could not be relied upon to suppress weed development, and in some circumstances could actually en courage certain species.