Safflower (Carthamus tinctorius) Density and Row Spacing Effects on Competition with Green Foxtail (Setaria viridis)

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Abstract
Field experiments were conducted in 1990 and 1991 at Lethbridge, Alberta, to determine if narrower row spacings and increased plant densities than commonly practiced would improve safflower's competitive ability with weeds. A factorial set of treatments of safflower at two row spacings of 11 and 22 cm and six densities ranging from 10 to 192 plants m−2 grown weed free or infested with green foxtail (Setaria viridis (L.) Beauv.). Decreasing safflower row spacing from 22 to 11 cm slightly improved competition with green foxtail, but increasing safflower density had a much greater effect. Weed-free safflower biomass and seed yield plateaued at 70 and 84 plants m−2 in 1990 and 1991, respectively. However, safflower infested with 500 plants m−2 of green foxtail increased in biomass and seed production up to 100 plants m−2 in 1990 and 156 plants m−2 in 1991. At these high densities, weedy safflower yielded less than weed-free safflower but three to four times more than achieved at lower stand densities. Concurrently, high safflower density reduced green foxtail biomass up to 72% and seed yield up to 85%. Dense safflower stands developed a closed canopy early in the season and shaded green foxtail more than thin stands.