EFFECT OF CROP GROWTH STAGE ON TOLERANCE TO LOW-DOSES OF THIFENSULFURON-TRIBENURON

Field studies were conducted at Morden, Manitoba, in 1994 and 1995 to investigate the effect of crop growth stages on canola and sunflower t olerance to sublethal doses of thifensulfuron:tribenuron (2:1). Thifen sulfuron:tribenuron at doses of 0, 0.23, 0.45, 0.9, 1.8, and 3.6 g ai ha(-1) plus a nonionic surfactant at 0.5% v/v were applied to canola a nd sunflower at the two- to three-leaf, four- to five-leaf, and six- t o seven-leaf stages. Crop leaf stage at the time of application affect ed tolerance of both crops to thifensulfuron:tribenuron. Crop injury w as lowest, and flowering, seed yield, and seed oil content were least affected when low doses of thifensulfuron:tribenuron were applied at t he two- to three-leaf stage. At the highest dose, there was little pra ctical difference among growth stages since yield of both crops was se verely reduced. Producers with drift-affected canola or sunflower can expect less effect on crop yield when thifensulfuron:tribenuron injury occurs during early crop growth.