Herbicide-resistant crops: yield penalties and weed thresholds for oilseed rape (Brassica napus L.)

An expérimental procedure was designed to provide a simple model for types of analyses necessary to determine weed density thresholds for advantageous use of crop plants engineered for herbicide resistance. Oilseed rape (Brassica napus L., cv. Tower) biotypes resistant (RES) and susceptible (SUS) to atrazine were used as model crop plants, and wild oat (Avena fatua L.) was used as the model weed. Along a wild oat density gradient equivalent to 0–128 plants m^?2, RES plants consistently experienced biomass and yield reductions of approximately 10–20% compared to SUS plants. When atrazine was applied at 1.5 kg ha^?1 to control wild oats competing with RES plants, RES biomasses and yields were stabilized at the same level as that where 25–30 wild oats m^?2 reduce yields of SUS plants. This implies that with wild oat densities of 25–30 plants m^?2, it becomes agronomically advantageous to crop with RES plants plus atrazine rather than to crop with higher-yielding SUS plants.