Competitiveness and yield impact of volunteer oilseed rape (Brassica napus) in winter and spring wheat (Triticum aestivum)

Volunteer oilseed rape is a relevant weed in many crop rotations due to seed persistence and dormancy. With the intro-duction of imidazolinone-tolerant oilseed rape varieties in Europe, herbicide-based control strategies of volunteers in cereals may become ineffective in the future. Experiments were conducted on experimental fields and in outdoor pots in Bingen (Germany) to quantify the effect of oilseed rape volunteers on yield and quality parameters of wheat. To simulate competition, oilseed rape was sown into wheat plots at a range of 2 to 320 plants m−2. Both spring and winter varieties of oilseed rape and wheat were used in the experiments. Crop yield parameter were expressed as the number of heads m−2, net yield as kg ha−1 and hectolitre−1 weight. Wheat moisture content, percentage of dockage, wheat thousand-kernel weight, and crude protein content were determined. Significant negative correlations were detected between the yield parameters heads m−2 and yield on the one hand, and oilseed rape density on the other hand. Moisture content of wheat and percentage of dockage were positively correlated with the volunteer density. No clear correlation was found between volunteer oilseed rape density and thousand-kernel weight, hectolitre weight and crude protein content of wheat. The highest volunteer density of 261 plants m−2 caused a maximum yield loss of 68% in winter wheat. Based on a non-linear regression analysis, a single volunteer oilseed rape plant m−2 is able to cause a yield loss of 0.74 to 1.61% in the field, which corresponds to 26.3 kg ha−1 (spring oilseed rape in spring wheat (SOSR × SW)) and 147.3 kg ha−1 (winter oilseed rape in winter wheat (WOSR×WW)), respectively. However, the introduction of imidazolinone-tolerant oilseed rape varieties will challenge farmers in terms of volunteer management. Control of volunteers is important to reduce yield losses. Accurate, delayed tillage after oilseed rape harvest and control of imidazolinone-tolerant volunteers with herbicides with other modes of action than those grouped into HRAC B will contribute to the successful avoidance and removal of imidazolinone-tolerant volunteers from subsequent crops.