Brassica napus L. (oilseed rape) is known as one of the most important volunteer crops worldwide. Brassica napus volunteers can emerge for more than 10 years in subsequent crops. Three field experiments were carried out to compare the competitive ability and reproduction of conventional and Clearfield volunteer B. napus in spring barley under different herbicide treatments in field conditions. Herbicide efficacy, aboveground biomass and seed production of volunteer B. napus and the barley yield were measured. All studied variables (herbicide treatment, year and B. napus variety) showed a significant effect on these characteristics and all together counted for 67.8% of the total variation in the studied data. Most of the variability was explained by herbicide treatment (37.2%), followed by year (26.7%) and B. napus variety (3.9%). The most effective herbicide controlling both types of volunteer B. napus was 2,4-D (efficacy more than 95%). Florasulam used together with tritosulfuron was the most effective of ALS inhibiting herbicides against CL volunteer B. napus (efficacy 58–83% depending on year). Competitive ability of volunteer B. napus in spring barley and herbicide efficacy were significantly affected by weather conditions during the growing season (low yield of spring barley in dry year). Reproductive ability of both tested varieties of volunteer B. napus was very low. In crop rotations with imidazolinone-tolerant oilseed rape varieties, it is necessary to concentrate on suitable herbicide strategies for CL volunteer B. napus control in subsequent crops and to adapt harvest date and tillage type to keep volunteer densities at the lowest levels possible.