Three experiments have investigated the effect of the timing of control of infestations of volunteer barley (Hordeum vulgare), Stellaria media and Galium aparine on the growth and yield of winter oilseed rape (Brassica napus). Although the experiments used conventional herbicides to achieve the different timings of control, the work was done in the context of the commercialization of herbicide-tolerant oilseed rape, where treatments could be applied at any time from autumn to spring. In the three seasons studied, oilseed rape growth was particularly vigorous in the autumn and, as a consequence, the competitive impact of the weeds was lower than anticipated. Untreated volunteer barley and G. aparine reduced yields in one and two experiments, respectively. However, in all experiments volunteer barley reduced crop growth in winter and spring severely, even though January treatments prevented yield loss in these vigorous crops. Delayed control of this weed would not be advisable if the weed was particularly dense or the crop less vigorous. In contrast, the G. aparine had no effect on crop growth and was only really apparent in the crop in late summer, so delaying treatment until even March would not put yields at risk. In one year, S. media markedly reduced crop growth in late winter but in the second experiment this did not occur. Consequently, as with the G. aparine, delayed autumn control would be unlikely to jeopardize yields. Thus, if herbicide-tolerant crops are commercialized in Europe, there will be flexibility in timing of application of herbicides to control broad-leaved weeds in winter rape but there would be a risk of yield loss from delayed control of volunteer cereals.