Abstract Harvest weed seed control (HWSC) is a weed management technique that intercepts and destroys weed seeds before they replenish the soil weed seedbank and can be used to control herbicide-resistant weeds in global cropping systems. Wild radish (Raphanus raphanistrum L.) is a problematic, globally distributed weed species that is considered highly susceptible to HWSC, as it retains much of its seed on the plant during grain harvest. However, previous studies have demonstrated that R. raphanistrum is capable of adapting its life cycle, in particular its flowering time, to allow individuals more time to mature and potentially shed seeds before harvest, thereby evading HWSC interception. This study compared the vegetative growth plus physiological and ecological fitness of an early-flowering R. raphanistrum biotype with an unselected genetically related biotype to determine whether physiological costs of early flowering exist when in competition with wheat (Triticum aestivum L.). Early flowering time adaptation in R. raphanistrum did not change the relative growth rate or competitive ability of R. raphanistrum. However, the height of first flower was reduced in the early flowering time–selected population, indicating that this population would retain more pods below the typical harvest cutting height (15 cm) used in HWSC. The presence of wheat competition (160 to 200 plants m–2) increased flowering height in the early flowering time–selected population, which would likely increase the susceptibility of early-flowering R. raphanistrum plants to HWSC. Overall, early-flowering adaption in R. raphanistrum is a possible strategy to escape being captured by the HWSC; however, increasing crop competition is likely to be an effective strategy to maintain the effectiveness of HWSC.

BACKGROUND Multiple-herbicide resistance in Lolium rigidum and other weed species is increasingly exerting pressure on herbicide discovery research for solutions against resistance-prone weeds. In this study we investigate: (i) the responses of L. rigidum populations and wheat to the new herbicide cinmethylin in comparison with other pre-emergence herbicides, (ii) the effect of seed burial depths on cinmethylin efficacy and crop selectivity, and (iii) the basis of cinmethylin selectivity in wheat. RESULTS Cinmethylin at 400 g ha(-1) controls herbicide-susceptible and multiple-resistant L. rigidum, with a reduction of >85% in plant emergence and 90% in aboveground biomass. Cinmethylin provides effective control of a large number of field populations of L. rigidum with evident resistance to trifluralin. When the wheat seed is buried >= 1 cm below the cinmethylin-treated soil surface, the emergence of crop seedlings is not different from the untreated control. The organophosphate insecticide phorate synergizes cinmethylin toxicity in wheat, with an LD50 of 682 g ha(-1) in the absence of phorate versus 109 g ha(-1) in the presence of phorate (84% reduction). The synergistic effect of phorate with cinmethylin on herbicide-susceptible L. rigidum appears smaller (a 44% reduction in the LD50 of cinmethylin). CONCLUSIONS Cinmethylin is effective in controlling multiple-resistant L. rigidum and appears safe for wheat when the seed is separated at depth from the herbicide applied to the soil surface. The basis of this metabolism-based selectivity is likely regulated by cytochrome P450 monooxygenases.