Enhanced weed‐crop competition effects on growth and seed production of herbicide‐resistant and herbicide‐susceptible annual sowthistle ( Sonchus oleraceus )

Enhanced crop competition could aid in the management of annual sowthistle (Sonchus oleraceus L.), a dominant weed of Australian cropping systems. A two-year pot study was conducted to evaluate the effect of wheat (Triticum aestivum L.) planting densities (0, 82, and 164 wheat plants/m) on growth and seed production of glyphosate-resistant (GR) and glyphosate-susceptible (GS) biotypes of annual sowthistle. Without competition, both biotypes produced a similar number of leaves and biomass, but the GS biotype produced 80% more seeds (46,050 per plant) than the GR biotype. In competition with 164 wheat plants/m, the number of leaves in the GR and GS biotypes was reduced by 62 and 61%, respectively, in comparison with the no-competition treatment, and similarly, weed biomass was reduced by 78 and 77%, respectively. Compared to no-competition treatment, the seed production of GR and GS biotypes was reduced by 33 and 69%, respectively, when grown with 82 wheat plants/m, but increasing wheat density from 82 to 164 plants/m reduced the number of seeds only in the GS biotype (81%). Both biotypes produced greater than 6,000 seeds per plant when grown in competition with 164 plants/m, suggesting that increased crop density should be integrated with other weed management strategies for efficient control of annual sowthistle.