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The rapid spread of herbicide-resistant weeds coupled with the decline in the number of new herbicide modes-of-action under development poses a serious threat to the future productivity of canola (Brassica napus L.). Crop competition is now an essential component of integrated weed management to reduce weed impacts and retard the spread of resistance. Canola plays an important role in any cereal-dominated crop rotation by providing diversification to broaden options for weed management, as well as other agronomic benefits for disease, pest and nutrition management. This review examines the opportunities to manipulate canola agronomy and increase the competitive ability by choice of cultivar, seeding rate, row spacing and orientation, and fertilizer use. The integration of such options for weed management prolongs the effectiveness and availability of herbicides.

Conservation cropping systems with no-till and stubble retention improve soil condition and water conservation. However, tillage is replaced by herbicides for weed control in these systems, increasing the threat of herbicide resistance. In the medium to high rainfall zones of the southern wheatbelt of Australia and under irrigation, wider row spacing is used to enable seeding into heavy stubble loads and to avoid stubble burning. Some evidence suggests that wider rows lead to reduced crop competitive ability and crop yields, greater herbicide dependence, and increased spread of resistance. Our aim was to test the hypothesis that increasing seeding rate compensated for reduced competitive ability at wider row spacings, especially when herbicide performance was suboptimal. We examined the impact of two wheat row spacings (18 and 36cm) and five seeding rates (resulting in a range of densities of 80-700 plants/m2) on control of Lolium rigidum with five rates of post-emergence application of diclofop-methyl (Hoegrass), ranging from label rate to lower rates, over two growing seasons. In the presence of L. rigidum, wheat grain yield was unaffected by row spacing but was significantly reduced at low seeding rates, especially at lower herbicide rates. Lolium rigidum was suppressed at higher crop densities but was also unaffected by row spacing. Grain yield was maximised when post-emergence herbicide was applied at 60-100% of the recommended dose at wheat densities >300 plants/m2. Significant levels of the weed remained in the crop at anthesis in all treatments. Weed dry matter ranged from 525g/m2 at low crop densities and with no herbicide to 150g/m2 with the recommended rate of herbicide and high wheat densities. The implications of manipulating crop competitive ability to improve weed control are discussed, especially in conditions where herbicide performance is unreliable due to weeds developing herbicide resistance or adverse environmental conditions.

Premise of the study: Solanum elaeagnifolium (silverleaf nightshade), having originated in the Americas, is now a serious summer-growing, perennial weed in many countries, including Australia. Most surfaces of the plants have a dense covering of trichomes

Herbicide tolerance of wheat and barley cultivars has been tested at Wagga Wagga in southern New South Wales since the 1980s and on the Darling Downs in southern Queensland since 1999. Although each project tests the cultivars and herbicides specific for their region, a number of herbicides and cultivars were common to each project. Overall, response between the two projects was similar for approximately 50% of the common cultivar × herbicide combinations. Crop damage was greater from testing in southern New South Wales than in southern Queensland for over a third of the common cultivar × herbicide combinations. This reinforces the need for regional testing for variations in cultivar tolerances to herbicides