The impact of weeds on crop yield can be reduced and the effectiveness of weed control procedures increased by improving crop competition. The rate and extent of crop canopy development are key factors influencing a crop’s competitive ability with weeds. A crop that rapidly establishes a vigorous canopy, intercepting maximum sunlight and shading the ground and inter-row area, will provide optimum levels of competition.

Canopy development can be influenced by:

Crop and variety. A competitive crop improves weed control by reducing weed biomass and seed-set. Crops can be roughly ranked in competitive ability. Oats are the most competitive crop against annual ryegrass. Chickpeas and lupins have been shown to have limited ability to compete against weeds.

Row spacing, sowing rate and sowing depth. At any sowing time increasing sowing rate can result in earlier crop canopy closure and greater dry matter production, improving weed suppression and the effectiveness of other weed management tactics. Narrow crop rows usually deliver much better crop competition than do wider rows. Maximum competitive ability will come from a crop sown at optimum and uniform depth to get rapid and uniform establishment.

Crop nutrition. Matching fertiliser inputs of both macro- and micronutrients to crop target yield and quality will maximise the crop’s competitive ability against weeds.

Foliar and root diseases, and nematodes. A healthy crop will best compete with weeds

Levels of beneficial soil microbes such as vesicular arbuscular mycorrhiza (VAM)

Environmental conditions including soil properties and rainfall. Matching the crop (and variety) to the soil type can improve crop vigour and biomass production, which in turn will optimise crop competitive ability. Time of sowing also has a direct impact on the competitive ability of a crop. Delaying sowing beyond the optimum window recommended in a given district will reduce early vigour, extend the time taken to reach canopy closure and reduce overall dry matter production.

Each will in turn affect plant density, radiation adsorption, dry matter production and yield. Early canopy closure can be encouraged through good management addressing the above factors.