Crop canopy development and structure influence weed suppression

Weed suppression characteristics of different winter cereal species and cultivars were studied in two field experiments in two successive years. Two cultivars each of barley, oats and wheat were studied using the weed Galium aparine L. at a range of densities. Significant differences were found in the suppressive abilities of the crop species: oats being the most suppressive, followed by barley and then wheat. Percentage yield loss was related to G. aparine density using a rectangular hyperbolic model. In 1995-96, significant differences between the two wheat cultivars were found, with cv. Avalon being able to suppress G. aparine growth much more effectively than cv, Spark. However, in 1996-97, high rainfall in May, June and July enabled G. aparine to escape suppression by the cultivars, Cultivar competitive ability was associated with high overall leaf area, resistance to loss of tillers under competitive pressure, greater height, canopy structure and development. The oat cultivars were more competitive at early growth stages than could be accounted for by their canopy structure; the possible contribution of allelopathic exudates to their suppressive ability is discussed.