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McDonald AJ, Riha SJ & Ditommaso A (2010). Early season height differences as robust predictors of weed growth potential in maize: new avenues for adaptive management? Weed Research 50, 110–119. Weed interference in annual cropping systems can be highly variable from year-to-year, as well as spatially heterogeneous. These factors have confounded efforts to develop simple methods for predicting competitive outcomes early in the growing season. A 2-year study was conducted with maize in competition with four annual weed species ( Setaria faberi, Abutilon theophrasti, Chenopodium album and Amaranthus retroflexus) to: (i) characterise the relationship between early crop–weed height differences and weed growth potential, and (ii) assess threshold-type decision rules based on these relationships. Analysis of 742 weed individuals suggests that early season height disparities are robust predictors of season-long growth. For example, weeds ≤8 cm tall when maize canopy was 60 cm tall had a very low probability ( c. 2%) of growing larger than 150 cm and this applied across weed species, relative emergence dates and spatial proximity to the maize row. In our dataset, weeds less than 150 cm tall were associated with low crop yield losses and weed fecundity. Precision management of weed individuals based on early season height differences may provide a reliable and practical basis for selective post-emergence weed control that is relevant to both yield loss and longer-term seedbank considerations. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) is considered an important driver of crop-weed interactions, yet the mechanisms involved have been only partially explored, especially with respect to early-season growth, when competitive hierarchies are formed. This study characterises the effects of different N levels on biomass accumulation and plant morphology for maize ( Zea mays), and four important weed species ( Amaranthus retroflexus, Abutilon theophrasti, Setaria faberi, and Chenopodium album). Under glasshouse conditions, plants were grown in separate pots and irrigated with nutrient solution at four N concentrations (0.2, 0.5, 2, 5 μm L−1) until 57 days after emergence. Except for S. faberi, which was unresponsive to N, the relative biomass growth rates (RGR) of maize and the broad-leaved weeds were positively and similarly affected by increasing nitrogen. At all N levels, maize had a height advantage by virtue of its larger seed size, which conferred early growth benefits independent of RGR. At low N, biomass growth was instrumental to S. faberi’s improved competitive position, whereas height development per unit biomass improved the competitive position of A. theophrasti, C. album and A. retroflexus. The approach presented could be applied to other crop-weed systems to evaluate environmental impacts on competitive outcomes. [ABSTRACT FROM AUTHOR]