Early season height differences as robust predictors of weed growth potential in maize: new avenues for adaptive management?

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.