Interest in using crop competitiveness as an integrated weed management tool is increasing. Our objective was to describe traits that could be sources of the competitiveness we previously observed in grain sorghum grown in association with shattercane, which is a common annual weed and a close relative of the crop. Such information could aid in developing management practices for cultivated sorghum to improve its competitiveness with weeds. A bioassay was conducted to compare emergence of the crop and the weed in the greenhouse, and vegetative growth was monitored for 31 d in a within-row competition study. Results described a crop that competed well with the weed and other crop plants and agreed with studies showing that relative time of emergence influenced competitiveness. The mechanism by which grain sorghum emerged before the weed was a by-product of domestication that reduced glumes surrounding the wild-type seeds. This could be shown experimentally by hulling shattercane seeds, which then emerged almost as quickly as the grain sorghum. When planted in the grain sorghum row, shattercane plants from hulled seeds decreased the number of leaves and the root mass of the crop. Similarly, the time between emergence of the crop and emergence of shattercane was lessened by planting shattercane seeds early, and this increased the leaf number of the weed and shoot mass of the crop. It might be possible to increase weed suppression in grain sorghum by using management practices, such as more equidistant crop planting patterns that exploit the competitiveness already present, but which is being lost to interactions among crop plants.
Nomenclature: Shattercane, Sorghum bicolor (L.) Moench SORVU; sorghum, Sorghum bicolor (L.) Moench ‘Pioneer 8310’, ‘Pioneer 8771’.