Application of nitrogen (N) fertilizer to sorghum at planting is a common practice that could confound competitive relationships of the crop with weeds. We studied the competitiveness of grain sorghum (Pioneer Brand 8333) relative to that of the annual weed shattercane and the perennial weed johnsongrass. The taxa are closely related, so survival requirements should be similar thus increasing the likelihood of finding differences associated with traits of the crop vs. weediness. Objectives of this research were to establish a competitive hierarchy for this crop–weed complex and to determine if relative competitiveness was affected by added N. A replacement design experiment was used in which plants were grown for 31 d in soil-filled pots placed outdoors. Taxa were planted in monocultures and 50:50 mixtures, representing all possible combinations of taxa, at a total density of 16 plants pot⁻¹. Soil moisture was maintained at field capacity by daily additions of water or 30 μg ml⁻¹ N in the form of an inorganic salt solution (KNO₃). There was no response to the solution containing exogenous N likely because the amount of N in soil was greater than demand. Actual shoot and root dry weights in mixtures were compared with the expected dry weights, which were calculated as 50% of the root and shoot dry weights in monoculture. For grain sorghum, actual dry weights in mixture were often better than expected. Replacement series indices calculated from dry weight data described grain sorghum as competitively superior to its weedy relatives. These results indicate that further research on N management for cultivated sorghum, as a means of increasing crop competitiveness relative to that of weeds, may be unwarranted. However, a better understanding of other competition mechanisms inherent in grain sorghum might suggest management alternatives to enhance crop competitiveness with weeds.

Nomenclature: Sorghum, Sorghum bicolor (L.) Moench ‘Pioneer Brand 8333’; shattercane, Sorghum bicolor (L.) Moench SORVU; johnsongrass, Sorghum halepense (L.) Pers. SORHA.