Control of southern crabgrass by quinclorac was influenced by plant growth stage. A three-parameter nonlinear regression was used to describe the influence of quinclorac concentration on each growth stage and to compare growth stages. A SAS® program for these comparisons is described. Pseudo R2 values for fit were above 0.98 for all tests. Flowering crabgrass plants had the highest GR50 value (50% growth reduction on a dry weight basis) whereas preemergence, three- to five-true-leaf, and two- to four-tiller stages were lower.