Cotton (Gossypium hirsutum L.) yields have declined since 1965 in spite of improvements in technology and introductions of higher yielding cultivars. Scientists have been unable to identify exact causes of the yield decline. One suggested cause is the increased use of herbicides and their toxic effects on root growth in the herbicide incorporated zone. In this study, the computerized crop simulation model GOSSYM was used to predict cotton performance under several levels of herbicide-induced root reduction. Various environmental conditions were simulated in a Leeper clay loam soil. The model predicted a small increase in yield when root reduction was moderate and at shallow depths. When the depth of root reduction increased to 200 mm or the percentage of root reduction increased, however, there was a predicted decrease of up to 40% in lint yield. This decrease in lint yield was much larger when decreased root length was combined with reduced soil water permeability in remaining roots. Predicted yield was also reduced under unfavorable weather conditions. An increase in the depth of root reduction or increase in the percentage of root reduction, along with permeability reduction, caused a decrease in root growth. This decrease caused low nitrogen uptake throughout the season and increased water and nitrogen shortages.