If adoption of herbicide-resistant seed and adoption of conservation-tillage practices are determined simultaneously, adoption of herbicide-resistant seed could indirectly reduce soil erosion and adoption of conservation-tillage practices could indirectly reduce residual herbicide use and increase farm profits. Our objective was to evaluate the relationship between these technologies for Tennessee cotton production. Evidence from simultaneous estimation of a trinomial logit model for adoption of no-tillage, other conservation-tillage, and conventional-tillage practices and a binomial logit model for adoption of herbicide-resistant and conventional cotton seed suggests a simultaneous relationship. The elasticity for acreage in herbicide-resistant seed with respect to the probability of adopting conservation-tillage practices was 3.98. The elasticities for acreages in no-tillage practices and other conservation-tillage practices with respect to the probability of adopting herbicide-resistant cotton seed were 0.34 and 0.10, respectively. Adoption of herbicide-resistant cotton seed in Tennessee reduced soil erosion by 9.2 million tons through its effects on adoption of conservation-tillage practices. By 2004, increases in adoption of conservation-tillage practices increased adoption of herbicide-resistant cotton seed by 445 thousand acres, substituting non-residual herbicides for residual herbicides on those cotton acres.