Technical Abstract: Populations of four perennial herbaceous species that were genetically modified for altered lignin content (or associated forage digestibility) by conventional plant breeding were evaluated for agricultural fitness in three Midwest USA environments for over four years. Reduced lignin concentration or increased digestibility resulted in increased winter mortality in three of four species. Selection for survival in one species appeared to break this apparent genetic correlation, suggesting that it is not mechanistic. Results indicate that perennial plants genetically engineered with altered lignin concentration or composition for use in livestock, pulp and paper, or bioenergy production should be evaluated for fitness prior to use in agriculture.