Development of forage grass genotypes which maintain a high level of performance over a wide range of environments is a goal of most breeding programs. In this study the additive main effects and multiplicative interactions (AMMI) model analysis was used to understand the complexity of genotype by environment interaction and to evaluate the adaptability and yield stability of some tall fescue genotypes and their selected polycross progenies. Replicated forage yield data of 72 genotypes (24 parental, 24 early flowering and 24 late flowering progenies) from six main cropping seasons (2008–14) at two locations and under two levels of irrigation were used for this purpose. The AMMI-1 analysis results accounted for 47.6% of the genotype by environment interaction. Interaction patterns revealed by AMMI-1 biplots indicated that most of the tall fescue genotypes were narrowly adapted and among all evaluated genotypes, only four genotypes (G22, G50, G62 and G65) with yield performance above the average were considered broadly adapted. The AMMI-1 mega-environment analysis indicated that all the environments in Lavark were grouped in one mega-environment, except for E1 and E2. For this mega-environment the winning genotypes were the genotypes G9, G48 and G72. The environments in Isfahan location, except for E13, were grouped in another mega-environment. The genotypes G23, G8 and G15 were the winners in this mega-environment.