[en] Highlights: • The long term effective half-lives T_eff-slow with their uncertainties are estimated. • The characteristic time of the decrease of ¹³⁷Cs bioavailability is determined. • Comparison of T_eff-slow among monitoring sites and literature data are discussed. • Old contaminated pastures have a very small soil to plant transfer factor (TF). - Abstract: Long term radioactivity monitoring programs contribute to the understanding of the behavior of radionuclides in the environment. This work aims to investigate the long term behavior of Cesium-137 in pasture ecosystem (root soil, grass and cow's milk) by using of more than twenty five years monitoring data collected at ten of French pasture sites contaminated by atmospheric fallouts from Chernobyl and nuclear atmospheric tests. We estimated with a simple exponential model the long term effective half-lives of radiocesium in root soil, grass vegetation and cow's milk along with their associated uncertainties. The average values of the effective half-lives over all the investigated sites were determined as 17, 11 and 9 years for soil, grass and milk respectively. Those values compare favorably to those estimated in previous studies in literature. These findings further enable us to quantify the decrease of ¹³⁷Cs bioavailability which ranges from 0.008 to 0.044 year⁻¹ with an average value of 0.026 year⁻¹ (i.e. effective half lives ranging from 16 to 87 years with an average value of 26 years in soil).