In the eastern Amazon, new grasslands cultivated after deforestation often suffer fast weed invasion, which is a significant problem for the sustainability of new farming systems. The authors study the links between farming systems and grassland deterioration using farm survey data. They distinguish two main farm trajectories - a changing trajectory in which farmers do not maintain a good balance between grassland and cattle, leading to deterioration; and a stabilized trajectory in which farmers implement various grassland management measures ranging from continuous grazing to complex rotation, with low weed invasion. The conditions for stabilization of farming systems and the choices available for grassland management are discussed.