In the mountainous areas of Vietnam, agroforestry is being developed in response to new market opportunities, the need to improve forest cover, and to reduce soil erosion. These resulted in changes in land use and affect the animal mobility and feeding patterns of ruminants, which represents an economic opportunity and contributes to the trade balance on meat. Using the example of the Dien Bien district in northwestern Vietnam, and based on a modelling of crop livestock integration and biomass flows in the area, this study assesses the effect of these changes on livestock systems, helping to define strategies for the sustainable development of livestock systems. The development of agroforestry contributes to the intensification of feeding systems, or leads to a cessation of livestock activities. Agroforestry systems intercropping forage within tree plantations increase the areas dedicated to forage production and the quantities and quality produced. Depending on their objectives, farmers maintain livestock activities to maintain soil fertility from avoiding over grazing and have access to manure as an organic fertilizer source for crops, trees and forage. They adapt their livestock systems, by stalling the animals, recycling grasses produced elsewhere and ensure feed supply in the whole year, thanks to silage. Animal housing, harvesting, transport and forage conservation technics can also represent a significant workload for farmers who then abandon livestock farming. The increase in forage production with agroforestry offers opportunities to develop local forage and silage markets and to participate in local livestock development, while providing opportunities to improve the livelihoods of farms without livestock. Agroforestry with forage production, combined with forage conservation, offers opportunities for sustainable intensification of livestock systems in the Mountainous area of Vietnam.