Feed production represents more than 70% of several environmental impacts estimated by Life Cycle Assessment (LCA) in pig (P) and broiler (B) productions. Yet, least cost feed formulation (LCF) is not relevant to reduce the environmental impacts of P and B feeds. Optimising feed composition while taking into account environmental impacts of feedstuffs (FS) is a way to do so. This study describes a new method, multiobjective formulation (MOF), based on the simultaneous use of an economic index (based on feed cost) and an environmental index (using LCA impacts) in a single objective function to minimize. A weighting coefficient (α) allows giving more or less influence to these indexes. The best trade-off was considered to be reached for an optimal weighting coefficient (αopt) set to be the coefficient beyond which the marginal increase of the economic index exceeds the marginal decrease of the environmental index. In the actual French context of FS availability, important reductions in impacts of P and B feeds were achieved with MOF in comparison with LCF. In B, at αopt, reductions by 12, 18, 7, 4, and 12% were achieved for ‘climate change’ (CC), ‘non-renewable energy use’, ‘eutrophication’, ‘acidification’ and ‘phosphorus consumption’ impacts, respectively, for an extra cost of 3%. In P, similar reductions of these impacts were achieved at αopt: -14, -13, -11, -7, and -6%, respectively, for an extra cost of 1%. For ‘land occupation’ impact, MOF had little effect in B (+4%) whereas in P, it decreased by 13%. LCA impacts at farm gate of one kg of live weight of B and P fed with these eco-feeds were also assessed. They confirmed that MOF reduces the environmental impacts of P and B productions (e.g. for CC, -7 and -10%, respectively) without pollution swapping and with moderate extra costs (e.g. +1% and +2% in P and B, respectively). Greater reductions of impacts are even possible when FS availability is less limiting (e.g. -19% for CC in B), questioning the actual availability of low-impact FS such as pea or sorghum.