Highlights: • Sheep/cattle mixed grazing can improve lamb liveweight gain through forage niche sharing (FNS) and/or parasite dilution (PD). • We modelled the gains for lambs in terms of metabolizable energy (ME) and crude protein (CP) from PD and FNS. • Gains in ME from PD were often higher than those from FNS, whereas the opposite was true for CP. • ME requirements are the most difficult to cover, which suggests a prominent role of PD in sheep/cattle mixed-grazing. • Using common metabolic metrics helped assess the relative weights of biological mechanisms of contrasting nature. Abstract: CONTEXT: Mixed-grazing by sheep and cattle is the simultaneous or sequential grazing of a pasture by the two species. It is an agroecological practice known to improve lamb liveweight gains through parasite dilution (PD) and/or forage niche sharing (FNS). OBJECTIVE: We assessed the combined and relative strengths of the two mechanisms by developing a metabolic approach (French uplands context). METHODS: We used recently published equations to model the infection cost of gastrointestinal nematodes in metabolizable energy (ME), and crude protein (CP). By comparing infection levels in mixed and monospecific grazing, we quantified the gains of PD in ME and CP. We also used feed value tables to assess the gains in ME and CP resulting from FNS sheep diet improvement. We interpreted these gains in light of ME and CP requirements of lambs. We applied this approach to the dataset of a mixed-grazing experiment, involving sheep monospecific grazing and mixed sheep/cattle grazing, without veterinary treatment. We also applied it to a generic situation where we studied the relative gains in ME and CP, along gradients of increasing strength of PD and FNS. RESULTS AND CONCLUSIONS: The approach applied to our experimental data revealed that i) infection by gastrointestinal nematodes can represent 100% of ME and 57% of CP requirements in monospecific grazing, ii) mixed-grazing can reduce these costs to 23% and 12%, respectively and iii) PD was more important than FNS in terms of ME gains, whereas the opposite was true for CP. However, meeting CP requirements was less constraining than meeting ME requirements on the studied grassland type, which puts into perspective the importance of CP gains. With the generic approach, most of the modelled situations also identified PD as the main mechanisms of ME gain (79%), whereas it was FNS for CP (70%), with the same observation that CP requirements were less difficult to meet. Both approaches suggest that in our modelled context, PD matters more often than FNS in mixed-grazing, due to the greater difficulty in meeting ME requirements. SIGNIFICANCE: We proposed a novel approach to assess the roles of two biological mechanisms of contrasting nature, with the help of common metrics. This approach made it possible to evaluate their combined and relative strength. It has the potential to improve our understanding of the impact of interacting biological mechanisms involved in agroecological grazing systems, and identify prominent ones.