A ruminant model, MINDY, was used to explore the impact of grazing patches of tannin-containing legumes on environmental impact by cows grazing on grass-dominated rangelands swards. MINDY was initialized as a pregnant beef cow grazing on a grass (Bromus biebersteinii; meadow brome-Control), with the addition of a patch with tannin-containing legumes (Lotus corniculatus-birdsfoot trefoil-Tre, or Onobrychis viciifolia-sainfoin-Sain). Twenty-five scenarios (treatments) were established as a product of frequency (weekly, fortnightly, and monthly) and dietary proportions (0.05, 0.10, 0.15, and 0.20 of total forage allocation) of either Sain or Tre. Methane (CH4) production and nitrogen (N) excretion increased with legume availability. As legume proportion increased, dry matter intake (DMI), animal performance, CH4 production and urinary nitrogen (UN) excretion increased with a greater magnitude for Sain. Methane yield was lower than the Control at weekly allocation of legumes. Environmental costs in terms of CH4 emission increments per unit of production relative to Control were greater at monthly allocations when the levels of legume in the diet were low (0.5 – 0.10). Legumes in the diet at 0.15 – 0.20 yielded the lowest costs regardless of frequency of allocation. Environmental costs in terms of UN increments per unit of production relative to Control were lowest at 0.15 – 0.20 of legumes in the diet regardless of frequency of allocation. This study suggests that strategic allocation of either sainfoin or birdsfoot trefoil when beef cattle graze grass-dominated rangelands swards can reduce environmental impact, while increasing secondary production of the system at the same levels of resource allocation.