Mineral supplementation is vital for optimum production and efficiency in livestock, but it is an added feed cost. We hypothesized a reduction in supplemental minerals in a ration may have minimal effect on rumen function and performance of lambs fed Ru-Mend, a product designed to enhance nutrient absorption. Sixty Katahdin wethers (average BW 13 ± 3 kg) 55 d of age were fed an ad lib corn silage plus alfalfa haylage-based diet with Ru-Mend, and either adequate (FULL) or reduced (RED) mineral supplements for 52 d. The total mixed rations were analyzed and RED diet found to be 30% lower in Co, Cu, Mn and Zn, 15% in S, 10% in Ca and Mg, and 5% in P as compared with the FULL diet. Diets were balanced for K, Na and Cl, but Na was 10% lower in the RED diet. Lambs were penned in groups of 15 with 2 pens per treatment. ADG was calculated for each animal from their initial and 44 d weight. DMI was calculated from pen daily intake. Lambs were euthanized on d 52, and tissues collected for analysis. Data were analyzed by ANOVA completely randomized design. Pearson correlations were used for comparison. Final rumen content pH was between 5.2 and 6.0 for all lambs. Lambs fed the FULL diet had higher DMI (640.71 vs. 513.13 g/d; P < 0.0001) and ADG (102.15 vs. 72.17 g/d; P = 0.003) than RED lambs. FULL lambs’ ADG correlated with lower rumen ammonia (P = 0.02). The rumen fluid contents of FULL-fed lambs had higher total amino acids (P = 0.004) and essential amino acids [arginine, isoleucine, lysine, phenylalanine, threonine and valine] (P ≤ 0.03) than RED-fed lambs. The lower ammonia and higher amino acids may indicate positive effect of minerals due to increased DMI and reduced protein degradation. Rumen osmolality and acetate were higher (P = 0.05) for FULL lambs. RED lambs had lower rumen and ileum contents Co, Cu and Mn (P ≤ 0.02) and rumen Fe, Mg, P and S (P ≤ 0.03). Lower ADG of RED lambs correlated with reduced ileum K, P and S (P ≤ 0.05) and numerically less rumen Mg, Ca and Co (P ≤ 0.10). This may indicate lambs fed the RED diet did not benefit from Ru-Mend, thus minerals needed for energy metabolism, growth and weight gain were unavailable.