Kelp meal (KM) is a supplement made from the brown seaweed Ascophyllum nodosum known to bioaccumulate iodine (I). A. nodosum is also the richest of phlorotannins and contains other bioactive compounds that may boost animal production. However, we are not aware of any study that evaluated the effects of KM compared with another I source in cows fed soybean meal (SBM) or canola meal (CM). Our goal was to assess the interactions between 2 sources of I and RDP on production and urinary excretion of nitrogenous metabolites. Sixteen multiparous Jersey cows (93 ± 58 DIM) were used in a factorial replicated 4 × 4 Latin square design (21-d periods). Diets were formulated to yield similar concentrations of CP and I and consisted (DM basis) of 55% baleage and 45% concentrate. Dietary CP and NDF concentrations averaged 18.5 and 35.6%, respectively. Experimental treatments were: 10% SBM + 110 mg/d of ethylenediamine dihydroiodide (EDDI) or 114 g/d of KM and 12.5% CM + 110 mg/d of EDDI or 114 g/d of KM. Kelp contained 775 mg/kg of I. Data analysis included the main effects of RDP and I source and interaction. Compared with CM, SBM significantly decreased DMI (20.7 vs. 21.7 ± 0.82 kg/d) and MUN (10.4 vs. 12.9 ± 0.4 mg/dL) and increased milk yield (22.7 vs. 21.5 ± 0.81 kg/d), thus improving feed efficiency (1.19 vs. 0.99 ± 0.03 kg/kg). Milk N efficiency was also greater (P < 0.01) in SBM than CM cows. Reduced milk yield and MUN was likely caused by atypical concentrations of NDICP (23.6%) and ADICP (15.5%) in the CM used. Concentrations of milk fat (5.44 vs. 5.08 ± 0.14%) and protein (3.64 vs. 3.52 ± 0.05%) were significantly greater in cows fed CM vs. SBM. Feeding KM significantly increased milk yield compared with EDDI (22.4 vs. 21.8 ± 0.81 kg/d), while DMI tended (P = 0.09) to improve in KM cows. Neither RDP nor I source affected the urinary excretion of total purine derivatives. In contrast, urinary excretion of N (% of N intake) decreased (26.2 vs. 31.2 ± 1.92%; P < 0.05) with feeding CM rather than SBM diets. Overall, SBM and KM increased DMI and milk yield, and feeding SBM further improved feed and milk N efficiency.