Our objective was to determine the effects of dietary neutral detergent fiber (NDF) level and source on Energy (E) and nitrogen (N) balance of Holstein and Jersey. Mid-lactation Holstein (n = 4) and Jersey (n = 4) were used in a split-plot triplicated 4 × 4 Latin square with breed and diet as the main and subplots, respectively. Treatment factors (as 2 × 2 arrangement) were: forage NDF (FNDF) level [19 (LF) and 24% (HF) of dry matter (DM)] and FNDF source [alfalfa silage (AS):corn silage (CS) NDF at 70:30 and 30:70 ratio]. Diets had 17% crude protein, 23% starch and 4.38 Mcal/kg gross E (DM basis). Total collection was performed from d 24 to 27 in each 28-d period and methane (CH₄) was measured with GreenFeed. Model included breed, FNDF level and source, and all possible interactions as fixed terms; square within breed, and cow within breed and square were the random terms. Significance was declared at P < 0.05. Holstein had greater DM intake (DMI, 34%), outputs of fecal DM (37%) and urinary DM (20%), and fat-protein corrected milk (FPCM, 31%) than Jersey. LF-fed cows had 8% greater N intake but 17% lower urinary N loss resulting in 11% decreased manure N excretion than HF-fed cows. Nitrogen balance (N intake - N in milk and manure) was not affected by breed and FNDF source but was greater for LF than HF-fed cows (194 vs. 139 g/d). These unusually high values may have reflected cumulated errors and accumulation in various N pool in the body. Neither E intake nor E output variables (% of gross E intake) were affected by breed and FNDF level. CS-fed cows lost 15% less urinary E and 10% less CH₄ E, resulting in a slight (3%) but significantly greater metabolizable E than AS-fed cows. Yet, FPCM/DMI was 4% lower for CS than AS-fed cows and the former had 12% greater loss of non-maintenance heat E than the latter. Nitrogen excretion and balance were not affected by breed and FNDF source but LF-fed cows had lower urinary and manure N loss resulting in greater N balance than HF-fed cows. Breed and FNDF level affected none of the E variables but CS-fed cows had lower losses of urinary and CH₄ E than AS-fed cows.