The objective of this study was to evaluate effects of 2 different sources of rumen available nitrogen on cattle performance, rumen nitrogen dynamics, and rumen microbial metabolism in lactating dairy cattle. Eight ruminally cannulated multiparous Holstein cows averaging 60 ± 10 DIM and 637 ± 38 kg of BW were assigned to one of 2 treatment sequences in a switchback design. Diets contained (DM basis) 44% corn silage, 13% alfalfa haylage, 12% ground corn, and 31% protein premix containing either a control mix of urea and wheat middlings (CON) or Fermenten at 3% diet inclusion rate (EXP). Both diets provided similar level (DM basis) of aNDFom (31%), CP (16.5%), RDP (8.2%) and metabolizable energy (64 Mcal ME/d). Diets were formulated to provide approximately 115% of rumen NH₃-N requirement as predicted by the CNCPS. The trial consisted of 3 28 d experimental periods, where each period consisted of 21 d of diet adaptation and 7 d of data and sample collection. Digestion markers were infused continuously during the sampling period and composited omasal samples were used to calculate nutrient flows. All data were analyzed using the Proc Mixed procedure in SAS. Dry matter intake was 25.5 and 25.2 kg/d for CON and EXP, respectively (P = 0.73). Energy corrected milk yield was 42.2 and 43.2 kg/d for CON and EXP, respectively (P = 0.43), with no treatment differences (P > 0.05) in milk fat and protein yield or content. Compared with CON, EXP increased milk urea nitrogen (10.7 vs. 13.5 mg/dL; P = 0.01) and rumen NH₃-N (4.4 vs. 5.3 mg/dL; P = 0.03). Rumen bacteria and omasal protozoa N content tended to increase with EXP inclusion (P < 0.1), while rumen protozoa and omasal bacteria N content increased (P < 0.05) with EXP inclusion. Effects of Fermenten inclusion on rumen microbial metabolism, especially microbial N content, were consistent with previous observations of increased microbial growth and turnover associated with supplementation of rumen available non-NH₃-N in N efficient diets.