Diets with 2 contents of nonfiber carbohydrate (NFC) containing corn stover were formulated to compare the lactation performance and rumen fermentation characteristics in lactating cows fed alfalfa hay. Twelve Holstein cows in mid-lactation (159 ± 15 d in milk) were randomly assigned to 1 of 3 dietary treatments: (1) low-NFC (NFC = 35.6%, L-NFC), (2) high-NFC (NFC = 40.1%, H-NFC), and (3) alfalfa hay (NFC = 38.9%, AH). In both L-NFC and H-NFC diets, corn stover was included at 15% of total dietary DM. The experiment was conducted according to a replicated 3 × 3 Latin square design with 21-d periods each, with the first 14 d for an adaptation. Milk yield and milk composition were recorded during d 15–21, and rumen fluid samples were taken on d 19 of each period. The data were analyzed using PROC MIXED of SAS. Intake of DM was lower for cows fed H-NFC compared with L-NFC and AH (20.1 vs. 21.5, and 21.9 kg/d; P < 0.01), while milk yield was higher in AH than in H-NFC and L-NFC (24.8 vs. 22.8 and 23.2 kg/d; P < 0.01). Thus, feed efficiency (milk yield/DM intake) were higher for cows fed H-NFC and AH than the L-NFC fed cows (1.15 and 1.15 vs. 1.08; P < 0.01). Milk fat content was higher for cows fed H-NFC and L-NFC compared with AH-fed cows (4.11 and 4.25 vs. 3.90%; P < 0.01). The NFC digestibility was higher in cows fed H-NFC and AH than those fed L-NFC (92.7 and 92.7 vs. 91.9%; P = 0.03). Concentration of milk urea N was lower for cows fed H-NFC and AH than those fed L-NFC (18.3 and 18.1 vs. 20.3 mg/dL; P < 0.01), indicating an increased N conversion for cows fed H-NFC and AH. The concentrations of rumen acetate (77.5 vs. 69.5 and 72.7 mM; P = 0.03), propionate (24.8 vs. 20.3 and 22.0 mM; P < 0.01) and total volatile fatty acids (120 vs. 106 and 111 mM; P = 0.02) were higher for cows fed AH than those fed H-NFC and L-NFC, with no difference between cows fed H-NFC and L-NFC (P > 0.05). From the results obtained in this study, it is inferred that the increased NFC content can improve feed efficiency in diet containing corn stover, and is beneficial for the N conversion.