Our objective was to assess 2 levels of forage neutral detergent fiber (FNDF) in the diet primarily as either alfalfa silage (AS) or corn silage (CS) on feeding behavior of Holstein (H) and Jersey (J) cows. Twent-four lactating primiparous cows (12 H, and 12 J; 606 ± 40 and 407 ± 43 kg of BW; 106 ± 17 and 112 ± 15 DIM, respectively) were the experimental units in a triplicated split-plot Latin square design with 2 × 2 factorial arrangement of treatments to evaluate 4 diets: Low FNDF (LF; 20% of diet dry matter (DM)) and high FNDF (HF; 26% of diet DM) with AS and CS NDF in ratios of either 70:30 or 30:70. All diets contained 30% NDF, 16.5% crude protein, 1.52 Mcal NEl/kg, and 24% starch (DM basis). On d 18 and 19 of each 28-d period, samples of TMR and individual orts were taken to determine particle size distribution using the Penn State Particle Separator (3-sieve and pan). Rumination time (RT) was measured using Hi-Tag rumination collars and data flow software with a resolution of 2 min. As expected, H had greater DM intake (DMI) than J cows (22.7 vs. 17.1 kg/d). However, as a percentage of BW, DMI tended to be greater for J than H cows (4.1 vs. 3.7, P = 0.08), and was greater for cows fed LF than HF diets (4.1 vs. 3.8), as well as for cows fed CS compared with AS diets (4.0 vs. 3.8). Particle size distribution as a percentage of DMI (estimated by the difference of DM retained on each sieve for TMR offered and orts) did not differ between breeds and FNDF sources, but physical effective DM (particles >1.18 mm) was greater for cows fed HF than LF diets (80.8 vs. 71.2% of DMI). Breed did not influence RT (492 ± 23 min/d), but J had greater RT than H cows when expressed as min per kg of DMI (31.3 vs. 20.6), FNDF intake (137 vs. 90), and BW (1.2 vs. 0.75). There were no effect of FNDF source on RT, but cows fed HF had greater RT than cows fed LF diets (511 vs. 473 min/d, 28.0 vs. 23.9 min/kg of DMI, 120 vs. 108 min/kg of FNDF, and 1.1 vs. 0.96 min/kg BW). In conclusion, breed and FNDF source did not influence feeding behavior, but HF diets resulted in greater intake of large particles and RT.