The objective of this study was to determine if behavior, reticulorumen health, and production in early lactation dairy cows varies with TMR straw particle size. For 28 d after calving, 40 multiparous Holstein cows, housed in free-stalls, were individually fed (with 24 h/d feed access) either 1 of 2 TMR with 8.8% wheat straw (DM basis) chopped to: 1) 2.54 cm (Short; n = 20) or 2) 5.08 cm (Long; n = 20). Enrollment in the trial was on a rolling basis; cows were evenly distributed between treatments by parity and previous 305-d milk yield. Wireless telemetry boluses were used to measure reticulorumen pH. Automated systems recorded TMR DMI, milk yield, and rumination activity. TMR and orts samples were collected every 3 d to determine feed sorting. A particle separator was used to separate feed samples into 4 fractions: long (>19mm), medium (<19mm, > 8mm), short (<8mm, > 4mm), and fine (<4mm) particles. Feed sorting was calculated as: actual intake of each particle fraction expressed as a % of its predicted intake. Data were analyzed in mixed-effect linear regression models and were modeled with polynomial functions over the 28 d of observations. The fitted data indicated treatment differences in linear slopes (P < 0.01), quadratic coefficients (P < 0.01), and cubic coefficients (P < 0.01) for DMI (Long: kg/d = 1.6d – 0.1d² + 0.002d³ + 10.6; Short: kg/d = 1.1d – 0.06d² + 0.001d³ + 11.8), milk yield (Long: kg/d = 4.0d – 0.2d² + 0.005d³ + 17.7; Short: kg/d = 2.9d – 0.2d² + 0.003d³ + 22.3), and rumination time (Long: min/d = 52.9d – 3.5d² + 0.07d³ + 265.7; Short: min/d = 38.5d – 1.9d² + 0.03d³ + 251.14). Mean reticulorumen pH decreased at a greater rate from calving to 10 DIM for cows on the Long treatment (pH = −0.079d + 0.0055d² – 0.0001d³ + 6.53) than for cows on the Short (pH = −0.076d + 0.0055d² – 0.0001d³ + 6.44), as indicated by differences in linear slopes (P < 0.01). Cows tended to sort the longest TMR particles differently by treatment (P = 0.06); on the Long treatment cows sorted against long particles (94.1 ± 1.7%; P < 0.01), while on the Short treatment cows did not sort for or against these particles (98.8 ± 1.7%; P = 0.5). These results suggest that cows fed a diet with longer straw particles selected against physically-effective fiber, which may have contributed to greater fluctuations in rumination time, reticulorumen pH, DMI, and milk production in early lactation.