The objective of this experiment was to evaluate the use of a 3 screen Penn State Particle Separator box (19.0-, 8.0-, and 4.0-mm screens; PSPS4mm) to estimate physically effective factor (pef) and physically effective neutral detergent fiber (peNDF) of forages and total mixed rations (TMR) using the Ro-Tap method as the gold standard. Samples of corn silage (CS; n = 21), alfalfa silage (AS; n = 21), and TMR (n = 20) were collected from 20 Midwest dairy farms in summer/fall 2014. Four gallons of each sample were collected and mixed thoroughly. Samples were split and analyzed for pef using PSPS4mm and Ro-Tap with standard shaking methods. Sample nutrient composition was determined at Dairyland labs (Arcadia, WI). The pef was calculated as percentage of sample retained above 4.0-mm and 1.18-mm screens for PSPS4mm and Ro-Tap, respectively. The peNDF (%DM) of each sample was calculated as pef x NDF. Dry matter intake, milk yield, and milk protein and fat content were recorded for each herd. The REG procedure of SAS was used to determine the relationship between pef and peNDF estimated using PSPS4mm and Ro-Tap. The Stepwise Selection procedure was used to determine variables that affect herd milk components. The PSPS4mm was a good predictor of pef and peNDF in TMR and forages (R² = 0.93 and 0.98; slope = 0.86 and 0.91, respectively). For the CS, AS, and TMR samples, average pef estimated with Ro-Tap was (mean ± SD) 87.4 ± 3.99, 83.5 ± 4.34, and 64.3 ± 6.24, respectively, and average pef estimated with PSPS4mm was (mean ± SD) 88.4 ± 4.58, 87.6 ± 2.92, and 64.2 ± 6.35, respectively. Significant variables for predicting herd milk fat content were pef, concentrate intake, and milk yield (model R² = 0.55). Significant variables for predicting milk protein content were pef and forage intake (model R² = 0.42). The PSPS4mm is a useful tool to estimate pef and peNDF of forages and TMR. The pef within and across sample type varied, so estimating pef of individual samples on-farm will allow for more precise formulation of ration peNDF, which affects herd milk components.