Our objectives were to determine (1) the best method to estimate the fast degradable fraction (A); (2) a time point to measure the undegraded fraction (C); and (3) the viability of using less time points to estimate starch fractional disappearance rate (kd) of mature corn ground through 4 grinding sizes (1, 2, 4, and 6 mm). Fraction A was determined by rinsing in a bucket or washing machine, rumen immersion followed by bucket or washing machine, warm water immersion for 30-min followed by bucket or washing machine. Ruminal in situ incubations were performed at 48, 72, 96, and 120-h to determine fraction C, and at 0 (A; washing machine), 3, 6, 12, 18, 24, and 48-h to determine kinetics of starch disappearance. Models were used with either 2 or 3-pools and kd determined by the slope of the ‘ln’. Effective ruminal disappearance (ERD) was calculated as: A + B [kd / (kd + kp)], kp = 7.47% h⁻¹. Data were analyzed using PROC MIXED with the fixed effect of method (either washing or model), grinding size, and their interaction and the random effect of run or cow. Correlation between estimates calculated using all time points or combinations of 2 and 3 time points were determined using PROC CORR. Fraction A was affected by grinding size (P < 0.01; 23.5, 15.7, 10.4 and 8.5%, for 1, 2, 4, and 6-mm); but not (P > 0.10) washing method. Samples ground at 6-mm had greater (P < 0.05) fraction C than other grinding sizes at 48 (14.7 vs. 2.45%), 72 (9.1 vs. 1.7%) or 96-h (2.9 vs. 0.3%); but not (P > 0.10; 0.1%) at 120-h. Models differed (P < 0.01) by 1.8%-unit for fraction B. As grinding size increased from 1 to 6-mm, greater fraction B (66.0, 73.8, 74.9, and 79.4%), but reduced kd (5.4, 5.1, 4.9, and 3.4% h⁻¹) and ERD (59.1, 53.7, 48.8, and 40.5%) were observed (P < 0.05). Based on correlation analysis the 2-pools model, and the incubation times of 0, 3, and 48-h were suitable (P < 0.001, r = 0.998) to evaluate starch in mature corn. Ruminal in situ incubations of 0, 3, and 48-h for starch disappearance kinetics using a 2-pool model are suggested for mature corn. Grinding size affects starch disappearance kinetics but not fraction A method.