In vivo measurements are the gold standard against which in vitro procedures digestibility estimates must be judged. To quantify the relationships of in vitro ruminal disappearance to total-tract digestibility of starch and NDF, samples were obtained from the top string of lactating cows from 32 commercial herds in the Upper Midwest. Silage, TMR, grain, and fecal samples were assayed at a commercial laboratory for some of the following: starch, 7h in vitro starch digestion, NDF digestibility at 24 and 240 h, and lignin at. For calculating in vivo digestibility, lignin and UNDF served as indigestible markers. All cows were fed kernel processed corn silage (25 to 40% DM) plus dry ground or high moisture ear corn or grain; diets averaged 26% starch and 31% NDF. Except for 2 herds, starch digestibility exceeded 95% being related closely to fecal starch concentration (R² = 0.98). However, in vitro starch disappearance at 7 h was not related to total-tract starch digestibility (R² = 0.00). Total-tract digestion averaged 55% for NDF or 60% for available NDF (NDF minus UNDF240). In vivo NDF digestibility of the TMR tended to decline as its UNDF content increased (R² = 0.21), but 24 h in vitro NDF disappearance of the TMR proved to be poorly related to NDF digestibility in vivo (R² = 0.13). Kernel processing score was not related to starch digestibility, but all silage had fermented more than 6 mo before feeding. Processing score tended to be greater and presence of whole and half kernels in silage was less with shredlage than conventional kernel processing rolls but total-tract starch digestibility was not altered by processing roll type. Dry matter digestibility calculated from UNDF differed by as much as 10% from digestibility calculated from lignin. Total-tract starch digestibility was not correlated with surface area of particles or mean particle size of the dry ground corn fed. To obtain reliable and accurate estimates of energy availability for ruminants, nutritionists should rely on total-tract digestion measurements.