The in vitro incubation time intervals selected to predict in vivo digestion of feeds at commercial laboratories have been based on ruminal retention time of undigested particles. To determine the specific in vitro incubation time that numerically matched and correlated best with ruminal and total-tract digestion of NDF and starch, site of digestion of 7 non-BMR corn silages ensiled for 4 mo was measured in 6 periods using 12 growing Holstein heifers (300 kg) equipped with duodenal cannulas in a 6 by 12 Youden square. Diets contained 90% of DM from corn silage plus a supplement with chromic oxide, an indigestible marker. For each corn silage, in vitro disappearance of NDF and starch also was measured at a commercial laboratory at multiple time points up to 240 and 48 h, respectively. In vivo ruminal and total-tract digestion of NDF ranged from 16 to 34% and from 26 to 43% numerically matching mean in vitro disappearance at 12 to 20 h and at 18 to 28 h, but the in vitro to in vivo correlations at these times were low (R2 = 0.07 to 0.20). Total-tract NDF digestion was correlated best with in vitro NDF disappearance at 240 h (R2 = 0.48). Ruminal and total-tract starch digestion ranged from 78 to 95 and from 95.5 to 99% numerically matching mean in vitro starch disappearance at 12 to 18 h and at 24 h. The incubation time best correlated with ruminal starch digestion was 7 h (R2 = 0.36). Ruminal NDF disappearance of individual corn silages was overestimated by 6 to 139 percentage points by in vitro incubation for 24 h while ruminal starch digestion was underestimated by 48 to 72% by in vitro incubation for 7 h. Correlations with in vivo ruminal digestion for NDF and starch digestion at these incubation times were 0.19 and 0.36. None of the incubation times tested gave disappearance values that correlated well with ruminal or total-tract digestion of starch or NDF from corn silage by growing heifers.