Objectives of our work were to assess the relationship between chemical components and forage fragility, when measured as grinding energy (GrE), and between fragility and digestibility of forages. In Experiment 1 28 forages were analyzed for DM, total N, starch, NDF, ADF, ADL, 24 h ivNDFd, total phenols (TP) and non-tannic phenols (NTP). Initial particle size (IPS) was determined after pre-grinding the samples with a 2 cm fitted screen knife mill. The GrE was measured using an ultra-centrifugal and a hammer mill, fitted with a 1 mm screen and final particle size (FPS) was determined. Difference between the 2 mills was determined using a t-test for 2 dependent samples. Multiple regressions were used to best predict observed GrE measurements from the set of parameters obtained. Energy measurements ranged from 34.9 to 356 J/g with the ultra-centrifugal mill consistently using less energy than the hammer mill (P < 0.001). The independent variables selected were able to explain 77 to 89% of the GrE and IPS consistently accounted for most of the variation, showing the difficulty of having a homogenous IPS. Forage resistance to grinding did not depend on the fiber quantity alone, but on the different quality and interaction of its components. In Experiment 2 GrE was measured on 34 forages, including corn silages, alfalfa and eragrostis hays. In vitro NDF digestibility was obtained from 0 to 240 h. Correlations between GrE and NDFd at various time points and NDF rates of digestion (kd) were calculated and the best predictors among fiber, its components and digestibility values were selected. The results showed high negative correlations of NDFd and kd with GrE (P < 0.001). In particular 18 and 24 h NDFd had the highest correlation values (−0.65 and −0.77) across samples and species, showing the important relationship between fragility and fiber digestibility. The IPS, FPS, cellulose (ADF-ADL) and kd explained up to 60% of the variation. Grinding energy can be used to rank forages to account for different fragility, but IPS represents a limitation. Forages with similar peNDF may then be adjusted to account for different GrE and fragility.