Technical Abstract: During the storage of hay or haylage, heating damage may occur and lead to losses of available protein and digestible nutrients. Recent research indicates that losses of TDN may be more significant economically than losses of available protein. Our objectives for this study were to establish a near-infrared (NIR) model to estimate losses of total digestible nutrients (TDN) caused by heating damage during storage, to test the model on farmer samples obtained from commercial forage testing laboratories, and then to determine the frequency with which significant losses of TDN occurred within these producer samples. Samples from a previous study in which internal bale temperatures were measured daily were selected to build into the model. The external validation coefficient of determination (R2) was 0.88, which is relatively high. The calibration was tested on three unknown sets of samples from commercial forage testing laboratories. The three sets contained spectra of 1000, 618 and 1994 forage samples submitted by producers. A very small number of samples in Sets 1, 2, and 3 exhibited H values >3 (21, 11 and 66 samples, respectively), which indicates that the developed equation was appropriate for use on the commercial samples. The predicted TDN losses from producer samples exceeded 4 TDN units 50% of the time, and exceeded 8 TDN units 16% of the time.