Composition Factors Predicting Forage Digestibility by Horses

Forages play a key role in the equine diet; however, assessing the feeding value of forages through in vivo digestibility studies is costly and time-consuming. The ability to evaluate feeding value through a model relating chemical composition to forage digestibility could be a useful tool in the equine industry. The objective of this study was to relate forage chemical composition to in vivo dry matter digestibility (DMD) using previously published data, with the expectation that forage DMD would be most significantly related to neutral detergent fiber (NDF) and acid detergent fiber (ADF) content of the forage. Simple linear regression results indicated more variation in forage digestibility was explained by NDF and crude protein (CP) concentrations than ADF (r2 = 0.6017, 0.5402, and 0.4643, respectively). The best two-variable model for equine forage DMD was related to NDF and CP concentrations (DMD = 65.81 + 0.7207 × CP − 0.3514 × NDF; DM basis; P < .0001, R2 = 0.6690, adj. R2 = 0.6583). Model fit was marginally improved with the inclusion of three predictor variables. Comparison of these equations to independent data sets not used in model development showed promise in predicting forage digestibility within the range of forages used to generate the equations (CP: 2.9-22.6, ADF: 25.4-52.4, NDF: 32.8-83.3, DM basis). Further study is needed to expand the equations for a broader range of forages, but equine professionals and horse owners can use NDF and CP to make relative comparisons of forage value with typical hays fed to horses.