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Forage improvement programs often select for increased crude protein and dry matter digestibility. Additionally, breeding programs may be interested in selecting for enhanced transpiration efficiency or water use-efficiency. Forage crude protein and dry matter digestibility are commonly determined by near infrared reflectance spectroscopy (NIRS), whereas water use-efficiency is estimated from 13C discrimination values obtained from isotope-ratioing mass spectrometers. If NIRS could predict 13C discrimination then W could be determined simultaneously with quality components at a much lower cost. To test this possibility, leaf samples of alfalfa (Medicago sativa L.) and several cool-season perennial grasses were analyzed with a dual-inlet, double collector gas isotope mass spectrometer, and values of 13C discrimination were calculated. Subsamples were scanned with monochromators that collected spectra from 400 to 2,500 nm or 1,100 to 2,500 nm, and absorption data were regressed with values of 13C discrimination. Standard errors of calibration for regressing 13C discrimination with NIRS absorption values were higher for grasses than for alfalfa. Coefficients of variation for all validation sample sets used for prediction of 13C discrimination by NIRS were less than 3%, and NIRS correctly identified 77 to 82% of the samples with the lowest 13C discrimination values as determined by mass spectrometer analysis. This level of predictability may be acceptable for identification of genotypes with high water use-efficiency during the early phases of forage improvement programs. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020