NEAR-INFRARED REFLECTANCE SPECTROSCOPY ESTIMATION OF C-13 DISCRIMINATION IN FORAGES

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 C-13 discrimination (Delta) values obtained from isotope-ratioing mass spectrometers. If MRS could predict Delta, 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 Delta 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 Delta. Standard errors of calibration for regressing Delta with NIRS absorption values were higher for grasses than for alfalfa, Coefficients of variation for all validation sample sets used for prediction of Delta by NIRS were less than 3%, and NIRS correctly identified 77 to 82% of the samples with the lowest Delta 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.