Crude protein in grasses is an essential nutrient that affects the growth of dairy cattle and the quality of milk produced by the cattle. Moreover, there has been demand for non-destructive measurement technology for measuring grass crude protein content. In this study, we investigated the relationship between the crude protein content and vegetation index of Rhodes grass (Chloris gayana Kunth.). A fertilization experiment was conducted using chemical fertilizers at six levels of nitrogen input in 2018. We used an indoor spectrophotometer to measure the reflection spectrum of Rhodes grass fresh leaves and calculated the two types of vegetation indices, normalized difference vegetation index (NDVI) and green normalized difference vegetation index (GNDVI). The nitrogen content was measured using the Dumas method, with the instrument of a total nitrogen analyzer and total carbon analyzer. We clarified the relationships among the fertilization date, nitrogen input level, and harvesting date. The crude protein content varied from 5.8% to 22.1%. The coefficients of determination of GNDVI and NDVI were 0.88 and 0.73, respectively, and the root mean squared errors of GNDVI and NDVI were 1.7 and 2.6, respectively. Therefore, we demonstrated the potential of estimating crude protein content in Rhodes grass by spectral reflectance.
