Stable isotopes are useful for estimating livestock diet selection. The objective was to compare δ¹³C and δ¹⁵N to estimate diet proportion of C₃–C₄ forages when steers (Bos spp.) were fed quantities of rhizoma peanut (Arachisglabrata; RP; C₃) and bahiagrass (Paspalumnotatum; C₄).Treatments were proportions of RP with bahiagrass hay: 100% bahiagrass (0%RP); 25% RP + 75% bahiagrass (25%RP); 50% RP + 50% bahiagrass (50%RP); 75% RP + 25% bahiagrass (75%RP); and 100% RP (100% RP). Feces, plasma, red blood cell (RBC), and hair were collected at 8-days intervals, for 32 days. Two-pool mixing model was utilized to back-calculate the proportion of RP based on the sample and forage δ¹³C or δ¹⁵N. Feces showed changes using δ¹³C by 8 days, and adj. R² between predicted and observed RP proportion was 0.81 by 8 days. Plasma, hair, and RBC required beyond 32-days to reach equilibrium, therefore were not useful predictors of diet composition during the study. Diets were best represented using fecal δ¹³C at both 8-days and 32-days. By 32-days, fecal δ¹⁵N showed promise (R² = 0.71) for predicting diet composition in C₃–C₄ diets. Further studies are warranted to further corroborate fecal δ¹⁵N as a predictor of diet composition in cattle.