Stable isotopes of C and N differ in their ability to reconstruct diets of cattle fed C(3)–C(4) forage diets

Stable isotopes are useful for estimating livestock diet selection. The objective was to compare δ(13)C and δ(15)N to estimate diet proportion of C(3)–C(4) forages when steers (Bos spp.) were fed quantities of rhizoma peanut (Arachis glabrata; RP; C(3)) and bahiagrass (Paspalum notatum; C(4)).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 δ(13)C or δ(15)N. Feces showed changes using δ(13)C by 8 days, and adj. R(2) 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 δ(13)C at both 8-days and 32-days. By 32-days, fecal δ(15)N showed promise (R(2) = 0.71) for predicting diet composition in C(3)–C(4) diets. Further studies are warranted to further corroborate fecal δ(15)N as a predictor of diet composition in cattle.