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.