Stable isotopes of C and N differ in their ability to reconstruct diets of cattle fed C3–C4 forage diets

Authors: Jaramillo, David; DUBEUX, JOSE - University Of Florida; RUIZ-MORENO, MARTIN - University Of Florida; DILORENZO, NICOLAS - University Of Florida; VENDRAMINI, JOAO - University Of Florida; SOLLENBERGER, LYNN - University Of Florida; MACKOWIAK, CHERYL - University Of Florida; QUEIROZ, LUANA - University Of Florida; GARCIA, LIZA - University Of Florida; SANTOS, ERICK - University Of Florida; KIEKE, BURNEY - Marshfield Clinic Research

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2022
Publication Date: 10/13/2022
Citation: Jaramillo, D.M., Dubeux, J., Ruiz-Moreno, M., Dilorenzo, N., Vendramini, J., Sollenberger, L., Mackowiak, C., Queiroz, L., Garcia, L., Santos, E., Kieke, B. 2022. Stable isotopes of C and N differ in their ability to reconstruct diets of cattle fed C3–C4 forage diets. Scientific Reports. 2022(12):17138. https://doi.org/10.1038/s41598-022-21051-4.
DOI: https://doi.org/10.1038/s41598-022-21051-4

Interpretive Summary: Stable isotope techniques can be useful tools to track carbon and nitrogen pathways in livestock research. The objective of this study was to evaluate the potential use of stable isotopes of carbon (13C) and nitrogen (15N) to estimate the dietary proportion of C3 or C4 forage species when beef steers were fed known quantities of rhizoma peanut (Arachis glabrata; RP), a C3-legume, and bahiagrass (Paspalum notatum), a C4-grass. Treatments were proportions of RP with bahiagrass hay: 1) 100% bahiagrass hay (0% RP); 2) 25% rhizoma peanut + 75% bahiagrass hay (25% RP); 3) 50% rhizoma peanut + 50% bahiagrass hay (50% RP); 4) 75% rhizoma peanut + 25% bahiagrass hay (75% RP); 5) 100% rhizoma peanut hay (100% RP). Feces, plasma, red blood cell (RBC), and hair samples were collected at 8-d intervals, for a total of 32 d. A two-pool mixing model was utilized to back-calculate the proportion of RP based on the sample and forage 13C or 15N. The dietary proportions were better represented using 13C rather than 15N. Overall, 13C of feces may be advantageous for determining dietary proportions in C3-C4 binary mixtures since sample collection does not require additional equipment or expertise. These results contribute to implementing techniques to further understand dietary proportions and selection in grazing livestock systems.

Technical Abstract: Stable isotope techniques can be useful tools to track carbon (C) and nitrogen (N) pathways in livestock research. The objective of this study was to evaluate the potential use of d13C and d15N to estimate the dietary proportion of C3 or C4 forage species when beef steers (Bos spp.) were fed known quantities of rhizoma peanut (Arachis glabrata; RP; C3) and bahiagrass (Paspalum notatum; C4). Treatments were proportions of RP with bahiagrass hay: 1) 100% bahiagrass hay (0% RP); 2) 25% rhizoma peanut + 75% bahiagrass hay (25% RP); 3) 50% rhizoma peanut + 50% bahiagrass hay (50% RP); 4) 75% rhizoma peanut + 25% bahiagrass hay (75% RP); 5) 100% rhizoma peanut hay (100% RP). Treatments were laid out in a randomized complete block design with five replicates. Twenty-five beef steers were used, and total dry matter intake was recorded daily. Feces, plasma, red blood cell (RBC), and hair samples were collected at 8-d intervals, for a total of 32 d. A two-pool mixing model was utilized to back-calculate the proportion of RP based on the sample and forage d13C or d15N. Feces showed dietary changes using d13C by Day 8, and adj. R2 between predicted and actual RP proportion was 0.81 by 8 d. Plasma and hair d13C were adequate predictors of dietary changes over the 32-d collection period (adj. R2 above 0.70 between observed and predicted RP proportions), however their use required a longer time period for detection because the physiological turnover is much longer than for feces. The dietary proportions were better represented using d13C rather than d15N. Physiological processes in cattle often increased discrimination of d15N, such as in plasma, which occurred to a different extent for each of the diets. Longer-term studies are required to have accurate calibrations for the turnover and the effectiveness of different tissues through time. Overall, d13C of feces may be advantageous for determining dietary proportions in C3-C4 binary mixtures since sample collection does not require additional equipment or expertise.