LABELING OF FORAGES WITH C-13 FOR NUTRITION AND METABOLISM STUDIES

Alfalfa was labeled in the field with 99 atom % (CO2)-C-13 and cut eit her the same day (C 1) or 30 d after labeling (C30). The Cl alfalfa co ntained 84% of the C-13 label in cell contents, whereas C30 alfalfa co ntained 47% of the C-13 label in cell contents. In two separate trials , Cl and C30 alfalfa were dosed to two or four Suffolk ewes fed natura l abundance alfalfa diets. Carbon isotope ratios (C-13/C-12, expressed as deltaC-13[parts per thousand] vs Pee Dee Belemnite standard) were determined for breath, feces, blood, and blood serum from ewes fed Cl alfalfa and blood and feces from ewes fed C30 alfalfa. In the Cl trial , carbon isotope ratios of respired CO2 peaked 4 h after feeding, then declined to baseline levels by 40 h after the dose. Fecal samples inc reased in C-13 only slightly from 12 to 40 h after the meal. Blood ser um values increased by approximately .5 parts per thousand from 0 to 4 h after the dose and remained relatively constant thereafter. In both trials, carbon isotope values from whole blood were constant. In the C30 trial, fecal samples peaked in carbon isotope value approximately 30 to 36 h after dosing, then declined; the time of this peak correspo nded closely to that from a concurrent study that used a pulse dose of Yb-labeled alfalfa hay. Thus, when incorporated into cell wall materi al, the excretion pattern of C-13 in feces was similar to that of Yb-l abeled hay, but little C-13 enrichment in feces was found when C-13 wa s primarily in cell contents of the labeled forage. When the soluble c ell contents were enriched in C-13, the marker was detected in respire d CO2 soon after feeding, which is consistent with the results of prev ious marker studies. These results demonstrate the feasibility of usin g forage labeled with the stable isotope C-13 in nutrition and metabol ism studies. Carbon-13, not subject to the regulatory constraints asso ciated with C-14, provides a useful alternative when a carbon tracer i s desired.