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.