Technical note: labeling of forages with 13C for nutrition and metabolism studies.

Alfalfa was labeled in the field with 99 atom % 13CO2 and cut either the same day (C1) or 30 d after labeling (C30). The C1 alfalfa contained 84% of the 13C label in cell contents, whereas C30 alfalfa contained 47% of the 13C label in cell contents. In two separate trials, C1 and C30 alfalfa were dosed to two or four Suffolk ewes fed natural abundance alfalfa diets. Carbon isotope ratios (13C/12C, expressed as delta 13C/1000 [parts per thousand] vs Pee Dee Belemnite standard) were determined for breath, feces, blood, and blood serum from ewes fed C1 alfalfa and blood and feces from ewes fed C30 alfalfa. In the C1 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 increased in 13C only slightly from 12 to 40 h after the meal. Blood serum values increased by approximately .5/1000 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 corresponded closely to that from a concurrent study that used a pulse dose of Yb-labeled alfalfa hay. Thus, when incorporated into cell wall material, the excretion pattern of 13C in feces was similar to that of Yb-labeled hay, but little 13C enrichment in feces was found when 13C was primarily in cell contents of the labeled forage.(ABSTRACT TRUNCATED AT 250 WORDS)