Disposition of oral neomycin in calves was determined using C-14-label
ed neomycin. The influences of age, diet, and method of administration
were observed. All calves were killed 96 h after a single oral dose o
f [C-14]neomycin (approximately 30 mg/kg) and the distribution of C-14
in excreta and tissues was determined. As indicated by urinary excret
ion, absorption of neomycin was greater in 3-d-old calves (11.1 +/- 1.
8% of the dose) than in 54- to 64-d-old nonruminating calves (1.5 +/-
.58% of the dose) dosed similarly. Absorption of neomycin was similar
in nonruminating (1.5 +/- .58%) and ruminating (2.13 +/- .62%) calves
when the doses were administered in solution via a nippled bottle. In
ruminating calves, absorption was somewhat less when the dose was admi
nistered on feed via a gelatin capsule (.5 +/- .06% of the dose) than
when given in solution via a nippled bottle (2.13 +/- .62% of the dose
). In calves dosed at 3 d of age, C-14 concentration in the kidneys re
presented 55 +/- 4.9 ppm of neomycin equivalents. The next highest con
centration occurred in the livers, which contained less than 5% of the
level in kidneys. Tissue concentrations of C-14 were related to absor
ption (as indicated by urinary excretion). Isolation and characterizat
ion: (positive-ion fast atom bombardment mass spectroscopy and nuclear
magnetic resonance spectroscopy) of C-14 compounds,in kidneys of calv
es dosed at 3 d of age indicated that at least 90% of the C-14 was pre
sent as neomycin. Neomycin was also the major C-14 compound in feces o
f all calves (70 to 80% of the C-14 present); Results suggest that use
of neomycin in veal production at recommended levels would cause kidn
ey residues of neomycin above the .250 ppm tolerance currently observe
d in the United States.