Microbiological Screening Tests to Detect Antibiotic Residues in Cull Dairy Cows

Two microbiological procedures were evaluated to detect antibiotic residues in dairy cows at slaughter. Inhibition of Bacillus subtilis growth was used for determining the presence or absence of antibiotic residues. The two tests differed only in the concentration of B. subtilis used. The Swab Test on Premises (STOP) was used to detect antibiotic residues in kidney and muscle tissue and the Live Animal Swab Test (LAST) was used to detect residues in urine of cull dairy cows. Kidney samples from 3% of cull dairy cows were positive. Confirmation by standard reference procedures and a subsequent investigation on antibiotic residues in urine from 317 cows and heifers with known antibiotic treatment histories suggest that a high percentage of false-positive readings occurred in urine. In addition, 23 % of urine samples were difficult to interpret in that B. subtilis growth surrounding Swabs dipped in urine was reduced. While producer response was generally favorable for an on-farm screening test for antibiotic residues detection in cows going to slaughter, interpretation problems, difficulty in collecting urine samples, and concerns over the complexity and sensitivity make it unlikely that the acceptance of the LAST will be widespread on dairy farms.