A study was conducted to determine the persistence of antibiotic residues in milk beyond the recommended withdrawal period. Composite milk samples (n = 122) were collected from 58 lactating dairy cows in the university herd receiving antibiotic treatments for any reason but only when administered as a single drug. Samples were obtained 72 h posttreatment and sampling continued every 24 h until result for antibiotic residue was negative by the Bacillus stearothermophilus disc assay. The antibiotic (n = 7) administered accounted for significant variation in drug persistence; however, route of administration, case number (for cows treated for more than one episode), number of days treated, animal's body weight, lactation number, and daily milk production did not affect drug persistence. Chi-square analysis indicated that 21% of milk samples were positive for residues beyond the recommended withholding period. Milk samples from cows treated with cephapirin and penicillin were the only samples that exceeded recommended withdrawal times. Often doses administered exceeded label directions.

A study was conducted to determine the effectiveness of on-farm tests for detection of antibiotic residues in milk and urine. Composite milk samples (n = 122) were taken from 58 cows receiving antibiotic treatment with a single antibiotic by any route of administration. Samples were obtained 72 h posttreatment, and sampling continued every 24 h until all residue tests showed a negative result for antibiotics. Antibiotic residues were detected by the Bacillus stearothermophilus disc assay, the Delvotest P, and the Penzyme test. Urine of cull cows was collected and tested for residues with the Live Animal Swab Test for antibiotic residues present in slaughter carcasses. No significant difference was found between the Bacillus stearothermobilus disc assay and either on-farm test. Chi-square analysis indicated that with both the Delvotest and the Penzyme test, 17% of samples gave positive results when the disc assay showed negative readings. An inordinately high percentage (69%) of urine samples from cull cows exhibited antibiotic contamination, although all animals had completed the specified withholding periods. Fifteen of these positive results were obtained from primiparous cows that had never been treated with any antibiotic.

The Virginia Tech dairy herd was used in a 10-mo study to determine the effect of intramammary antibiotic therapy of quarters with elevated SCC on milk production, subsequent DHIA SCC, and infection status. Cows were assigned randomly to experimental or control groups. Animals in both the control and experimental groups with SCC scores greater than or equal to 5 for the first time during that lactation were quarter sampled, milk was cultured to detect presence of mastitis pathogens, and SCC was determined. All experimental cows with quarter SCC greater than or equal to 5 were treated with an intramammary cephapirin product only in those elevated quarters (DHIA SCC greater than or equal to 5), regardless of clinical symptoms. Control cows received antibiotic therapy when symptoms were clinical, regardless of SCC. Treatment group had no significant effect on milk production, SCC, or infection status of the cow. Treatment of cows in the experimental group cured 70% of infected quarters, whereas only 50% of infections in the control group were eliminated.

Detectable concentrations of antibiotic residues in milk supplies are illegal. They interfere with manufacturing of some dairy products, may cause hypersensitivity or resistance to drug therapy in humans, and are perceived by consumers as undesirable. Antibiotics are used for intramammary, intramuscular, oral, or reproductive therapy to counter acute or subacute diseases. Residues can result for many reasons, including poor records of treatment, failure to observe recommended label withdrawal time, prolonged drug clearance, treated animal identification problems, products not used according to label directions, lack of advice on withdrawal period, and others. Several effective and useful on-farm residue screening tests are available and should be used to monitor cows treated with an antibiotic for any reason. Herds that use drugs without label directions or that do not follow label directions should find these tests valuable. Producers must understand how to interpret the results of these tests. Also, dairy farmers must recognize if the selected test method is capable of detecting those antibiotics in use and what are to be monitored.