The relationship between antibiotic residue violations and somatic cell counts in Wisconsin dairy herds

The objective of this retrospective observational study was to characterize somatic cell counts (SCC) on Wisconsin dairy farms and to determine the relationship between SCC and the risk of antibiotic residue violation. Monthly official state regulatory data were used when both the bulk tank SCC value and antibiotic test results were available for the same date. Data were collected from Wisconsin dairy farms from January 1995 through November 1998 and consisted of results of tests performed on 805,772 grade A and 176,763 grade B milk samples. Herd-year SCC averages were used to classify herds (less than or equal to 250,000; 251,000 to 400,000, 401,000 to 550,000, 551,000 to 700,000, >700,000), and the relative risk of antibiotic residue by SCC class was determined. Arithmetic mean SCC values were 334,634 and 480,029 for grade A and grade B milk, respectively. SCC values were significantly higher for samples with positive antibiotic residue tests for grade A milk during all 4 yr tested. The SCC values were significantly higher for samples with positive antibiotic residue tests for grade B milk for 3 of 4 yr. The rate of antibiotic residue violation per 1000 herd-years increased with SCC class for both grade A and grade B milk. The relative risks of antibiotic residue violation by SCC class were 1.0, 1.43, 2.38, 2.78, and 7.10 for grade A milk and 1.0, 1.11, 2.67, 4.33, and 5.43 for grade B milk. Programs to reduce the level of subclinical mastitis on dairy farms may have an additional benefit of reducing the risk of antibiotic residue violations.