Analysis of a national sample of animal drugs in commercial feed in an attempt to statistically determine the causes of poor assay recovery

The objective of this study was to determine if laboratory assays of medicated feeds differed from expected drug levels and to determine if differences found could be related to pelleting temperature or pellet size. Twelve animal drugs (amprolium, apramycin, chlortetracyline, lasalocid, melengestrol acetate, neomycin, oxytetracycline, penicillin, pyrantel, sulfamethazine, sulfathiazole and carbadox) were selected by the American Feed Industry Association (AFIA) for survey and evaluation. Twelve multi-plant feed manufacturing corporations representing 263 medicated feed manufacturing establishments participated in a 1-year survey of animal drug assays. Complete data for drug, animal species, class, product form, pellet size, pelleting temperature, expected drug level and assay level of the drug were provided for 2148 observations. Two measures of difference were calculated: drug level — assay level (DIFF) and assay level/drug level ∗ 100 (PCTEXP). Paired t-tests were used to determine if DIFF differed from zero. Correlation and regression procedures were used to examine relationships among variables. Mean DIFF differed from zero (P < 0.01) for eight of the 12 drugs and was positive for all drugs. Correlations between pelleting temperature and pellet size and both measures of difference were low (r < 0.25), although a few were different from zero (P < 0.01). Analysis of covariance did not reveal any consistent relationships between measures of difference and pelleting temperature or pellet size. In summary, there was a difference between drug level and assay level for most drugs examined, and in all cases mean drug level was higher than mean assay level. However, there was no strong evidence that pelleting temperature or pellet size was the cause of these differences.