Evaluation of AOAC microbial diffusion procedure for analysis of chlortetracycline in high mineral feeds

The performance of the AOAC microbial diffusion assay procedure for the analysis of chlortetracycline was evaluated in mixed feeds and premixes as well as laboratory-prepared feeds, all with various mineral contents. In 100 mixed feeds with a calcium content ranging from less than 1 to greater than 22%, no relationship was shown between calcium content and incidence of chlortetracycline deficiency. In 106 premixes, a relationship was shown between chlortetracycline content and adherence to guarantee: below 10 g/lb, the incidence of deficiency was high; in premixes containing 20 g chlortetracycline and above, the incidences of deficiency were less than 5%. In laboratory-prepared feeds containing 12-15% calcium, grinding and storage at both 4 degrees C and at room temperature (20 degrees C) caused no decrease in the chlortetracycline concentration. Only storage of the ground feed at 37 degrees C caused a distinct loss of activity. The temperature-related loss was attributed to the epimerization of chlortetracycline.