Tetracycline resistance genes The concentrations of tet (B) , tet (C) , tet (M) and tet (W) in fecal deposits were affected learn more by both treatment and time of exposure (P = 0.05, Figure 2). Numbers of copies of tet (B) in A44 and AS700 fecal deposits were greater than control and T11 fecal deposits but did not differ between A44 and AS700 treatments. Compared to day 7 levels, the concentration of tet (B) increased by day 42 (P = 0.01) approximately one order of magnitude and remained

greater than day 7 levels up to day 112 (P = 0.03), decreasing thereafter. Similarly, the concentration of tet (C) increased from initial amounts and was greater between days 42-70 when compared to day 7, but all other time points were not different from day 7. Treatments A44, AS700, and T11 all resulted in greater concentrations of tet (C) compared to the control fecal deposits, with AS700 having more copies than all other treatments. The control fecal deposits contained less tet (W) compared to the other treatments, but unlike tet (C), the T11 fecal deposits buy Lumacaftor had the highest concentration of tet (W). After 28 days, the amount of tet (W) decreased below the concentration on day 7. Only time (P = 0.0001) affected the concentration of tet

(L) in fecal deposits, which decreased from the initial concentrations on day 7, after 175 days of exposure. An interaction between treatment and time influenced the concentration of tet (M). By day 175, copies of tet (M) were less in all fecal deposits compared to those on day 7 (P = 0.05), with the exception 17-DMAG (Alvespimycin) HCl of control samples. There were no differences in tet (M) numbers in A44, AS700 or T11 deposits, and all had greater amounts of tet (M) on day 7 as compared to control deposits. However, by day 112, the fecal

deposits had similar tet (M) concentrations. Although not analyzed statistically, the concentrations of tet (M) and tet (W) were greater than other tetracycline resistance determinants. Figure 2 Persistence of tetracycline resistance genes in cattle fecal deposits under field conditions. The treatments were (N = 3; plus standard error): Control, no antimicrobial agents added to the diets of steers from which fecal deposits originated; A44, chlortetracycline (44 ppm); AS700, chlortetracycline and sulfamethazine (each at 44 ppm); T11, tylosin (11 ppm). Sulfonamide resistance genes An interaction between treatment and time affected the resistance determinant sul1 in fecal deposits (P = 0.0001, Figure 3). Concentrations increased 1-2 order of magnitude Log10 copies (g DM)-1 within the first 56 days of the experiment, across all treatments, and remained greater on day 175 than the starting concentrations on day 7 (P = 0.05). The exception was the A44 treatment, which had similar levels of sul1 on day 7 and day 175.