Sulfadimethoxine (SDM) was fed to laying hens for 5 days at the dietary levels of 0, 100, 200 and 400ppm, and thereafter, dietary SDM was withdrawn by changing the diet to SDM free diet for another 14 days. Contents of SDM in eggs were determined using high performance liquid chromatography. The detection limit was 0.01ppm for SDM.
The behaviour of SDM in whole egg could be well described by following equation,
y=0.003663•xT•e^-0.6636t
where, y is SDM content (ppm) in whole egg laid at T or t days, x is dietary SDM level (ppm), T is days on SDM feeding (T≤3), and t is days after the withdrawal of dietary SDM. During SDM feeding, t should be zero, therefore e^-0.6636t=1. After the withdrawal of dietary SDM, x and T are fixed values.
Amount of SDM transferred into whole egg was estimated under fixed condition of administration (x=100, T=3, and t=0), which was several times more than those of other antibiotics and synthetic anti-bacterials estimated under the same condition. The co-efficient, -0.6636, in the above equation is a characteristic of SDM indicating the speed of disappearance from whole egg after the withdrawal of dietary SDM. Biological half life of SDM was estimated to be 1.04(=log_e2/0.6636) days with 95% confidence interval from 0.97 to 1.13 days, which was shorter than those for antibiotics and synthetic anti-bacterials.