EFFECT OF DIET ON THE EXCRETION PROFILE OF IVERMECTIN IN CATTLE FECES

Two groups of 26 cattle (cross-bred, 3-8 years old, weight 450-650 kg) were placed onto 2 feed regimens, pasture-fed and grain-fed, The past ure-fed cattle were kept on irrigated pasture with hay supplement ad l ibitum, whereas the grain-fed cattle were kept in a feedlot and fed ha y and lupin grain in a 40:60 ratio, Half the cattle within each feed g roup (n = 13) were given a subcutaneous injection of ivermectin (Ivome c(R)) at the dose rate of 200 mu g/kg Liveweight. Over 5 times higher levels of ivermectin were excreted in the faeces of grain-fed cattle c ompared with pasture-fed (grazing) cattle, Peak excretion levels of 0. 36 mg/kg (grain-fed) and 0.09 mg/kg (pasture-fed) of ivermectin were r ecorded at 6 and 8 days post-injection (dpi), respectively, Ivermectin was detected in faeces for up to 13 dpi (grain-fed) and 14 dpi (pastu re-fed). Faecal pH in grain-fed cattle was more acidic (6.43) compared with pasture-fed cattle (7.28) and the levels of nitrogen in faeces f rom grain-fed cattle were higher (0.42%) compared with pasture-fed cat tle (0.20%). Treatment with ivermectin had no influence on either the pH or nitrogen levels of faeces from either feed regimen. The impact o f diet on the excretion profile of ivermectin is discussed in the cont ext of studies attempting to demonstrate ecotoxic effects of ivermecti n residues in cattle faeces on dung beetles.