ABSTRACT:The high prophylactic doses of ZnO commonly used to control post-weaning diarrhoea (PWD) in piglets have become an environmental problem. The possibility of partial replacement of ZnO by sodium humate (HNa) in PWD treatment was investigated in this study. Weaned piglets (32) were challenged with two enterotoxigenic E. coli strains (ETEC/O149/F4/LT and ETEC/O147/F18/LT) and allocated into four treatments maintained for three weeks: C (no supplementation); ZnO2.5 (2.5 g ZnO/kg diet); HNa + ZnO1.0; HNa + ZnO1.5; HNa + ZnO1.7 (20 g HNa and 1.0, 1.5 and 1.7 g ZnO/kg diet, respectively). The effects on incidence, severity and duration of diarrhoea, faecal shedding of total E. coli and both ETEC strains, growth rate and selected blood parameters were investigated. In contrast with ZnO2.5 and HNa + ZnO1.7, high daily diarrhoea scores, incidence and duration and mortality due to severe dehydration were seen in C, HNa + ZnO1.0 and HNa + ZnO1.5 groups. The administration of ZnO and HNa did not affect the faecal shedding of the challenged ETEC strains for eght days, even in clinically healthy piglets in ZnO2.5 and HNa + ZnO1.7 groups. Signs of growth depression were found in C; HNa + ZnO1.0 and HNa + ZnO1.5 groups during the first week. No difference in growth performance was observed in ZnO2.5 and HNa + ZnO1.7 piglets. Most of the selected biochemical and haematological parameters did not differ significantly among the treatments. However, a significantly higher serum Zn as a result of high dietary ZnO intake in the ZnO2.5 group compared to the control and HNa groups was detected. Significantly lower serum P in ZnO2.5; HNa + ZnO1.7 and HNa + ZnO1.0 groups compared to the control group was most likely induced by the increased Zn in serum. The results indicate the possibility of reducing the high pharmacological levels of ZnO in the prophylaxis of PWD through partial replacement with HNa. Such a treatment maintains the favourable prophylactic effect while lowering the Zn content in faeces.Keywords: humic; enterotoxigenic E. coli; faecal shedding; growth performance; blood chemistry List of abbreviations: ALP = alkaline phosphatase, ALT = alanine transaminase, ANOVA = analyses of variance, AST = aspartate aminotransferase, BWG = body weight gains, CFU = colony forming units, DDS = daily diarrhoea score, ETEC = enterotoxigenic Escherichia coli, FCR = feed conversion ratio, FI = feed intake, HNa = sodium humate, HS = humic substances, PWD = post-weaning diarrhoea