Monensin and
lasalocid are
polyether ionophores commonly used in the beef and poultry industries for the prevention of coccidial
infections and promotion of growth. These
ionophores can exhibit higher toxicity than many other
antibiotics; thus, evaluating their fate in the environments associated with concentrated feed operations is important. Sorption of
monensin and
lasalocid was measured in eight
soils of varying physiochemical composition. Organic
carbon-normalized sorption coefficients (log Koc) ranged from 2.1 to 3.8 for
monensin and from 2.9 to 4.2 for
lasalocid and were inversely correlated to equilibrium soil-
solution pH. Degradation of
lasalocid and
monensin in two contrasting
soils with and without
manure amendment was measured in moist
soils at 23 degrees C and 0.03 MPa moisture potential. The half-life of both compounds in the fresh nonsterile
soils was less than 4 d, for which
monensin degraded slightly faster than
lasalocid. Fresh liquid
manure amendments did not significantly alter degradation of either compound. Based on parallel 60Co-sterilized soil experiments, some abiotic degradation of
monensin was apparent, whereas
lasalocid only degraded in the presence of microbes. Analysis of beef-derived lagoon effluent used for irrigation confirmed that
monensin can be present at low-ppb to low-ppm concentrations in the aqueous and suspended solids fractions, respectively; however, subsequent analysis of drainage water in a nearby ditch suggested that attenuation by soil after land application will greatly reduce the amount entering surface waters.