Ionophore residues in eggs in Northern Ireland: incidence and cause.

Monensin, salinomycin and narasin were detectable in six, two and one, respectively, out of 161 eggs surveyed in Northern Ireland in 1994. In all cases, the concentrations detected were less than 2.5 ng/g. Lasalocid was detectable in 107 eggs at concentrations ranging from 0.3 to 129 ng/g. Cross-contamination of unmedicated feeds with monensin during feed manufacture (up to eight batches of unmedicated feed contaminated with monensin) was similar to that previously observed for lasalocid (up to nine batches contaminated). Therefore differences in the incidence in eggs could not be explained by differential carry-over during feed manufacture. In a feeding trial it was shown that the relative ability of monensin, salinomycin and lasalocid to accumulate in eggs was in the ratio 0.12:3.3:63 ng/g egg per mg/kg feed, respectively. This indicated that the potential for monensin and salinomycin to cause residues in eggs was very low, by comparison with lasalocid. In 1995, a granular formulation of the lasalocid premix was introduced into the United Kingdom that decreased the carry-over of this drug from medicated to unmedicated feed. Six months after the introduction of this formulation, the incidence of lasalocid residues in eggs (21%) was lower than that found (66.5%) in an earlier survey (1994) carried out, and published, by this laboratory.