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. I n all cases, the concentrations detected were less than 2.5ng/g. Lasal ocid was detectable in 107 eggs at concentrations ranging from 0.3 to 129 ng/g. Cross-contamination of unmedicated feeds with monensin durin g feed manufacture (up to eight batches of unmedicated feed contaminat ed with monensin) was similar to that previously observed for lasaloci d (up to nine batches contaminated). Therefore differences in the inci dence in eggs could not be explained by differential carry-over during feed manufacture e. 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. Th is indicated that the potential for monensin and salinomycin to cause I residues in eggs was very low, by comparison with lasalocid In 1995, a granular formulation of the lasalocid premix was introduced into th e United Kingdom that dea eased the carry-over of this dr drug from me dicated to unmedicated feed. Six months after the introduction of this formulation, the incidence of lasalocid residues in eggs (21%) was lo wer than that found (66.5%) in an earlier survey (1994) carried out, a nd published, by this laboratory.