Quantifying Carryover of Animal Feed Additives in Feed Mill Bucket Elevators Using Magnetic Iron Tracers as a Screening Method

Carryover or cross-contamination can occur when medicated and non-medicated feeds are produced in the same equipment, potentially contaminating subsequent feeds with drug residues or other unwanted additives. United States Food and Drug Administration Guidance 272 recommends sequencing, flushing, and equipment cleanout to minimize carryover, with flushing validation helping ensure residues remain below detectable levels. No published studies were identified that have used magnetic iron tracers for cross-contamination assessment. This study aimed to quantify cross-contamination in feed mill bucket elevators using Microtracer® F-blue and to determine the influence of flush amount, feed composition and physical properties on carryover. Two bucket elevators (4.3 m3/h, 1.9 m/s, 5.97 rad/s) with 410 vented buckets (0.3 × 0.2 m, 0.2 m spacing) retaining about 0.49 kg mash per boot were evaluated in three tracer carrier, flush and carryover trials using 0.45 kg or 0.34 kg Microtracer® F-blue (3.54:1) in 1814 or 1361 kg mash batches. Three samples were collected from each feed type, tracer particles were magnetically recovered and counted, and carryover (%) was calculated from carryover tracer counts relative to unrecovered tracer, with controls yielding zero particles. The particle size of flush feed ranged from 0.0015 m to 0.0028 m. Using Microtracer® F-blue, cross-contamination remained well below 1% across all flush levels, although a significant difference was observed (p = 0.0004), with 1814 kg of flush feed producing the highest carryover (0.11 ± 0.11%). The effect of flush feed on carryover was influenced by ingredient type, but not by particle size or bulk density. Results were validated with a marker additive (Calsporin®). Mean cross-contamination remained well below the 1% threshold recommended by European agencies.