Avoiding Drug Carryover During Feed Processing and Delivery, Part 2
Sequencing, Flushing and Equipment Clean-out When working with a drug that requires a withdrawal time before the meat animal goes to market or when manufacturing and delivering feed for several animal species, one must be careful to follow the label and use GMPs to avoid drug cross-contamination. The GMPs state that adequate procedures shall be established and used for all equipment used in the production and distribution of medicated feeds to avoid unsafe contamination of medicated and nonmedicated feed. Three techniques to avoid crosscontamination include sequencing, flushing, and equipment clean-out.
The ordering sequence in which feed rations are processed and delivered determines the likelihood of drug carryover and tissue residue. It is an excellent practice to schedule the production of all medicated feeds having the same drug(s) in sequence with the higher levels first and ending with a low level. This sequence should be followed by a nonmedicated feed for the same animal. Individuals manufacturing feed for a single species such as swine, in which a withdrawal drug is fed to young animals, should generally mix feed in the following order: nursery ration containing the withdrawal drug, sow feed, grower, and finishing ration.
Place cull sows in the finishing pen prior to sending them to market if this sequence is followed. When using a sequencing pattern to avoid cross-contamination, it is imperative that feed production records are kept and are detailed enough to denote the last batch/ ration. Otherwise, the sequencing pattern could be violated by the next individual preparing feed. In most feed mills, sequencing feed will reduce carryover enough to eliminate the potential for tissue residue. However, sequencing may not reduce carryover to a sufficiently low level if maintenance or design problems exist in the mill as described in Table 2 below:
Table 2. Some Common Corrective Actions for Carryover
| Mode of Carryover | Corrective Actions |
| Electrostatic hang-up | -ground wire to affected equipment -purchase nonelectrostatic form of premix -use liquid ingredient to control dust -vibrators to shake hang-up loose |
| Delayed or extended dust return | -adjust air velocity at collection points -allow more time for dust to clear system -use liquid ingredient to reduce dustiness -collect and discard dust following production of medicated feeds (or retain for next run of like medicated feed) -remodel dust system |
| Mixer residues | -adjust ribbons or paddles -install plastic wipers on ribbons -install air sweep jets for cleaning -remodel discharge for more complete cleanout -add drug when mixer is 1/2 to 3/4 full (may affect mixing time required for good mix) |
| Surge bin, conveyor residues | -adjust for more complete cleanout -remodel bin or discharge |
| Elevator residues | -adjust bucket clearance in boot (if possible) -install air sweep jets -remodel boot for more complete cleanout |
| Bin residues | -manual inspection and cleaning when changing kind of feed stored -install vibrator or air sweep jets |
| Pellet mill and dryer residues | -flush blender and dies -adjust dryer for more complete cleanout |
| Entire system | -use production scheduling -allow time between kinds of feed for manual cleaning of system - use flush material about 5% of mixer capacity, but not less than 200 lbs. (should be established by actual tests) |
| Bulk truck | -establish cleanout procedure for truck -require a sample from the first product discharged at point of delivery -analyze delivery samples randomly and let driver know that samples are being analyzed |
Flushing involves taking a known ingredient, usually ground grain, and moving a quantity through the system to 'flush' out any medicated feed that remains. The amount of flush material depends on the system (about 5-10 percent of mixer capacity) but should not be less than 200 pounds of ground grain. Once the material has passed through the feed processing/ conveying system, it must be stored in a separate bin for use in an identical medicated ration.
Flushing a portable grinder-mixer poses several difficulties, since it would either require transporting several hundred pounds of ground grain (in sacks) to the bulk feeder or storing ground grain in a covered container near the bulk feeder. Flushing procedures for this system include the following: - add 300 pounds (or 5 percent of mixer capacity) of ground grain to the mixer through the charging chute (note: to compensate for the addition of 300 pounds of grain used to flush the mixer, deduct that amount from the feed ration),
- run the mixer for 30 to 60 seconds before discharge,
- discharge the flush material into the bulk feeder containing the feed most recently mixed. A simpler option may involve cleaning the mixer by discharging carryover feed out the bottom port in the vertical mixer. Some portable systems do not contain clean out ports; in this instance flushing may be essential.
Equipment clean-out is often the least used, but potentially most effective, method of avoiding drug carryover during feed processing and delivery. Cleaning the mixer, conveying system, pellet cooler, and sack-off bin or delivery truck between runs to remove residual feed is recommended under high risk situations. These may include the following: working with a high potency form of a drug (making premixes); sequencing cannot be incorporated into the production schedule; feed processing systems have large carryovers between batches (e.g. portable grindermixers); or when physical properties of drugs are such that sequencing and flushing is not sufficient to prevent carryover. The GMPs stipulate that all equipment shall be designed, constructed, installed, and maintained so as to facilitate inspection and use of clean-out procedures. Scheduled cleaning of mixers is required where liquid ingredients (molasses or fat) are added to the feed ration in the mixer.
