Feed Manufacturing: Avoiding Drug Carryover

by Teresa Acklin
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Guidelines for feed processors to improve product quality and help ensure safe food supplies

   Drug carryover is a form of feed contamination that may result when the substance in question has been transferred (carried) from an acceptable location or feed to an unacceptable location or feed. Carryover of an animal drug can occur during feed manufacturing, handling or delivery.

   Carryover of a drug that requires a withdrawal period (Category II drugs) into a finishing ration may result in a tissue-residue problem in meat animals. This residue can lead to significant market losses for producers experiencing this problem in their animals.

   Even drugs that do not require a withdrawal period (Category I drugs) can create serious problems. For example, carryover of many drugs into a batch of feed intended for an off-label species can lead to illness or even death of the animal.

   Avoiding drug carryover during feed processing and delivery is essential when using medicated articles/feeds. The U.S. Food and Drug Administration's Good Manufacturing Practices (GMPs) provide guidance for medicated feed manufacturers to ensure their products meet the necessary identity, strength and quality standards with respect to drug content. These regulations stipulate adequate procedures be established and followed for all equipment used in the production and distribution of medicated feeds to avoid unsafe contamination of medicated and non-medicated feeds.

Risks and Causes

   It is the responsibility of the feed processor to ensure that the correct amount of the desired drug is properly incorporated and that no cross-contamination of an unwanted or unspecified drug is present in that feed. Whether the manufacturing system is simple or complex, it is possible to avoid drug carryover by following the GMPs.

   The type of drug (Category I or II), number of species and feed delivery system determine the degree of risk associated with drug carryover. Feed processors who manufacture products for one species and use only Category I drugs experience the least amount of risk associated with cross-contamination and tissue residue. Since there is no withdrawal time associated with this product, it may be used until the time of slaughter.

   Manufacturers who use a Category II drug can avoid carryover and tissue residue problems by using separate delivery systems for these feeds and by sequencing, flushing and cleaning feed processing equipment. Mills that produce feed for multiple species experience more risk of cross-contamination by medicated articles (either Category I or II) than mills manufacturing products for one species. When changing feed rations from one species to the next, the risk of cross-contamination is minimized through sequencing, flushing and equipment clean-out of feed processing and delivery equipment.

   Drug carryover may occur for many reasons, as outlined in Table 1 on page 41. Significant amounts of the drugs or medicated feed may remain in the production system and contaminate the following batches of feed. Contamination may occur in one piece of equipment, or it may result from a combination of residues throughout the entire system.

   Once the source of carryover is known, corrective action can be taken. Often, adjustment of equipment will markedly relieve the problem. Repairs, remodeling or replacement of worn components may be necessary. A few of the more common preventive/corrective measures are listed in Table 2 on page 41.

Preventive and Corrective Actions

   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. Three techniques to avoid cross-contamination 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 non-medicated 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.

   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 on page 41.

   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% to 10% of mixer capacity) but should not be less than 90 kilograms 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 require either transporting several hundred kilograms 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 135 kg (or 5% of mixer capacity) of ground grain to the mixer through the charging chute. To compensate for the addition of 135 kg of grain used to flush the mixer, deduct that amount from the feed ration;

   • run the mixer for 30 to 60 seconds before discharge; and

   • 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.

   Clean-out may be desired when working with a high potency form of a drug (making premixes); when sequencing cannot be incorporated into the schedule; when feed processing systems have large carryovers between batches (e.g. portable mixers); or when physical properties of drugs are such that sequencing and flushing are 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.

Handling and Delivery

   Segregation can occur in ingredients and mixed feeds, and a number of sites in the processing, handling and transit of feed and feed components can produce conditions favorable to segregation, as shown in Table 3 on page 43. Segregation of the drug from the medicated feed may lead to carryover and non-uniform concentration of the medication, and both situations may result in violative tissue residue in the market animal.

   Mixed feeds are subject to segregation because of differences in particle size, shape and density. For example, feed ingredients can be evenly dispersed following the mixing process, become slightly segregated as they drop into the surge bin, experience some remixing during transport from the surge bin auger to the elevator leg, become segregated as feed is discharged from the leg and undergoes a free-air fall into the holding bin over the pellet mill, and be partially remixed as material is transferred into the conditioning chamber above the pellet mill. When pellets are formed, the segregation process ceases.

   The preparation of a meal feed ration may experience segregation and partial remixing throughout the entire handling process until the feed is consumed.

   Particles tend to segregate when combined in a complete feed and there is a large size difference between ingredients. Particle shape affects the movement of a material through the air in a free-fall situation, as flat particles will tend to fall slower and remain where they fall, whereas particles that are round or cuboidal will fall faster and tend to roll toward the storage wall.

   Particles with high density will be less affected by free-fall air resistance than those of low density. The less dense particles will tend to be carried toward storage walls by the air currents created in the bin.

   Management opportunities exist to reduce the amount of segregation that may occur in feed and feed ingredients, as seen in Table 4 on page 44. Most feed rations contain between 60% and 70% ground grain; consequently, the particle size reduction process is critical. Routinely monitoring the grain after grinding will ensure that the desired particle size and uniformity are achieved.

   Many of the other feed ingredients are delivered in granular form. Purchasing specifications should be placed on particle size for potential problem ingredients, such as limestone, dicalcium phosphate, salt, etc.

   One processing method that will reduce ingredient segregation involves the production of feed pellets. Another commonly used technique to reduce segregation is to add a liquid — usually molasses, fat or water — to the feed formula. These liquids act to unite small and large particles into agglomerates, which maintain their homogeneity through the subsequent processing and handling.

   However, liquid addition may create as many problems as it solves. Feed containing liquid may adhere to equipment and bins, creating clean-out difficulties and possible feed cross-contamination problems. Proper application, the correct equipment and location of the spray bar on the nozzle can help avoid thes problems.

   This article is based on information provided by the Cooperative Extension Service, Kansas State University, Manhattan, Kansas, U.S.

Table 1 — Sources of Carryover

EquipmentMode of Carryover
Dust systemdelayed return of dust to production line
excessive pickup of drug and carrier
hang-up (electrostatic or moisture)
Mixerresidual feed remaining in mixer
buildup of material on ribbons and walls
electrostatic hang-up on walls and top
leaking mixer gate (not fully closed)
Surge binincomplete clean-out
electrostatic or moisture hang-up
Conveyorssame as surge bin
Elevatorsresidual feed remaining in buckets and boot
electrostatic or moisture hang-up
residual feed from incomplete cleanout
Bulk truckerror in bin chart records
incomplete clean-out
bridging and hang-up

Table 2 — Common Corrective Actions for Carryover

Mode of CarryoverCorrective Actions
Electrostatic hang-upground wire to affected equipment
purchase non-electrostatic form of premix
use liquid ingredient to control dust
use vibrators to shake hang-up loose
Delayed or extended dust returnadjust 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
remodel dust system
Mixer residuesadjust ribbons or paddles
install plastic “wipers” on ribbons
install air sweep jets for cleaning
remodel discharge for more complete clean-out
Surge bin, conveyor residuesadjust for more complete clean-out
remodel bin or discharge
Elevator residuesadjust bucket clearance in boot (if possible)
install air sweep jets
remodel boot for more complete clean-out
Bin residuesmanual inspection and cleaning when changing kind of feed stored
install vibrator or air sweep jets
Pellet mill and dryer residuesflush blender and dies
adjust dryer for more complete clean-out
Entire systemuse 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 90 kg
Bulk truckestablish clean-out 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

Table 3 — Sources of Segregation Problems

SourcesPossible Segregation Problems
Mixer surge binfree-fall from mixer
air pressure relief
mill or equipment vibration
electrostatic hang-up
Bucket elevatorfree-fall at elevator discharge
Pneumatic conveyingsegregation at cyclone collector
free-fall from collector to bin
feed angle of repose segregates particles by size
Binsfree-fall through air
funneling during discharge accentuates segregation
vibration of mill or equipment
electrostatic hang-up
Dust collecting systemvery fine particles tend to aspirate off
residue dust is not thoroughly mixed back into feed

Table 4 — Overcoming Segregation Tendencies of Mixed Feed

Particle sizegrind ingredients to a uniform particle size range
use liquids to agglomerate
Particle shapeprocess to more uniform particle shape range
if shape is a desired characteristic, use fat or molasses
to agglomerate
Particle densityagglomeration is the most common way to overcome
density differences
finer particle size reduces tendencies to segregate because
of density differences