Mixing times that are necessary to process diets will vary with the type of mixer, the relative capacity of the load within the mixer, and the maintenance condition of the mixing equipment. A feed mixture is acceptable when the variation between several samples is less than 10%. (Samples for analyses can be collected at periodic intervals as the mixer is emptied.) Horizontal mixers will generally reach the 10% variation standard within three to four minutes after the final component is added, whereas the single-screw vertical mixer generally requires eight to 12 minutes. If the mixer has a large amount of wear on its ribbons or screws, or if the mixer is filled above its rated capacity, the variation may be higher (20 to 30%). There is a direct relationship between the RPM of the mixer and the time required to mix a diet. The faster the RPMs, the sooner will the 10% variation be achieved.
The proper sequencing of feed ingredients into the mixer helps in manufacturing diets of good and uniform quality. A suggested mixing procedure is:
Diets should be mixed in a sequence that prevents drug residue problems. This process is termed "batching." To avoid drug residue problems with market animals, never mix the finishing diet immediately after a medicated feed (e.g., starter pig diet). Drug residue can be a problem with many on-farm mixers where 20 to 30 lbs. of residual feed often remains in the bottom of the mixer after the feed is discharged. This residue is often the source of most contamination problems between feed mixes. Removal of the clean-out plate and the residual feed after every medicated diet is mixed is essential in preventing the carry over of drugs to the next diet mixed. Many exit ports on feed mixers are not easily accessible, so the problem can be minimized by mixing diets in a correct batching sequence. This procedure uses the principle of mixing feeds that contain potential residue problems followed by diets that will be fed to animals not intended for market at least in the near future.