As a general rule, 10-15 minutes is the minimum mixing time for (single-auger) vertical feed mixers. Feed that is not uniform may result in below-optimal animal performance. Shortened mixing times may not be a serious problem for older animals consuming large quantities of feed; however, a lack of complete feed mixing for young, fast-growing animals could result in an unbalanced feed intake. Even for larger animals, reduced feed efficiency and slight reductions in growth rate may occur but go unnoticed.
As part of Australian Pork Limited's FeedCheque program, mixing standards were assessed on 10 farms. Feed was mixed for various lengths of time and sampled, with uniformity reported as the coefficient of variation of salt concentration. This information is based on those assessments.
In commercial feed manufacturing, the standard for uniform mixing is a coefficient of variation (CV) of 10% or less. If the CV is over 10%, the mixing time needs to be increased and/or the system inspected for factors that caused the poor ingredient distribution (e.g. overfilling, worn equipment).
Salt is a commonly used micro-ingredient to test mixer performance. Salt is common in most feeds and it comes from only one source. Physical characteristics that make salt an attractive ingredient to test mixing uniformity include:
- it is denser than most feed ingredients
- its shape is generally cubic rather than spherical
- it is smaller than many other particles.
If the mixer will uniformly incorporate salt, those ingredients with more typical physical properties (shape and density) should pose no problem.
Ingredients were added in the usual mixing sequence except for salt, which was added as the last ingredient. Mixing time intervals were every two or three minutes, with total mixing time being 15 minutes.
When access to the interior of the mixer was feasible and safe, three or four samples were taken at each interval. These samples were taken toward each side of the mixer and from the discharge slide. This technique was used for most of the vertical mixers tested. When access to the interior of the mixer was not feasible, 10 samples were taken at discharge to the feed bin.
The average salt concentration and variation between samples for each time interval was calculated to arrive at a single value, the CV.
Shortened mixing times appeared to be a common practice, with many producers unloading almost immediately after adding the last ingredient. Mixing times varied from one to 10 minutes.
The graph in Figure 1 is typical of the vertical mixers tested. The CV dropped rapidly in the first three minutes but did not move into the acceptable mixing range (below 10%) until 13 minutes had elapsed. This supports the recommendation that 10-15 minutes should be the minimum mixing time for (single-screw) vertical mixers. Some of the older vertical mixers did not reach 10% CV until 15 minutes of mixing had elapsed. An example of an older mixer is shown in Figure 2, which shows the results from a 27-year-old vertical mixer. The CV remained above 40% after 15 minutes with virtually no mixing of the feed.
Poor mixing may be due to worn equipment (e.g. old mixer), the particle size of grain ingredients or the sequence of adding ingredients. A particle-size distribution test of the feed in the Figure 2 study showed that the grain was very coarse due to wear of the roller mill. Coarsely ground grain can have a detrimental effect on a feed's mixing properties. For example, ground grain with a large particle size reduces the likelihood of uniform incorporation of micro-ingredients compared with grain ground to an average particle size.