As previously stated “the objective of feeding a total mixed ration (TMR) is to provide a consistently uniform mix, with the intended ingredient and nutrient composition, across the entire length of the feedbunk with every batch.” And while a number of factors such as ingredient variability and moisture content, consistency of mixing procedures, operator error, and weighing system variability contribute to batch-to-batch variation, it is the characteristics of the mixer itself that essentially control within batch variation, and uniformity across the feedbunk. In this article we will discuss TMR mixer characteristics that affect mix uniformity and a sieving technique that can be used to evaluate it and make adjustments to improve animal performance.

Within-batch TMR variation: For a TMR mixer to create a uniform mix, it must be capable of completely dispersing feed particles that vary widely in size, shape and density both horizontally and vertically within the body of the mixer and the mix contained therein. Manufacturers of TMR mixers achieve this in various ways, and with varying degrees of success, using different designs and configurations of vertical or horizontal augers and/or reels and mixer tubs. These machines then interact with the feed ingredients, which, by their physical characteristics are resistant to flow and mixing (e.g. consider clumping of chopped forage), and are thus susceptible to differences in mix uniformity and performance.
Uniform mixing of forages with concentrates is a particular challenge; the long, narrow shape and lighter density of forage particles makes them naturally prone to separation from the smaller and denser grain particles, especially if there is little surface moisture. This natural tendency decreases as forage particle size is reduced, and the size distribution is made less diverse. Thus it is particularly useful for a TMR mixer to process longer particles and clumps into the mix without further reducing the size of the smaller more fragile particles, a common effect seen when there is excessive mixing force or friction. After auger design, success in forage processing and creating an optimal particle size distribution is largely dependent on knife technology.
A mixer’s ability to create a uniform mix will decrease as the augers and other internal mixing components wear, especially with horizontal mixers. With vertical mixers it is important that the attachments at the outer edge at the bottom of the auger that load the feed into the auger are replaced when they are worn beyond their useful life. Finally, all mixers require space within which ingredients can tumble for them to mix; thus overfilling any mixer beyond its design capacity will dramatically decrease mix uniformity.

Testing mix uniformity: Most TMR mixers discharge feed from the bottom of the mixer, such that what ends up in the feedbunk from the start to finish of feedout will be a reflection of the mix as it exists from the bottom to the top of a mix, respectively. Thus by analyzing a series of samples collected at equally spaced intervals along the feedbunk, one can develop a picture of the degree of consistency, or variability of the mix, as well as whether there is a bias in composition from the bottom to the top of the mix that may be due to particle size, shape and/or density. This is most easily done using a sieving technique that was originally developed by Penn State University for evaluating TMRs for lactating dairy cattle (for detailed information go to www.extension.psu.edu).

Penn State Particle Separator (PSPS): The PSPS consists of a stack of either 2 or 3 square sieves plus a pan that can be used to separate TMRs into 3 or 4 size fractions based on proportions of ingredients found in the sample. The standard series is as follows: Top Sieve: 3/4 inch round holes, Middle Sieve: 5/16 inch round holes, Lower Sieve: 1/20 inch (or 4/25 inch screen), and Bottom Pan. For evaluating TMR performance, we have found it best to use the 3-tray separator (eliminating the Lower Sieve: 1/20 inch or 4/25 inch screen), as the amount retained on the Pan has not varied appreciably when the 4-tray separator is used.

Sampling and sieving procedure: The sampling and sieving procedure is as follows:
1. Make a TMR and distribute it uniformly, in a single pass, down the feedbunk or alley.
2. Divide the row of feed visually into as many equal segments by length as samples that are to be taken, ideally a minimum of five and usually not more than ten.
3. Carefully take a grab sample (about one quart or liter) comprised of about 5 handfuls of TMR from the middle five feet of the first segment.
4. Gently lay the sample over the top sieve, taking care that the long particles do not have opportunity to drop through the holes on end.
5. Place the separator on a smooth, flat surface; hold it by grasping the diagonal corners of the top and middle trays; and then sieve the particles by sliding the trays sharply forward and back such that the particles on the top sieve slide back and forth a few inches across the holes of the sieve.
6. Repeat this motion for a total of five times, then rotate the trays 90 degrees and shake again, until the trays are shaken five times with each side facing forward, twenty times in all.
7. Weigh the amount retained on each sieve and the pan, and express it as a percent of the total weight.
8. Do steps 3 to 7 for each segment of feed along the row of feed, keeping the results in their order in the feed-bunk, and thus, feed delivery.
Correct use of the PSPS can take some practice, and it is especially important that sample size is limited so that the top sieve is not overloaded as it will interfere with the sieving action and results. Consistency of sample size and PSPS operation by a single operator will ensure the most accurate results.

Evaluation of sieving results: For the most part, if the proportion of TMR retained on each tray is similar (e.g. less than +/- 1 - 2 percentage points) across the length of the feedbunk, it indicates good mix uniformity and mixer performance, and one can be confident that there should be a similar proportion of all ingredients at all sites along the feedbunk. If there is high variability between samples, but no trend from start to finish along the feedbunk, it indicates there may be a problem with efficiency of mixing, lack of particle size reduction of baled or other long forage, or perhaps an interaction between ingredients that is preventing dispersion, such as balling. Finally, if there is a trend in the proportion of TMR retained on one or more tray, either increasing or decreasing as one moves from start to finish along the feedbunk, this may indicate either inadequate mixing time, or ingredient separation due to lack of moisture to prevent smaller concentrate ingredients from sorting out from longer, cylindrical forage particles.
Availability of the PSPS: The PSPS is available for purchase from Nasco (www.enasco.com). Commercial feed representatives and nutritional consultants generally include particle size analysis using the PSPS or equivalent as part of their services. Sieving analysis, or the use of a PSPS, may also be available through your local agriculture extension office. Jaylor has their own version of the PSPS called the Jaylor Shaker Box that is included with the purchase of each Jaylor mixer, and mixer evaluations can be performed by your local Jaylor representative. Once problems with mix uniformity have been identified, solutions can be developed and tested using the same sampling and sieving procedures to ensure optimal animal performance — “Because Nutrition Matters.”

Dr. Alan Vaage is a Ruminant Nutritionist with over 30 years
of experience in the beef industry, and currently provides technical support for Jaylor, in Orton, Ontario. Dr. Vaage can be contacted by email:
nutrition@jaylor.com.