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Design, Selection and Use of TMR Mixers David W. Kammel, Professor Biological Systems Engineering Department UW-Madison, 460 Henry Mall Madison, WI 53706

Introduction Mixer design is an art. Field testing, consumer feedback and experience is used to refine the design. Despite the information collected over years of use in the beef and dairy industry, and the yearly design changes, mixer designis still a mechanical art form. Feeding a dairy cow is an art. Total mixed rations (TMR) have become the major feeding system of the dairy industry. Experienced nutritionists and research trials allow us to build better rations. Despite the information collected in research feed trials, and the use of ration balancing software, feeding a dairy cow is still a biological art form. Mixer DesignThere are approximately 25 different mixer manufacturers in the market, and in general, the mixers seem to be doing an adequate job of mixing a TMR. There has been only a handful of cases where the mixer design failed to do the job, and those mixers quickly left the market. In other cases, the mixers were used improperly (over or under mixing) which caused nutritional problems for the dairy herds. Themixer design that works best for one farm may not be the best choice for a different farm. The question “Is there a best mixer design?” is open for discussion, debate and personal opinion. A better question would be “Which mixer is better for my situation?” There are certainly design differences in the mixers available. Design changes are driven by market and consumer demand. Take for example theconsumer demand that a mixer should be able to handle the addition of long dry hay into the ration. This has impacted the design changes of many of the mixers on today’s market. In less than 5 years this design goal has produced mixers that can process either a little hay or a lot of hay. This particular design change has caused another potential problem with misuse; particle size reduction withtoo long a mixing time. Mixer design is still primarily a trial and error process with due consideration given to prior experience. The manufacturer selects a specific mixer design that is expected to perform and field tests determine design changes and their effect on the mix.

In a summary paper on mixer design, these design and testing issues were identified: l Mixer design (type, geometry,power, time, speed, efficiency) l Define material changes (particle size reduction) l Define standards for comparison of mixers l Classify and measure degree of mixing (determine the quality of the mix) l Describe the mixing process l Correlate quality of mix with respect to time All of these are good research projects, and some manufacturers and researchers may have answers to some of theseproblems. Other issues may not ever be solved. As an industry, there is no effort to coordinate the research efforts or develop standards for testing. Function of a Mixer The definition of mixing is the putting together of two or more substances or groups, one with another so that the particles or members of each are diffused among those of the others. The function of the mixer is to blend uniformly,particles of different sizes, moisture content, and bulk density. “Perfect” mixing is the state in which any sample removed from the mixture has exactly the same composition. This never really occurs because of segregation due to differences in particle size, density, and shape. The manufacturers select mixer designs that provide for the needed “degree of mixing” in the desired time. The variationof the mix should be minimal as the feed is delivered out of the mixer. Samples pulled from the beginning, middle, and end of unloading should be the “same”. In reality, this means no significant difference to the animal. The mixer should allow the use of a variety of feeds to be blended into a ration that provides the desired nutritional requirements for the animal to be fed. The mixed ration...