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Mixing it up: Design, selection and use of TMR mixers

Brian Luck and David Kammel Published on 06 May 2014

In an effort to control particle size reduction, all components of the ration should be added to the mixer without the mixer being in operation.

TMR mixers are utilized for batch mixing feed rations for dairy cows. TMR mixers allow the use of a variety of feeds (dry hay, silage and haylage, grain, etc.) to be blended into a ration that provides the desired nutritional requirements for the animal.



Ideally, the mixed ration is uniformly blended, so when an animal takes a mouthful of feed, it receives a homogeneous sample of the combined ingredients. These mixers allow for better control of the feed ration and provide the ability to adjust the ration based on the nutritional needs of the herd.

Mixing requires motion and redistribution of the particles within the ration. This is done by moving particles mechanically with augers, reels or chains. The mechanical forces that mix the ration can also cause particle size reduction to occur.

This particle size reduction may or may not be a beneficial or desired function of the mixing operation. Mixer designs with the ability to mix in dry hay must be able to achieve the tearing and breaking of long hay pieces while, ideally, only moving ration components at smaller initial particle sizes.

It has been recommended that a properly mixed and processed dairy cow ration have 2 percent to 8 percent of the ration larger than 0.75 inches, 30 percent to 50 percent between 0.31 and 0.75 inches, 10 percent to 20 percent between 0.16 and 0.31 inches and 30 percent to 40 percent less than 0.16 inches when assessing particle size distribution using a four-pan Penn State Particle Separator.

It has also been recommended that particle size distribution of the ration be checked every two to four weeks to ensure uniformity. Temporal variations such as moisture content of ration components and machine wear can contribute to differences in particle size distribution with a consistent mixing regimen.


Sequencing feed components
The sequence in which feed components are added to the mixer plays a role in producing a consistent and uniformly mixed ration. A general method for adding ration components to a TMR mixer is outlined as the following:

• In an effort to control particle size reduction, all components of the ration should be added to the mixer without the mixer being in operation.

• The addition of grains and small-particle feeds should be added first, and longer-particle forages added last.

• The mixer should then be operated for five to eight minutes or until adequate mixing is achieved.

• Finally, the mixer should be stopped until the ration is ready to be unloaded into the bunk.

This method should be slightly modified when whole bales of long forages are being utilized in the ration. Long forages should be added first to the TMR mixer and processed until the desired particle size is nearly reached.


At this point, the mixer is stopped and the remainder of the ration is added to the mixer in the order of smallest particle size to largest. This method may not be ideal for each individual producer’s mixing requirement but should be considered in an effort to maintain consistency, uniformity and proper particle size distribution within the ration.

Auger mixers
Several differing mixer configurations are available for producers. These include horizontal auger, reel and vertical screw mixers. The auger mixer uses one, two, three or four augers to churn the feed in a hopper by moving feed along the flighting of the augers.

In one-auger and two-auger mixers, the flighting moves feed toward the middle of the mixer, where it flows to the top, then toward the sides and back down to the augers. Feed is also moved to the discharge door from both ends of the mixer.

In three-auger and four-auger mixer configurations, one or two counter-rotating augers and flighting move feed in the opposite direction of the other augers, generating a churning motion. Feed moves from end to end and from bottom to top.

The feed eventually moves toward the discharge door and is unloaded when the door is opened. In many auger mixer designs, notched auger flighting or knife sections attached to the flighting provide the ability to cut or tear long hay into 3-inch to 4-inch pieces and incorporate it into the ration.

Reel mixers
The reel mixer combines a set of augers and a reel, similar to a combine reel, in a hopper. Feed is lifted and tumbled by the reel, moving it toward rotating augers, which provide a mixing action. The augers move feed from end to end and to the discharge door for unloading.

Knife sections on the auger flighting cut or tear long dry hay into 3-inch to 4-inch pieces and incorporate it into the ration. An optional hay pan allows the hay to be metered into the mixer directly through the knifed augers, providing the ability to break up large portions of dry hay or baleage.

Vertical screw mixers
The vertical screw mixer consists of a large tub with a single or multiple vertical tapered screws centered transversely in the tub. Knife sections are attached to the flighting to cut material.

Moveable shear or restrictor plates on the tub wall provide a shear surface, increasing the ability to process and reduce the particle size of large packages of hay.

As the screw turns, feed is forced to rise in the center of the mixer and flow toward the side walls. An “edge deflector” creates space at the bottom of the hopper for feed to fall and be moved vertically again by the screw. These units can process rations with almost 100 percent dry hay with no prior processing of hay required. PD

Brian Luck, Ph.D., is in the Biological Systems Engineering Department at the University of Wisconsin – Madison.David Kammel, Ph.D., is in the Agricultural Engineering Department at the University of Wisconsin – Madison.

References omitted due to space but are available upon request. Click here to email an editor.

In an effort to control particle size reduction, all components of the ration should be added to the mixer without the mixer being in operation. Photo by PD staff.