|Total Mixed Ration (TMR) Mixer Sizing and Costs|
Appropriate size is one of the most basic decisions that needs to be made when buying a TMR mixer. The mixer size is dependent on several factors: the dry matter intake of the animal, the group size to be fed, the ration density, and the number of times per day the group is fed.
Dry Matter Intake
For milking cows, the dry matter intake is dependent on production level. Dry matter intake for different milk production levels was obtained from nutritionists and is shown in Figure 1. For heifers and dry cows, the dry matter intake is dependent on the animal size, and should be available from nutritionists.
The mixer size is also dependent on the animal group size. The larger the group of animals, the larger the total amount of feed needed per batch. Look at group size for not only the milk cows, but other groups as well. For example, the amount of feed needed for a heifer group might control the size of the mixer.
The ration density changes depending on the types of feed used in the ration. The ration densities in Figure 1 vary from 15 to18 lb/cubic feet (as-fed). The ration densities were calculated based on a 70% forage to 30% grain ratio, with haylage and/or corn silage for forage and high moisture ear corn as the grain. Four different rations are shown. As-fed bulk densities were assumed to be 12 lb/c.f. for hay silage @ 60% moisture, 14 lb/c.f. for corn silage @ 65% moisture, 46 lb/c.f. for high moisture ear corn @ 30% moisture, and 5 lb/c.f. for dry hay @ 12% moisture (moisture content is on a wet basis; c.f. = cubic feet).
There are differences in ration density when dry hay is substituted for silage. The addition of dry hay tends to bulk up the ration. The as-fed ration density decreases approximately 1 lb/c.f. for every 10% addition of dry hay. As the TMR density decreases, the ration volume per cow increases and greater mixing capacity is needed. A ration with 20% replacement of dry hay for silage is also shown. Substitution of dry hay beyond 20% of the ration will probably decrease the ration density as well, but it is unknown how much it will affect the density.
In one ration, in Figure 1, corn silage was substituted for some haylage. Since corn silage tends to have a slightly higher bulk density than haylage, the as-fed ration density increased approximately 1 lb/c.f. As the TMR ration density increases, the ration volume per cow decreases and less mixing capacity is needed.
Total Mixed Ration Volume for Milking Cows
Figure 1 shows the approximate volume of feed per cow needed at varying milk production levels. Please be aware that these values are estimates based on assumptions of a base total mixed ration and densities of feeds used in the rations. Many factors affect the final ration density. The volume read from the chart for a particular ration can be used to determine the mixing capacity needed for a group of cows. The resulting TMR ration volume estimates have been checked with several manufacturer's recommendations and seem to be close to what sales people suggest for sizing a mixer.
Figure 1. TMR volume per cow, as-fed.
Using Figure 1 for a ration which has 10% dry hay and has a ration density of approximately 16 lb/c.f., an 80 pound per cow milk production gives a ration volume of approximately 6.5 c.f./cow/day.
A group of 100 cows would require:
Feeding twice a day would require:
Number of Feedings per Day
The example gives the mixing capacity per batch of feed for this group of cows. A farmer considering feeding only once a day during some parts of the year would either need to buy a mixer large enough (650 c.f.) for once a day feeding, or mix two batches (325 c.f. each). Economics, labor availability, and feed bunk management will help determine the decision.
Always ask about the mixing capacity of the mixer you are considering buyingit might not be the same as the model number. It is also not always the total volume of the mixer. The struck-level capacity of a mixer is the total volume of the mixing compartment. Depending on the manufacturer, mixing capacity is 60 to 90% of the struck-level capacity of the mixer. If the mixing capacity of the mixer is not known or stated, use an estimate of 60 to 70% of the struck-level capacity of the mixer. Always follow the manufacturer's recommendations on mixer capacity. Overloading mixers beyond their rated mixing capacity increases the mixing time required to provide a uniform mix and can in some cases result in spillage and lost feed.
A survey was conducted to determine costs of mixers on the market in 1998. The cost data represent the manuacturers' suggested retail prices (MSRP). The MSRP values are for a trailer mixer with medium-priced electronic scale and magnet on the discharge, ready for use. The information includes 59 mixers from 11 different companies. Most companies have 4 to 8 mixer sizes in their line. Costs of the mixers ranging in size from 120 to 900 c.f. are plotted in Figure 2.
Figure 2. TMR mixer costs.