Most spring mixer orders in the Free State go wrong before anyone opens a brochure. The mistake happens earlier, at the point where a producer converts headcount into cubic metres and either uses a rough rule of thumb or skips the step entirely. By the time the machine arrives and spring calving loads are rising, there is no fixing it.
The LEO Agriculture app includes a vertical TMR mixer size calculator that is free to download and built specifically to prevent that. Most producers either do not know it exists or open it once and close it because they are not sure what numbers to put in. This piece walks through what the calculator actually needs, how to read what it tells you, and where the delivery wagon question changes the answer.
The Three Numbers That Drive Everything
The calculator is not asking for your average headcount. It needs your peak-load group size, your feeding frequency, and your ration density. Get those three wrong and the output is meaningless, regardless of how precise the tool is.
Peak-load group size means the largest single group you will mix for in one batch, not the whole herd averaged across the day. On a typical Free State cow-calf operation heading into spring, that is usually your close-up dry cows plus first-calf heifers if they run on the same ration. That group frequently gets undersized because producers count their milking herd and forget the transition animals.
Feeding frequency is straightforward but often entered incorrectly. If you push up twice a day but only mix once, enter one batch per day. If you split your milking cows and dry cows into separate batches, those are two separate runs even if they happen back to back. The calculator treats each batch independently, so the number you enter here directly sets the batch volume the machine needs to handle.
Ration density is where most producers get into trouble. Silage-heavy rations sit at roughly 130 to 180 kg per cubic metre (8 to 11 lb per cubic foot) when loaded. A high-roughage dry-cow ration with long-stem hay or oat straw can be lighter. If your spring transition ration leans heavily on straw and dry material, entering an average silage density will cause the calculator to underestimate the tub volume you actually need, because a lighter, fluffier ration occupies more space per tonne of dry matter. The calculator provides density feedback automatically once you have entered the other values, which is a useful check if you are unsure where your ration sits.
Forage percentage matters too, particularly if you are expanding or changing the ration structure for spring. The calculator includes a field for forage percentage and flags when you should upsize to accommodate hay expansion. For operations running bale-heavy rations through calving, that warning is worth paying attention to.
Reading the Output Against VT and HD Capacity Bands
The calculator recommends a specific LEO model and shows you the batch volume against what that machine can efficiently handle. This is where the common mistake is made: seeing a recommended size and immediately asking for the next one up as a safety margin.
Oversizing is a real cost. A machine running at 50 to 60 percent of tub volume does not mix consistently. The auger geometry is designed around a working load range, and how the auger is sized for each tub volume directly affects mix quality at the loads you actually run. A machine that is too large for your batches leaves material in the corners and produces inconsistent particle distribution, which defeats the point of a TMR.
The VT series runs from 6 to 21 cubic metres and covers most dairy, beef, and sheep operations in the Free State. The HD series starts at 16 cubic metres and runs to 45 cubic metres, built for larger-scale feedlot and commercial dairy work. The overlap between the top of the VT range and the bottom of the HD range is not just a capacity question. The HD machines are built for truck mounting and for operations where the daily throughput demands heavier construction. If the calculator puts you at 14 to 16 cubic metres, the question to ask is whether your daily batch count and your future herd plan push you toward the HD band, not just whether you want a buffer.
For producers planning for herd growth into the next season, sizing for current headcount and then picking one size up is not a strategy. Size for the batch you will actually run at 80 percent utilisation two or three years from now. The article on what a 21 m³ standard-duty vertical mixer can handle in practice gives a useful reference point for where the top of the VT range sits on real-world rations.
When the Bunk Line Is Far From the Mix Bay
Here is the variable most size calculators do not account for, and it changes the answer on a lot of Free State properties where the mix bay sits near the silage clamp and the bunks are spread across a large yard or out in a paddock feedlot.
If you are mixing and then driving the full load to the bunk yourself, the mixer needs to hold the entire batch. But if a delivery wagon is carrying the mixed ration from the mix bay out to the bunk lines, the calculation shifts. You can mix one full batch, offload into the wagon, and while the wagon is travelling and discharging, the mixer can be cleaning down and preparing the next load. For operations with long bunk lines or multiple feed points separated by distance, this changes the effective throughput of a smaller machine considerably.
LEO’s 25 Series feed delivery wagons run from 28 m³ to 40 m³ (1,000 ft³ to 1,400 ft³) and can be set up as trailer units or truck-mounted for operations where terrain or distance makes tractor-towing impractical. If the discharge door setup at your bunk lines is something you are working through, the detail on setting discharge doors for consistent bunk coverage is worth checking before you finalise the configuration.
The practical implication for sizing: if a delivery wagon is part of the system, you may not need to size the mixer for the total daily ration volume. You need to size it for the largest single batch you will mix and transfer in one cycle, accounting for the time the wagon is out on the run. Some operations find this allows them to stay in a smaller, more efficient capacity band rather than jumping to the next machine size purely to handle the daily volume in one shot.
Before You Place the Order
Late winter in the Free State is exactly the right time to run these numbers, because lead times matter. Getting the calculator inputs right now, before the spring calving flush pushes feed demand up and before dealers are quoting extended waits, gives you the information you need to order the correct machine rather than the convenient one.
The size calculator in the LEO Agriculture app is free. It runs on iOS, Android, and the web. Put in the peak group, the feeding frequency, the ration density, and the forage percentage. Read the output against both the recommended model and the utilisation figure it gives you. If a delivery wagon is part of your system, factor that into the batch volume before you decide whether to move up a size. Operations running multiple ration types through the same machine will find that scheduling multiple rations on a larger machine introduces its own set of sequencing decisions that are worth thinking through before spring.
The calculator does the maths. The job is making sure the inputs are honest.
