Buying a vertical TMR mixer is one of the larger capital decisions a feeding operation makes. Most producers spend real time comparing knife systems, gearbox ratings, and discharge options. Fewer spend enough time on the number that will quietly define every feeding day for the next decade: mixer volume.
Get that number wrong by even two cubic metres on the low side, and you feel it immediately. Get it wrong by five, and you end up splitting batches within eighteen months. Neither outcome is recoverable without buying another machine.
Why Capacity Math Starts at Your Peak Load, Not Your Average
The instinct when sizing equipment is to think about what you normally do. Average herd numbers, typical ration weights, a usual day at the feedbunk. That logic works for some things. For a vertical mixer, it leads you straight into trouble.
Vertical mixers have a rated fill capacity for a reason. Pack material above the auger tip and you are no longer mixing; you are just spinning product around the top of a pile. The auger flights work through a specific volume of material, and when you exceed that, the outer portions of the load simply do not circulate properly. You can run the machine an extra fifteen minutes and the ration still will not be uniform, because time does not fix geometry. The material above the auger’s working zone just sits there.
So the number you need to size to is your maximum single-batch volume, including your heaviest ration, at your largest anticipated herd size. Not what you mix on a quiet Tuesday in April.
What Happens When You Buy Borderline
Say you are feeding 280 cows today and your current operation calls for a mixer that lands right at the edge of a particular capacity class. You buy that size because it covers you now. Eighteen months later you add 60 animals. Suddenly your calculated batch volume exceeds rated capacity by ten or twelve percent.
At that point you have two choices. You can overfill and accept degraded mix quality, which hits milk production and feed efficiency in ways that are genuinely hard to trace back to the mixer. Or you can split into two partial batches per feeding, which means double the tractor hours, double the fuel burn, double the wear on the machine, and double the labour time for a job that should have been one cycle. Over a full season that is not a small number.
A 15% increase in cow numbers is not unusual growth over two or three years. Yet it is enough to tip a borderline machine into a daily problem. The next size up costs more at the point of purchase. It almost always pencils out over any realistic machine life.
This is the core argument for buying ahead: not to waste money on capacity you do not need, but to cover the growth that is most likely to happen before you replace the machine. If you are planning to expand, factor that into the calculation before you write the order, not after.
Running the Numbers Before You Commit
Sizing a vertical mixer properly requires four inputs: herd size, number of feedings per day, target mixer utilization, and ration density. Ration density is the variable that trips people most often. Heavy corn silage-based rations behave very differently from high-hay diets in terms of bulk volume per unit weight, and the difference can shift your required mixer size by a full model step.
The basic calculation divides your total daily dry matter requirement by your feedings per day, then adjusts upward by your utilization target and converts to volume using your ration’s bulk density. Running a mixer at 80% of rated capacity rather than 100% gives you a meaningful buffer against load variation and makes it easier to maintain mix quality across different ration types and seasonal forage changes.
LEO Agriculture publishes a free online Vertical TMR Feed Mixer Size Calculator that handles this math, factoring in herd size, feeding frequency, ration density, and utilization to return a recommended volume before you start talking price. That kind of tool used before the purchase order is worth considerably more than it is used afterwards.
Ration Density: The Variable Everyone Underestimates
Average ration density in North American dairy operations runs around 0.147 metric tons per cubic metre, which is the figure baked into most standard sizing references. But that average hides a wide spread. High-forage rations with significant amounts of dry hay can sit well below that figure, meaning the same batch weight occupies considerably more volume in the tub. If your ration runs heavily toward long-fibre hay, you may need a machine one class larger than a straight weight-based calculation suggests.
This is also where forage expansion matters. If you are planning to shift from a silage-heavy ration to more dry hay, or if you are growing your own hay and intend to feed more of it, you want to upsize for that change now rather than discover the volume problem after the equipment arrives.
The mix quality effects of oversized forage in an undersized machine go beyond just capacity. Long dry hay that sits above the auger tip does not get processed. It either comes out in the ration as long strands or it wraps and causes mechanical issues. Both outcomes are worse than they look on paper.
The VT and HD Series Range
The LEO Agriculture VT Series covers standard vertical mixing from 6 to 21 cubic metres across seven models, designed for dairy, beef, and sheep operations running two to three mixes per day. The HD Series runs from 16 to 45 cubic metres and is built for commercial operations handling twelve to eighteen hours of daily mixing. Truck-mount configurations are available in the HD range for large feedlot applications.
The overlap between the top of the VT range and the bottom of the HD range is intentional. An operation at 16 to 20 cubic metres daily needs to decide whether standard-duty cycling fits its schedule or whether commercial-duty construction and longer daily run times put the HD in the right category. Volume is one factor; daily machine hours is another.
Each machine in both ranges uses a capacity-specific auger rather than a shared design scaled across multiple sizes. The auger diameter, flighting geometry, and wall-to-auger distance are calculated for each model individually. That matters for mix consistency at the extremes of a machine’s working range, not just at the centre.
One Calculation That Pays for Itself
Do the capacity calculation with your peak numbers, not your current ones. Add a buffer for ration density if your diet runs heavy in dry forage. Check where your result lands against the available model range, and if you are sitting right at the boundary between two sizes, the economics of double-batching should settle the question quickly.
Ration quality after the mix leaves the machine is a separate problem worth attention on its own. But none of that matters if the mix was never uniform to begin with, and a mixer running above its rated volume will not give you a uniform mix regardless of what you do downstream.
