A vertical mixer’s stated volume is the tub. That is all it is. The number stamped on the plate describes how much air the bowl can hold before you load anything into it, and it says nothing about how a wet Gippsland spring ration will behave once the auger starts turning. Plenty of buyers find this out the hard way, somewhere between the second batch of the morning and a phone call to the dealer.
This time of year, with spring orders being placed and paddocks about to green up fast, it is worth getting the maths right before the machine arrives.
Tub Volume and Ration Volume Are Not the Same Thing
The cubic metre figure on a mixer spec sheet is gross bowl capacity. Usable capacity is something else. Most manufacturers will tell you to work at 70 to 80 percent of gross volume. That headroom exists for a reason: feed needs room to lift, tumble, and actually mix. Pack a tub too tight and the auger stops moving material. The mix comes out uneven and the motor works harder than it should.
Now add a spring flush ration into that picture. Fresh pasture is high in moisture and low in structural fibre compared with dry hay. When Gippsland producers start supplementing with silage and wet by-products in October and November, bulk density swings substantially. A batch that sat comfortably in a tub last August, when the ration was built around dry straw and grain, can suddenly overflow or pack solid. Same headcount, same tub, completely different behaviour.
The underlying reason is density. Dry hay and straw typically run around 80 to 100 kg per cubic metre in a mixer. Wet silage can push 600 to 700 kg per cubic metre. A ration heavy in silage, wet distillers grain, or fresh-cut pasture will weigh significantly more per cubic metre than a dry winter ration, which means you hit the weight limit of the machine long before you fill the tub. If you sized on volume alone, you sized wrong.
Long Fibre and the Northern Victoria Rebound Problem
Producers in northern Victoria who ran supplementary feeding programs through dry stretches face a different version of the same issue as they rebuild condition. Transitioning a herd back to a forage-heavy ration after a period of high-grain dry feeding means bringing long fibre back into the mix. Hay, silage, and roughage all occupy more physical space in the tub relative to their weight.
Long-stem material needs to be cut before it can mix properly. A mixer’s knives are doing real work here, and the tub geometry matters just as much as the knife count. A flat-sided decagonal tub forces material toward the auger rather than letting it ride the wall. Round-walled tubs let bales spin without being cut properly, which slows the whole process and leaves the batch taking up more space than it needs to for longer than it should.
The practical consequence: if you are running a ration with a significant proportion of long hay during the spring transition, build extra headroom into your capacity calculation. A tub running at 65 percent of rated volume on a fibre-heavy ration is doing exactly what it should. A tub running at 90 percent will give you problems with mix consistency and unload time before it ever gives you a warning light.
For producers working through how much additional capacity to plan for as herd numbers grow alongside spring supplementation needs, the detail on sizing a vertical TMR mixer when your herd is still growing is worth reading before you finalise any order.
Reading Capacity Charts Against Real Ingredient Densities
Manufacturer capacity charts are built around average assumptions. Usually American or European average assumptions, which means average ration densities that may not match what goes into your tub on a Gippsland spring programme.
Here is a simple way to check the number yourself. Take your target batch weight, add 20 to 25 percent for headroom, then divide by the estimated bulk density of your ration. That gives you a minimum tub volume. If that number sits above 80 percent of the mixer’s rated capacity, you are in trouble before you start.
Batch weight is the figure most buyers know. Density is the one they guess. For a mixed spring ration, a rough working figure for bulk density sits somewhere between 250 and 400 kg per cubic metre depending on silage inclusion rate and moisture content. A ration that is 50 percent pasture silage at 65 percent moisture will land toward the heavier end of that range. A ration that is 40 percent dry hay will land toward the lighter end, but it will take more physical space per tonne.
The calculation is not complicated, but it does require honest ingredient weights rather than estimates from last season’s dry feed program. This is where getting the sizing methodology right before purchase pays for itself every morning for the next decade. The principles behind why getting mixer sizing wrong costs you every day apply directly here, and the numbers involved are significant over a machine’s working life.
Unload Cycles and the Time Cost Nobody Budgets For
Gippsland dairies running two or three mobs often underestimate how batch frequency affects the effective capacity they need. If a ration change at spring flush adds 15 minutes to each mix cycle because the auger is fighting overfull material, that compounds across every feeding for the season. Three batches a day, seven days a week, across a four-month spring flush adds up fast.
A tub that is properly sized for the ration clears cleanly, unloads quickly, and cycles without drama. One that is marginal on a wet ration will leave residue, create sorting opportunities at the bunk, and demand more operator attention at the exact time of year when labour is already stretched thin.
Choosing a Tub for the Whole Year, Not the Easy Season
The practical rule is to size the mixer against the hardest ration you will run, not the average one. For Gippsland and the Riverina, that is usually a spring wet-feed ration with high silage inclusion and potentially significant pasture, not the dry July ration that makes the machine look plenty big enough.
LEO Agriculture’s VT series runs from 6 to 21 cubic metres, with each machine matched to a capacity-specific auger rather than a generic design scaled across multiple tub sizes. That distinction matters when you are cutting wet, fibrous spring forage rather than dry hay, because the auger geometry determines how efficiently the knife does its work at volume. Details on the full range and how different tub sizes handle different ration profiles are available through the guide to choosing the right TMR vertical feed mixer.
When you sit down with a capacity chart this spring, bring your actual ingredient list, your moisture estimates, and your batch frequency. Run the density numbers, not just the headcount. A mixer that handles 200 cows on a dry winter ration may leave you short on a 180-cow herd in November if the ration changed and the calculation did not.
