By the time the ryegrass flushes in Gippsland, the decision is already made. Mixers ordered in late winter are on-farm by August or September. Mixers ordered in October are still three months away. And a machine that is just barely big enough for your current herd in July turns into a bottleneck by November, when dry-matter intakes climb and you are suddenly running two batches where one used to do the job.
The hidden cost of a too-small mixer does not show up on the invoice. It shows up in the next twelve months of fuel bills, labour hours, and wear on a machine that was never sized for the load it ended up carrying.
How Spring Dry-Matter Demand Changes the Maths
Dairy cows in late lactation through a dry winter typically consume somewhere around 20 to 22 kg of dry matter per day. Once high-quality spring ryegrass comes into the system and cows are pushing peak production, that number moves. Pasture-based herds supplemented through the mixer might see effective TMR inclusions shift significantly as managers adjust the ration to offset grass energy and protein variation. Even if overall mixer-fed dry matter drops because the cows are spending time on pasture, the batches you do run need to be accurate and timed properly or you lose the benefit entirely.
For herds where the mixer carries the full ration year-round, spring is simply harder. Higher intakes, more frequent ration adjustments, and the risk that a machine running at or above its practical working volume starts producing inconsistent mix. Overfilling a vertical mixer compresses the material too much for the auger to work through cleanly. Undermixing is the result, and undermixing in a high-production herd has a direct effect on butterfat and milk solids.
The working rule most operators know is that a vertical mixer should run at 70 to 85 percent of its rated bowl volume for consistent results. Below that and the auger does not engage the material properly. Above it, you are asking the machine to do something it was not built for. If your peak spring batch sits at 90 percent or more of the bowl, you are already in trouble.
Translating Herd Size Into Cubic Metres
Take a concrete example. A 200-head dairy herd in Gippsland, fed a TMR ration through the mixer twice daily. Average batch size in winter might sit at around 4.5 to 5 cubic metres per mix, depending on ration composition and forage percentage. A machine rated at 6 m³ handles that comfortably in winter.
Then spring hits. Dry-matter intake climbs. You adjust the ration. The batch volume grows to 5.8 m³. You are now running the 6 m³ machine at nearly full capacity, twice a day, every day. When you need to upsize ingredients mid-season, there is no headroom. You either mix short and compromise the ration, or you run a third batch. That third batch, at current diesel prices and with an operator’s time factored in, is not trivial.
If you are still working through exactly how to convert your headcount and ration density into a bowl volume requirement, this breakdown of the calculation and its consequences covers the method in full.
What the Second Daily Batch Actually Costs
Most operators underestimate this because they add it up mentally rather than on paper. Run the numbers.
- An extra mix takes roughly 45 to 60 minutes including loading, mixing, and discharge.
- Diesel consumption for PTO-driven mixing, depending on tractor size, runs in the range of 8 to 14 litres per hour under load.
- At current fuel prices, that second batch costs somewhere between $15 and $30 in diesel alone, per day.
- Labour at a casual or overtime rate for that additional hour adds another $30 to $50 daily depending on your situation.
Call it $45 to $80 per day across a 180-day spring and summer feeding period. That is $8,100 to $14,400 in recurring annual cost, and that figure does not account for the additional wear on a machine running longer hours than it was sized for. Knife life shortens. Gearbox hours accumulate faster. Components that were designed for a certain duty cycle get pushed past it.
Compare that against the capital cost of stepping up one bowl size in the VT Series. The gap between a 6 m³ and an 8 m³ machine, or between an 8 and a 10, is a one-time purchase decision. The running cost of the undersized machine compounds every season.
The Auger Has to Match the Bowl
One thing worth understanding when you are comparing machines at different capacities: not every mixer scales the same way. A larger bowl with the same auger geometry as a smaller unit does not perform proportionally better. The auger diameter, flighting pitch, and sidewall clearances all need to be specific to the volume they are working in. LEO Agriculture machines, for instance, use capacity-specific augers across the range rather than fitting one standard auger design across multiple bowl sizes. That matters because it affects both mix quality and the torque loading on the drivetrain over the life of the machine. There is a fuller explanation of why auger design has to change with capacity if you want to go further on that point.
Check the Size Before You Order, Not After
Late winter is exactly the right moment for this calculation because there is still time to act. If your spring peak batch volume is going to push your current machine past 85 percent of bowl capacity, the case for stepping up is strong. If you are already at that threshold in winter, you have your answer.
The LEO Agriculture app includes a built-in TMR mixer size calculator that takes your cow numbers, feeding frequency, and ration density and outputs a recommended bowl size with real-time feedback. It also shows which bale sizes work in the recommended machine, which matters if you are running large round bales through a mixer in northern Victoria where bale dimensions vary. It is free to download from the App Store or Google Play, and there is a guide to using the mixer tools in the app if you want to see what it covers before you install it.
Run the calculator now against your projected spring numbers, not your current winter batch. If the recommended size is larger than what you have on order or on-farm, that gap is worth acting on before the ryegrass makes the decision for you.
Lead times for commercial mixers are running three to four months from some manufacturers. That window closes fast once August arrives.
