How is bulk milk somatic cell count calculated?

It is a volume weighted average, not a plain average of the cows. Every cow contributes her own cell count in proportion to how much milk she puts in the vat, so multiply each cow's litres by her cell count, add those together across the herd, and divide by the total litres. That weighting is why a high cow producing well affects the vat more than a high cow that is nearly dry.

Why do a few cows dominate the vat reading?

Because cell counts across a herd are not evenly spread: a small number of infected cows can sit ten or twenty times above the rest, and multiplying that by their litres gives them an outsized share of the total cells in the vat. It is why withholding or drying off a handful of identified cows can move a bulk reading a long way in a single milking, and why herd testing to find out which cows they are is usually the first step.

What does a high cell count cost?

Two ways. There is whatever your company charges in grade penalties or deducts from the price, which differs between processors and is not supplied here, and there is the milk the affected cows are not producing, since infection reduces yield well before anything shows in the vat. This page calculates the vat arithmetic and lets you enter your own penalty, rather than quoting a grade structure that may not be yours.

Somatic Cell Count Calculator NZ

This works out what your vat will read from the cows putting milk in it, and then the useful part: what it would read if the highest group were withheld or dried off. The key thing about bulk milk somatic cell count is that it is a volume weighted average and not a plain average of the herd. Each cow contributes her cell count in proportion to the litres she puts in, so a high cow milking well does far more damage to the vat than a high cow who is nearly dry, and a handful of infected cows can account for a startling share of every cell in the tank. That is why removing a small, correctly identified group can move a bulk reading a long way in a single milking, and it is the whole argument for herd testing: you cannot withhold the right cows until you know which ones they are. What this page does not do is quote a grade penalty. Penalty and grading structures differ between processors and change, so rather than print a schedule that might not be yours, the page does the vat arithmetic exactly and gives you a field to enter your own penalty against it.

Advertise on this page

The herd, in groups

Group	Cows	Litres each a day	Cell count, thousands	Share of the cells
The bulk of the herd				55.1%
Middling cows				14.3%
The high cows				30.6%

Milk price

/kgMS

Combined fat and protein

Your penalty, if you are graded

152

bulk milk somatic cell count, thousands per millilitre

Without the high cows108

The drop45

Milk withheld a day90 L

Value of that milk a day$72.43

Those cows are 1.9% of the milk and 30.6% of the cells. Withholding costs you the milk those cows are making. The comparison worth doing is that daily loss against the penalty you are carrying, plus whatever the infection is already costing in yield.

No grade thresholds or penalty amounts are supplied here, because they differ between processors and change. Enter your own from your supply agreement. The arithmetic on this page is exact and holds whatever those thresholds are.

How it works

Each group contributes cows multiplied by litres, which is its share of the vat, and cows multiplied by litres multiplied by cell count, which is its share of the cells. Add those across the groups and divide the cells by the litres, and you have the volume weighted bulk count. The share of the cells column shows how much of the total each group is responsible for, which is usually the number that surprises people. Removing the high group repeats the calculation without it, and the drop is the difference between the two readings.

Why the weighting is the whole story

A plain average of the herd would treat a cow making fifteen litres the same as one making thirty, and the vat does no such thing. Cells arrive in the tank attached to milk, so a cow contributes in proportion to what she puts in. That cuts both ways: a very high cow who is drying off is doing far less damage than her count suggests, and a moderately high cow milking hard may be doing more. It is also why the same herd can read differently between milkings without anything having changed medically, as the mix of who is milking well shifts.

Worked example

A herd of 260 cows where 240 average 18 litres at a count of 90, fourteen average 16 litres at 450, and six average 15 litres at 2,400 reads 152 thousand in the vat. Those six cows are 2.3 percent of the herd and produce 1.9 percent of the milk, but they account for 30.6 percent of every cell in the tank. Withhold them and the vat reads 108 thousand, a drop of 45 thousand, at the cost of 90 litres of milk a day, worth $72.43 at $9.25 and 8.7 percent solids. Whether that trade is worth making depends on your grade position, which is the number to enter above.

Related calculators

Milk Solids kgMS: converting litres into what you are paid for.
Dairy Payout Income: what the milk is worth.
Break Even Milk Price: where a penalty lands in the budget.
Herd Replacement Rate: culling the persistent offenders.

Data sources: no grade thresholds, penalty amounts or production loss relationships are supplied, because they differ by processor and by study. The bulk milk calculation here is a volume weighted average, which is exact arithmetic rather than an estimate, and the milk price default comes from the same maintained constant the other dairy pages use.