How do you measure a silage stack?

Length, average width and average height give the volume, and volume multiplied by density gives the weight. The averaging matters, because a stack has sloping sides and using the width at the base overstates it substantially. Measuring at mid height on each face is the usual approach. Density varies with how well the stack was compacted, the crop and the dry matter, so it is the figure worth being careful with rather than the dimensions.

How much silage is lost to wastage?

Enough that a stack calculation ignoring it is not much use. Losses happen on the surface where air gets in, at the shoulders, at the face if it is opened faster than it is fed, and in the feeding out itself. The share differs enormously with how the stack was made, sealed and managed, so this page takes it as an input rather than assuming one, and puts the figure before and after wastage side by side.

How long will a silage stack feed a mob?

Divide the dry matter remaining after wastage by the mob's daily dry matter demand. That answers the question that actually gets asked in autumn, which is not how big the stack is but whether it will get through to spring. Working it out before winter leaves time to buy in, which is a very different position from finding out in August.

Silage Stack Yield Calculator NZ

Measure the stack, and this tells you what is actually in it: tonnes as fed, kilograms of dry matter, what remains after wastage, what the feed is costing a kilogram and how many days it will feed the mob you are wintering. That last figure is the reason to do the exercise in autumn rather than in August, because a shortfall found in April can be bought out of and a shortfall found in August cannot. Two things need care. The first is the measuring: a stack has sloping sides, so the width at the base overstates it considerably and the average width taken at mid height on each face is what you want. The second is wastage, which is not a rounding error. Losses happen on the surface wherever air gets in, at the shoulders, at the face if it is opened faster than it is fed out, and again in the feeding out itself, and the share differs enormously with how the stack was made, sealed and managed. So this page asks you for it and shows the figure before and after side by side, because a stack calculation that quietly ignores wastage will tell you that you have enough feed right up until you do not.

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Stack length

Average width

Average height

Density as fed

kg/m3

Dry matter

Wastage

What the stack cost to make

Mob demand a day

kg DM

48 days

days of feed, after wastage

Tonnes as fed332.6 t

Dry matter in the stack99,792 kg DM

After wastage87,817 kg DM

Cost per kg DM fed$0.17

Wastage costs 11,975 kg DM here, which is 6 days of feed and was paid for in full. Wastage is taken off before the days are worked out, because the feed lost to spoilage was never available to the mob.

No standard density, dry matter or wastage figure is supplied. All three move with the crop, the compaction, the sealing and the management of the face, and the wastage figure in particular differs enough between well made and poorly made stacks that assuming one would be the least useful thing this page could do.

How it works

Length multiplied by average width and average height gives the volume, and volume multiplied by density gives the weight as fed. Applying the dry matter percentage gives the kilograms of actual feed in the stack, and taking off the wastage gives what will reach the mob. Cost per kilogram of dry matter divides what the stack cost to make by that net figure rather than the gross one, which is the honest version: the feed that spoiled still cost money to grow, cart and stack. Days of feed divides the net dry matter by the mob's daily demand.

Wastage is a cost, not a discount

It is tempting to treat spoilage as feed that simply was not there, but it was paid for in exactly the same way as the rest. Growing it, cutting it, carting it and stacking it all cost the same whether it ends up in a cow or in a heap at the shoulder of the stack. That is why the cost per kilogram here uses the net figure: a stack with 20 percent wastage has feed costing a quarter more a kilogram than the same stack with none, and improving the sealing is often the cheapest feed available on the farm.

Worked example

A stack 24 metres long, 9 metres wide on average and 2.2 metres high holds 475.2 cubic metres. At 700 kilograms a cubic metre as fed that is 332.6 tonnes, and at 30 percent dry matter it contains 99,792 kilograms of dry matter. Taking off 12 percent wastage leaves 87,817 kilograms actually available. Against a mob eating 1,800 kilograms of dry matter a day, that is 48 days of feed. At $14,500 to make, the feed that reaches the mob costs $0.17 a kilogram of dry matter, and every point of wastage avoided lowers that figure.

Related calculators

Hay Bale Weight: the same arithmetic for bales.
Winter Hay and Baleage: what the mob needs to get through.
Feed Conversion Ratio: what the feed turns into.
Stock Units: the demand behind the daily figure.

Data sources: no density, dry matter or wastage standard is supplied, because all three depend on the crop and on how the stack was made, sealed and managed. The geometry is exact; every other figure is yours.