After a lifetime working with those woolly creatures, I can now reveal that 5% is an optimistic figure and that it's not uncommon to lose 7%.
It seems that a sheep's only desire in life is to die, and their ability to find new ways to go about it never ceased to amaze me.
At this time of year, just before shearing, they have an infuriating habit of rolling over to rub an itchy back, and then getting stuck upside down. Gas quickly builds up in their stomachs and they soon die needlessly.
Then there are winter deaths from drowning or getting stuck in snow drifts, followed by the annual, heavy run of losses at lambing time. On top of that there are 101 different diseases that afflict sheep all year round.
Cattle on the other hand are content with life, so losses are much lower. That's just as well with good cows worth upwards of £2000, compared to a ewe that will typically set you back a more modest £140.
There is nothing more distressing than finding a dead sheep, or worse a dead cow.
Apart from the financial loss, there's often a nagging feeling of guilt that maybe more could have been done to prevent the death – still, those who keep livestock should expect dead ones.
My father used to say: "Never count your losses," and that was probably good advice as the tally some years can be quite alarming.
At one time we buried dead sheep on the hill where they had died. Those that died closer to the farm buildings and all dead cattle were hidden, discreetly out of sight to await collection by the local knackery – a service that was provided free of charge.
While we hid our dead animals, we couldn't conceal our losses from our neighbours.
The distinctive knackery lorry could be clearly seen from miles around, as it made its way up the farm lane, heralding another death to all and sundry.
Times change and it is now illegal to bury dead animals on the farm and they must all be collected by the knackery for proper disposal.
To add insult to injury, it now costs about £60 plus VAT to uplift a dead cow, and £16 plus VAT to dispose of a dead ewe.
The knackery business may be an unpleasant one, but you would be amazed at the way they process dead farm animals into useful products.
The first valuable product to be removed are the skins and hides that are sold to the Far East. Calf skins are used in car upholstery, while cow hides go towards heavy leather goods like rigger's boots. Sheep skins also make fine leather.
Fallen stock are classed as Category 1 waste that is not permitted to enter the human food chain. Cattle slaughtered in an abattoir that are deemed too old for human consumption, and certain specified risk materials from the slaughtering process are also classed as Category 1
Category I waste from abattoirs is also collected by knackers and ends up being processed along with the skinned, fallen livestock.
A combination of heat and chemistry produces a glutinous tallow that is used as a raw material in the production of bio-fuels like bio-diesel.
The end product that is left at the end of that process is dried meat and bone meal (MBM), and it is sold to power stations that at one time used to charge knackers to incinerate it.
Nowadays power companies recognise that MBM has a similar calorific value to coal, and pay for it to generate heat that is converted into electricity.
So next time you fill your car with diesel, forget about the horsepower under the bonnet, and remember that the engine is probably being partly fuelled by sheep and cattle.
Waste not, want not.