Scotland's sheep industry is stratified, with surplus breeding sheep from our high hills being sold to upland farmers who then produce prolific, crossbred ewe lambs that are sold to lowland farmers who mate them with meaty sires.

The system starts in the autumn when hill farmers select the best pure-bred ewe lambs to be retained as breeding replacements for their flocks. The next "draw" of ewe lambs, or mid-ewe lambs as we call them, that aren't quite good enough to be bred pure, are sold to upland farmers to breed crossbreds.

Those upland farmers who want better quality breeding stock can also buy the older "draft" ewes from the same high-hill flocks. Hill farmers sell those ewes when they are five-year-olds after they have had four "crops" of lambs. Such ewes are deemed to be too old to survive another year on a high hill, but are perfectly capable of breeding another couple of seasons on a kinder farm.

This is a time of year when all sheep farmers are constantly examining and working on their sheep. Hill farmers have a system of denoting the age of their ewes using saw marks on their horns, or notches in the ear of breeds that don't have horns. For instance, a ewe born in 2010 will have no marks, those in 2011 one mark, 2012 two marks, 2013 three marks, 2014 four marks and 2015 no marks - and so on.

While the age of a ewe is important to a high hill farmer, all sheep farmers pay particular attention to the teeth of their ewes, because sheep with slack or missing teeth find it hard to graze properly during the hungry winter months when grass is short.

Unlike carnivores which only need to eat meat once a day at most, and can go several days without a meal, herbivores like sheep spend a lot of time grazing because grass isn't as nutritious as meat.

All that grazing requires two important features in a breeding sheep - a good set of teeth and sound feet to keep it on the move.

Well-fed sheep without teeth can live into their teens but most need them to be profitable. Some start to lose their teeth at just five years of age, while most are "broken-mouthed" by seven.

Some farmers with better land successfully breed from broken-mouthed sheep, but for most that's a risky strategy. Without a lot of extra, expensive supplementary feed, they would be lean in the spring and struggle to provide enough milk for their lambs.

Sheep only have one set of incisors on the lower jaw, that bite the grass against a dental pad on the upper one.

There are many reasons for sheep losing their teeth prematurely. Badly-bred sheep with teeth that don't bite properly against the middle of the dental pad soon get long in the tooth. Long teeth get slack and fall out. That's why you will often see farmers and shepherds looking carefully into the mouths of breeding sheep, particularly rams, before they buy them.

Another cause of broken mouths is mineral imbalances. The immense drain of calcium on a ewe's metabolism while she is pregnant or suckling her lambs leads to early loss of teeth.

Sheep farmers and shepherds also check the udders of their ewes after looking at their teeth. Ewes with defective udders can't produce enough milk to rear their lambs properly, or they may produce so little that their lambs perish from hypothermia within a few hours of birth - milk is the fuel that generates warmth in a newborn lamb and then nourishes it to grow rapidly. Those with diseased udders are fattened for slaughter.

Once the breeding sheep have been selected, they then get their annual service. That usually involves being dosed with a wormer and dipped in a solution of chemicals that kill parasites like ticks, lice and sheep scab mites, as well as preventing them from becoming infested.

Dipping is not as popular as it once was, and many now prefer injecting their sheep or applying the chemicals in a concentrated form along their backs. That way the insecticide is incorporated into the lanoline next to the skin and, when done properly is as effective as the more laborious plunge dipping technique.

Another important task is to check that every hoof is sound, trim overgrown ones, treat diseased ones and run the flock several times through a footbath.