This is the time of year when sheep farmers start to sow the seeds for next year's lamb crop by putting rams out to ewes.

Most will have put a lot of effort into preparing for this big and important event in the sheep farmer's calendar.

Firstly, the breeding flock will have been dosed with anthelimintics and flukicides to kill off any stomach worms and liver fluke, as well as getting routine vaccinations. Lame ones will have had their feet pared, and the whole flock will have been run through a footbath at least once to help eliminate lameness.

Finally they will have been put onto better grazing to build up their body condition, or "flush" them as we say. Flushing encourages breeding ewes to produce more ova, which should result in more twins the following spring. While that was going on, rams will have had similar preparatory treatment.

Paring rams' hooves was always a sweaty, back-breaking job that I detested. Nowadays there are metal cradles that you back the ram into before turning it over and holding the ram securely upside down on its back while you work on its hooves. Back in the days of my youth, before such contraptions were developed, we used to have to sit them on their bottom and hold them in a "shearing" position. Every now and then, the ram, that could weigh at least double your own weight, would wriggle into a more comfortable position and throw you off balance.

It's vital that rams have perfect hooves, as lame ones are usually reluctant to seek out and serve ewes. Mating, or "tupping" as we call, it involves a lot of running about to check the whole harem.

Rams also need to be in tip-top condition because they have little spare time to graze properly, and as a result are pampered and fed special rations in the run-up to the breeding season.

One of the big worries at tupping time is that a ram will not be inclined to serve ewes, or be infertile and fail to get them pregnant.

One way round that problem is to fit "sire sine" harnesses to the rams. They are leather harnesses that are tightly buckled on around the chest and shoulders in such a way as carry a coloured chalk pad fitted securely against the brisket, between the forelegs. They can chaff the "armpits" of the rams, so other farmers prefer to catch their rams everyday and smear an oil-based, coloured chalk concoction on instead. It's a lot more hassle, but kinder to the rams.

The idea is that the chalk on the ram's brisket will colour the wool on the rump of the ewe when he serves her. If none of the ewes in a batch have been coloured by the ram at the end of a week-or-so, he can be replaced by another, known "worker".

Most farmers change their rams round different groups of ewes at the end of the first oestrous cycle, which can range from 13-19 days, but usually averages 17 days. At that stage, the colour of the chalk can be changed on the rams. So, if the colour of chalk for the first service was say blue, and that for the second service or oestrous cycle is red, then any ewes returning will be coloured red and blue. If that happens consistently in a group you can be fairly sure that the previous ram is infertile.

This simple practice has other uses. For instance, it allows you to know fairly accurately when ewes are due to lamb. Those that were coloured say blue in the first service period of 17 days, can be separated from those that either returned to first service or were subsequently served in the second 17-day period.

That allows the farmer to bring the first batch of ewes into the lambing shed, or sheltered field and lamb them before he turns his attention to the second batch. Those that were never marked with any colour can be assumed to be barren and sold.

Knowing when different batches of ewes are due to lamb also allows the farmer to feed different batches more accurately depending on their stage of pregnancy. Not only does that save on the cost of feeding, but it also allows farmers to have their sheep in ideal condition for lambing - neither too fat or too thin.

It's amazing how coloured chalk has allowed sheep farmers to develop sophisticated systems of husbandry.