MOST animals come into oestrus or heat - or season as farmers call it - in response to shortening or lengthening daylight. For instance, sheep and deer come into season in the shorter days of autumn so their offspring will be born in kinder spring weather when grass is growing again, while birds mate and lay eggs in the spring as days are getting longer so their chicks hatch in late spring or early summer when there is an abundant supply of insects to feed on.

Although cattle are not strictly seen as seasonal breeders, their fertility is also affected by daylight. Cows and heifers come into season less frequently between November and March compared to between April and October. Herds that are given supplementary artificial light in their sheds at night during winter come into season more readily than herds without night-time lighting that only experience natural hours of daylight.

Fertility is one of the major factors affecting the efficiency and profitability of any dairy herd. It can account for one of the major costs of production and also represents an area where significant improvements can be made.

My grandfather's generation got their cattle in-calf naturally using bulls, but modern dairy farmers rely on artificial insemination (AI), or the "bull with the bowler hat". AI enables farmers to select semen from top bulls that are proven to be suited to their herd's breeding programme. Nowadays, sexed semen is also available, enabling farmers to breed many more replacement heifers. Using AI sires to breed replacements also removes the need to replace stock bulls every two years or so to avoid fathers mating with their daughters.

The snag with AI is that the bull with the bowler hat has to identify when cattle are in season and ready for AI - something that comes naturally to real bulls.

Not all cattle make it obvious when they are in season and it takes an observant and skilled stockman to detect them. There are a few aids available, such as injecting maiden heifers with hormones to synchronise when they come into season and then giving all of them AI at the optimum stage of their oestrus. Others fit transponders to their cattle that record the additional activity generated by cattle in season. Those transponders are automatically read by electronic scanners in the milking parlour that then transmit the information to a central computer in the farm office. Maiden heifers that are not being milked can have their transponders read when they drink from a water trough.

Some farmers go to the expense of hiring skilled contractors in the main breeding season to observe the herd and AI those that are in season.

Another of the advantages of AI is that, within reason, you can determine when your cows are going to calve. For instance, if you don't want the worry of cows calving when you are away on a family holiday, you simply stop AI for an appropriate period about 10 months earlier.

AI is not commonly used on beef farms, despite being an excellent way of improving herd genetics without the expense of buying top bulls. The problem with beef cows is that they are not as docile as their dairy counterparts, and don't have the routine of being brought into the sheds to be milked twice-a-day. Separating individual beef cows in season from the main herd and bringing them into pens for AI is a lot more hassle than with dairy cows. Oestrus synchronisation of the whole herd is a way round that problem, but profits in the beef industry are wafer thin and few are prepared to go to that expense.

There are similar problems with sheep, although some top pedigree breeders synchronise their flocks to AI them with semen from top rams.

Unlike cattle, cross pleating inside the cervix of sheep prevents penetration with a normal AI pipette. The consequence is that it is not possible to move sufficient sperm through the cervix. To overcome that problem, vets or well-trained operators use a minor surgical technique called laprascopic AI, which deposits semen directly into each of the uterine horns by means of a pipette inserted through small incisions in the stomach wall.

Pigs are more predictable than cattle or sheep. After weaning, sows will come into season again as quickly as three days later, but four to seven days is more typical, with an average return to season of five days after weaning. So canny pig farmers wean their sows on a Friday morning to avoid the task of inseminating them over the weekend.