The hilarity of naming a film Meet the Fokkers, hinges on a sort of music-hall double entendre which caused hypocritical smirks in Hollywood. A certain section of US society is terrible prim about sex but realise that a bit of bonking sells films. And if you can get away with using the F word in a 'totally innocent' way, then !ka-ching! someone will be making money. While I can't use that word in such a polite medium as blogspot, its Dutch homologue is quite innocent: a fokker is a breeder and a paardfokker is a horse-breeder. Thoroughbred horse breeders seem to be wholly innocent of common sense about the reproductive biology of the principal in their way of making a living. It's like the Cat Fancy Association of America: they don't even know what colour their cats are.
For reasons of convenience, and historical inertia, a horse becomes a '1 year old' on the First of January following the foal's birthday. A lot hinges on this, in the same way as it affects a child's experience in school if s/he is the oldest or youngest in 3rd Class - ignoring puberty etc. in 6th Class. A horse born in mid-January will have had 350 days of growth and training before it becomes a 1 year old; a horse born in July will have less than half that advantage. There are at least 190 races for Two Year Olds, including the Maribynong Plate [$A151,000], Critérium de Maisons-Laffitte [€190,000], and the Colin Stakes [Can$125,000], which are, for these developmental biology reasons, not run on a level playing field.
For optimum success in the Prix des Chênes [first past the post = €40,000] this September, breeders will have had to take their pedigree mare to 'service' 340 days before 1st January 2014. But you have to pay attention to the mare because her estrous cycle is about 3 weeks in length, so you want to pick the time when she is fertile which is nearest to the 340 day cut-off. If you/she/he 'misses' then you've lost 3 weeks of growth and development at the other end of the pregnancy. And if you cut it too fine and the foal was born at Christmas 2013, then it was a 1 year old a week later and you haven't got a race winner. A horse pregnancy is 340 +/- 20 days and I don't think the precise length of the next pregnancy is accurately predictable, so breeders must allow some lee-way. So far so maths.
The biology of it adds an additional complication which we have experienced with our sheep: mares don't cycle in the Winter because their evolutionary history has found that foals born in Winter 11 months later don't do as well as foal born in Summer - when there is more grass and so more milk. A mare's fertile period is from 'early Spring until Autumn'. I'll bet that the fertility within these limits in Gaussian / normally distributed / bell curved. Mares served in June and July are much more likely to 'take' than those at the edges of the estrous cycle distribution. Deciding to use 1st January as the arbitrary cut-off between year cohorts has disastrous consequences for fertility in the bloodstock industry.
In the UK, for similar medieval history reasons, the personal tax year runs between successive 5th of Aprils. This strikes outsiders as peculiar-to-bonkers but that's a very modernist prejudice. The year used to start at/on the Spring equinox 21st of March . . . and why not? Then the British calendar had to catch up with the rest of Europe in 1752 but the tax year had to be 365 days and so the start-of-year shifted forward to 5th April. The Irish government gave up on that medieval quaintness in 2002 when, in parallel with adopting the Euro, they switched the tax year to start on 1st January. The sky did not fall! Revenue didn't get richer; people and corporations didn't over-pay their tax that year. Surely (with one leap they were free) the paardfokkin' blood-stock 'industry' could shift the year cut-off to later in the year and save everyone a lot of headache and heartache.