"Anyone who breeds knows the feeling of elation when your mare "gets it right"and gives you exactly what you ordered, be it filly or colt.
The earth spins around the earth and the earth spins around the sun, resulting in 4 phases of the moon, called quarters. This cycle takes 29.5 days and is known as the lunar cycle.
The moon is said to be new when it is directly between the sun and the earth, in complete shadow and invivible to us.
The first and the last quarter occur when the moon is on the opposite side of the earth to the sun, so that the suns rays are reflected arounde the earth to hit the moon. This casts the earths shadow on the moon, leaving only a crecent shaped sliver to recieve the suns rays.
When there is a full moon, the moon is completley outside of the earths shadow and is totally exposed to the sun's rays. A full moon rises at sunset and sets at sunrise, but other phases of the moon can start during the day and vanish before dawn.
In 2004, researchers in the UK published the results of a 14 year survey into the effects of the moon on thoroughbred mare fertility. It was not concerned with whether the mares produced live foals but concentrated only on conception rates. Fertility was defined as the proportion of coverings that led to conception and tended to peak around the full moon each month, decreasing around the first quarter, which occurs a week earlier.
The idea is that, just are highest on the day after the full moon and new moon each month, so to is the horses fertility also centered on the day after the full and new moons. Tables were generated, centered on the days after the full moon, new moon, and the first and last quarters. these tables showed that the mares tended to cycle less during the first quarter and oestrus and fertility peaked around the full moon and for about a week afterwards. The overall increase from the first quarter to the full moon was 29%.
The conclusion reached from this study was that peaks in mare coverings and in fertility tended to synchronise on or just after the full moon, meaning that mares are more fetile at or just after a full moon. This is valuable information if you are struggling to get your mare in foal and, with the use of modern hormone therapy, it is a simple matter to time her ovulation for the days after the moon is full.
The full moon period is apparently also the time to breed fillies. Depending on which quarter of the lunar cycle the mare concieves, you may be able to determine the sex of the foal before you breed the mare.
The optimum time to breed for a filly is from the day before or the day of the first quarter to a few days after the full moon and for colts, the day before or the day of the last quarter until a few days after the new moon. These times change by a day or two each month and there are 2 or three transitional days between each where you have a 50/50 chance of getting a filly or a colt."