The Arabs used to be the finest horse-breeders in Europe. Their subtle understanding of astrology may well have helped them in this breeding process. Then, in the eighteenth century, Britain out-bred the Arabs, creating the finest racehorses in the world. The UK is presently slowly losing this edge, partly because, with the price of top stallions at several million pounds apiece, they are being brought up by Japanese and American buyers.

Are horses, like most animals, more fertile around the Full Moon? Clearly this would be important as stud racehorse stallions are expensive to hire. This was the view of the farmers in the old Roman empire, as documented by Claudius Ptolemy:

"...[farmers] notice the aspects of the Moon, when at full, in order to direct the copulation of their herds and flocks, and the setting of plants or sowing of seeds: and there is not an individual who considers these general precautions as impossible or unprofitable."

Ptolemy, Tetrabiblos, Ch. III

To test this view, nine years of data were acquired from a thoroughbred stud farm on mare fertility: dates of horse covering (i.e., mating) through to live birth. This stud farm has several hundred matings a year, and racehorses are the only animal whose sex life is fully documented and published. Each mare is covered several times per season until it conceives, though each season nearly ten percent remain barren.

Like cows, horses ovulate every three weeks throughout the breeding season. with modern technology, vets can tell within forty eight hours if conception has occurred, so the record of mare coverings is also a record of when they are ovulating. There is a tendency for horses in a stud farm to synchronise their ovulation, so that the vets become more busy at three-weekly intervals. In this they resemble many creatures in the wild, where the females of a species tend to ovulate in synchrony.

Using hormones, vets have unnaturally shifted the season of oestrus (coming-on-heat) so that it begins in February. Gestation in horses takes eleven months and in the wild they would not foal until May, only dropping their young when the frosts were over. Normally they would be on heat in the summertime.

I published the results in 2000, claiming a positive result for lunar influence (1). No-one took any notice, and I have now collated a further five years of data from the same studfarm: this makes 2318 mating-pairs between 1986 and 1999, or 4898 mare coverings.

To analyse this data by Moon phase, coverings are grouped by 'lunar-day number' 1 - 29 when they happened, where New Moon is Day 1 and Full Moon is 14 or 15. The first diagram shows, how mares came on heat just before the Full Moon and stay that way for a few days. This is a statistical average, over the years of data. Surprisingly, the main dip appears in the first quarter, a week before the Full Moon. That is a swing of twenty percent or so - large enough to be of practical value for a vet.
 
 

Figure: Mare coverings for 14 years 1986-1999, grouped by lunar-day numbers, where Full Moon is day 14 or 15.

     The Full and New moon positions were compared in the article using five-day totals, centred on the day after the Full and New Moon. The idea is that, just as the tides are highest on the day after the Full and New positions each month, on average, so likewise the Moon's influence on horse fertility is centred on the day after these key points. All the coverings during the five lunar days 29,1,2,3 and 4 totalled 794, while all the coverings over the lunar days 14 - 18 came to 866. That's a ten percent excess of horses coming on heat at the Full position as compared to the New, over the full 14 years of data.

Summarising, three- and four-week rhythms of fertility interact in a horse population, as the mares come on heat in the spring. The former expresses a biochemical cycle within the horses, viz. the oestrus cycle, while the latter responds to the angle between Sun and Moon. Mare oestrus peaks just after Full Moon, about ten percent more than at New Moon.

Mare Fertility

We define horse fertility, as the proportion of mares which conceive after covering. This averages 40% - which is a lot higher than for humans! The graph shows this plotted around 360 degrees of the Sun-Moon angle, where Full Moon is 180 degrees and New Moon is zero. Every twenty-nine and a half days, as Sun and Moon come together in the sky, this angle returns to zero, from 360. Clearly, the big peak is at Full Moon. So, biologically, the racehorse mares - and perhaps all animals - are then more fertile. To estimate this ratio we first exclude the 'no-return' cases which are the coverings that did not have any outcome recorded - that's about seven percent of all mating-pairs each season. To get the hang of this trend, let us proceed as before, scoring the five consecutive lunar day numbers, this time including conceptions as well as coverings:.

                                 Coverings            Conceptions           Fertility

Full Moon  period           844                   369                        44%

New Moon period          767                    307                        40%

The overall mean value for mare fertility is 40%, so there is no minimum in fertility at New Moon, only a maximum at Full. But, a minimum does appear during the first quarter - a week before the Full moon. For comparison, the 'fertility' graph is constructed using a 500-point moving average which corresponds to about three days of the lunar month: so each point is an average of all the coverings within about three days of the lunar-month cycle.

A peak occurs one or two days after the Full Moon, with a ten percent difference of mares on heat varying with the lunar cycle. A Full Moon peak in coverings i.e. oestrus cannot remain in the same mare population, as they are cycling to a three-week rhythm; but we are not here dealing with a single population of mares, as horses enter the stud farm often for quite short periods. These results may be the only evidence ever presented of the effect of the lunar cycle upon mare fertility. There is more to the story, I believe, in terms of how the node cycle modulates the fundamental lunar-month cycle, and this node-cycle modulation could be the reason why the effect has not been earlier discovered. If the two nodes have a similar effect, then the node cycle is of nine-year periodicity. This is certainly a reason for wanting at least nine years of data to investigate the subject. A further account may deal with this matter.

(1)  N. Kollerstrom and Camilla Power, 'The Influence of the Lunar Cycle on Fertility on two Thoroughbred Stud Farms' Equine Veterinary Journal January 2000.
 

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