Puberty in Thoroughbreds

Puberty is a transitional period for horses, from a time of reproductive immaturity to a time where sexual behavior is demonstrated and sperm is ready for release in the colt, and the filly starts to cycle. Unfortunately, very little information is available about puberty in horses, including at what age and what weight most horses reach this important life stage.

Horses are long-day seasonal breeders, becoming sexually active as days lengthen in the spring and throughout the summer. In the Southern hemisphere, spring begins in October and fall arrives in February.

Thoroughbred foals typically arrive in two crops--spring (October-born) and autumn (February-born). Researchers from Massey University in New Zealand used this separation in time of birth to design a study to examine the effects of season, age, and body weight on the onset of puberty in Thoroughbred horses raised in the Southern hemisphere.

Fifty-nine pasture-bred Thoroughbred foals were examined from birth to 13 months (autumn-born) or 17 months (spring-born). Foals were examined daily, and body weight and blood samples were taken every two weeks. The blood was used to determine testosterone (colts) or progesterone (fillies) concentration. An increase in testosterone greater than two times the standard deviation above baseline was diagnostic for puberty. In fillies, a progesterone concentration of greater than 2 ng/ml and greater than or equal to three times the previous measure was indicative that ovulation had recently taken place.

Both groups of foals entered puberty  the spring after their birth; the autumn-born foals were about eight months old and the spring-born foals were about 11 months old. Autumn-born foals also weighed less at puberty onset, ranging from 611-759 pounds (277-344 kilograms), or approximately 49% of anticipated mature weight (1,268 pounds or 575 kilograms).

This suggests there is a weight threshold of about 617 pounds (280 kilograms) for the onset of puberty to occur. The spring-born foals reached this weight threshold in May/June, but this is a time when day length is decreasing, so there was no stimulus of increasing day length to bring about puberty. In contrast, autumn-born foals reached the threshold weight at the same time daylight was increasing, providing these foals with both stimuli at the same time.

Therefore, the researchers concluded that both season of the year and body weight are critical for foals to achieve puberty. Autumn-born foals in the Southern hemisphere, therefore, reach puberty at a younger age and a lighter body weight.

Brown-Douglas, C.G.; Firth, E.C.; Parkinson, T.J., et al. Equine Veterinary Journal, 36 (6): 499-504, 2004.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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