Onset of puberty in pasture-raised Thoroughbreds born in southern hemisphere spring and autumn.

Reasons For Performing Study: There is little information on age, weight and time of year of puberty in Thoroughbred horses, and the interpretation of such data is difficult due to the wide variety of descriptions of the onset of puberty.

Objectives: To examine the age, bodyweight and date of onset of puberty in Thoroughbreds born in spring and autumn.

Methods: Bodyweight data and blood samples were collected in 59 pasture-raised Thoroughbred horses. Five autumn-born and 18 spring-born colts and 3 autumn-born and 33 spring-born fillies were examined from birth to age 13 and 17 months. A testosterone concentration >2 s.d. above the baseline concentration was indicative of onset of puberty in colts; and progesterone concentration >2 ng/ml and at least 3 times greater than the previous progesterone concentration was indicative of a first ovulation in fillies.

Results: Spring-born fillies and colts were older and heavier than autumn-born fillies and colts at puberty. The age at onset of puberty in spring- and autumn-born foals was 291-408 days and 212-270 days, respectively. The weight at puberty in spring-born foals was 302-409 kg, and in autumn-born foals was 277-344 kg. However, the mean date at onset of puberty was not significantly different between spring- and autumn-born horses, with puberty occurring in October (New Zealand spring).

Conclusions And Potential Relevance: Seasonal changes in photoperiod affect the timing of onset of puberty, provided a minimum threshold bodyweight has been reached. Spring-born horses reached this threshold weight during the winter months and remained reproductively inactive until after the stimulus of increasing day-length occurred. The autumn-born horses reached the threshold weight to support puberty at the same time as stimulatory photoperiod and, therefore, reached puberty significantly younger and lighter than the spring-born horses.