What Are the Average Growth Parameters for Young Thoroughbreds?

Over a three-year period (1993-1995), a total of 350 Thoroughbred colts and 350 Thoroughbred fillies in central Kentucky were weighed and measured monthly to determine average growth parameters for young horses.

Horses were weighed on a portable electronic scale through 18 months of age. Wither height and condition score were also measured in about half of the foals. In order to estimate mature body size in these horses, 472 broodmares were weighed 60 to 90 days after foaling. In addition, 25 Thoroughbred breeding stallions were also weighed. The average body weight of the broodmares equaled 570 kg and the average weight of the stallions equaled 580 kg. At 14 days of age, colts and fillies weighed an average of 77.7 and 76.1 kg and had average heights of 107.3 and 106.3 cm, respectively. Colts were heavier and taller than fillies throughout the study and at 490 days averaged 9.9 kg heavier and 1.6 cm taller. The greatest difference in condition score between sexes occurred at four months of age when the fillies had an average score of 6.48 and the colts had a score of 6.0.

Compared to March foals, foals born in January and February were 6.8 kg lighter at 14 days of age. They remained smaller until about 9 months of age when they averaged about the same as the March foals. April and May foals were larger at 14 days of age than March foals, and remained slightly heavier until 6 months of age. Average daily gain (ADG) among the four groups was similar until about 7 months of age. ADG was 1.5-1.7 kg/d during the first month and declined linearly to about 0.70-0.80 kg/d at 7 months of age. After 7 months of age, ADG tended to be more variable and by 12 months, ADG was very different between the different months of birth. Foal growth rates were reduced during the winter months regardless of when the foals were born and increased during April and May of the foal’s yearling year. Growth rate in these yearlings was more a function of season of the year than age.

This report of KER’s 1996 research was published in Pferdeheilkunde.

Read the entire research report, titled A Summary of Growth Rates of Thoroughbreds in Kentucky.