The pattern of thoroughbred growth is affected by a foal's birthdate

Abstract

Understanding equine growth patterns is important when balancing the economic and perceived athletic benefits of large size and rapid growth against the potential associated risk of skeletal abnormalities. Existing Thoroughbred growth data indicate that the long and short term patterns of growth are different based on when a foal is born, however this difference has not previously been quantified for body weight (BW) and withers height (WH). Therefore, the objectives of this study were to (1) accurately and precisely characterize the short term patterns of equine growth separated from the long term sigmoidal pattern and (2) investigate the influence of the day of year on which the foal was born (DOB), age, and day of year (DOY) on the short term pattern of growth. Data for this study consisted of 35,044BW measurements and 25,987WH measurements from 2184 horses. Data were collected from 1977 to 2007 on farms located in the United Kingdom, Ireland and the United States. The BW and WH data were examined as changes over age or the DOY on which the measurements were taken. The data were further categorized based on DOB. The Richards sigmoid growth model was fit to the data and percentiles for BW and WH over age described. The long term pattern of growth was similar to that in previous studies of Thoroughbreds. The DOB influenced the short term patterns of BW and WH growth over time, such that at specific ages or on particular DOY foals born in the first 60 days of the year will be growing differently than those born between DOY 91 to 150. The data further indicate that early growth is more closely associated with age, while later growth may be more affected by environment. This study adds to the rich database of Thoroughbred growth data already available and adds significant detail regarding WH development and specific characteristics of short term growth. The data and results presented here should be useful to those managing growing stock and interested in more precisely moderating the pattern of growth. The study also raises questions regarding how the patterns of growth can be controlled and whether certain patterns may be more beneficial or detrimental than others regarding the future health and performance of the horse.