Relationship Between Age, Gender, Height, Body Condition Score, and Body Weight in Thoroughbred Yearlings Raised in Kentucky

Forty years ago, Henneke and coworkers developed a body condition score (BCS) system for horses that is widely used today. This system applies a score of 1 to an emaciated horse and a score of 9 to an extremely obese horse. The system was originally designed to measure BCS in Quarter Horse broodmares, but it has subsequently been adapted to measure body condition in horses of all breeds and life stages. The NRC (2007) suggests that weight gain of 16 to 20 kg will increase the body condition of a 500-kg horse from a 4 to a 5. There is little information regarding the relationship between BCS and weight gain in young horses. This study evaluated the relationship between age, gender, wither height (HT), BCS, and body weight (BW) in a large group of yearlings raised in central Kentucky over a 17-year period (2002–2018). Measurements were taken from colts (n = 2,048) and fillies (n = 2,032) during August of their yearling year. BCS was scored in 0.5 increments. Differences in measurements between genders were determined using an unpaired t-test. Colts (age 515.1 ± 34.7 d, BW 461.7 ± 34.7 kg, HT 155.3 ± 3.5 cm, BCS 5.88 ± 0.34; mean ± SD) and fillies (age 516.6± 34.0 d, BW 446.8 ± 32.4 kg, HT 153.7 ± 3.4 cm, BCS 5.94 ± 0.34; mean ± SD) were not different in age, but colts were heavier (P < 0.01) and taller (P < 0.01). Fillies had a higher BCS (P < 0.01). Over 95% of the yearlings had BCS of 5.5 to 6.5 with > 60% having a BCS of 6.0. Within each gender, multiple linear regression analysis using age, year of birth, HT, and BCS as independent variables and BW as the dependent variable resulted in regression coefficients for BCS of 15.1 ± 1.5 kg and 20.1 ± 1.4 kg (±SE) for colts and fillies, respectively. This suggests that weight gains from 15 to 20 kg will increase the body condition score of 450-kg yearlings from 5.5 to 6.5. The majority (>80%) of the yearlings measured in this study were being prepped for yearling sales where BCS of 6.0 is considered ideal. An understanding of how much BW gain is needed to affect BCS should be useful in the sales-prepping process.