Forelimb Conformation of the Growing Thoroughbred and the Impact of Birth Weight and Parental Conformation

In Thoroughbred juveniles, offset carpal and fetlock inward conformation was associated with higher birth weights.Carpal conformation was associated with the carpal conformation of both the sire and the dam. Forelimb conformation of juvenile Thoroughbreds seems to be affected by both genetic and environmental factors.

1. Introduction

Conformational deviations from a straight central axis when viewed frontally are very common in the forelimbs of young Thoroughbreds [1,2]. Many deviations are congenital, but limb conformation can change dramatically over the first 18 months of life and result in a very different yearling conformation. Veterinarians are often asked to evaluate the conformation of the forelimbs of foals, but there are little data about their development. The purpose of this study was to follow forelimb conformation during the first 18 months of life to better understand the development of the conformation of the forelimb of the Thoroughbred and to understand what factors may affect it.

2. Materials and Methods

Study foals (n = 119) were housed at four divisions of one farm and were born in 2002. Foals entered the study if their conformation could be graded before 36 days of age and until at least 120 days of age. Also recorded were birth date, weight, and age and weight at final evaluation. Conformational grading was performed at 30-day intervals during 2002 and at 60-day intervals in 2003, stopping in August. Grading was most commonly graded by the consensus of two observers.

The conformation at each grading period was summarized as straight, valgus, offset (carpal deviation resulting in the metacarpal long axis being parallel and lateral to the radial long axis), and outward rotation. Combinations of deviations were frequent. Fetlock conformation was graded as straight, inward deviation of one or both fetlocks (includes varus and inward rotation), or outward deviation of one or both fetlocks (includes valgus and outward rotation). The conformation of available dams (n = 116) and sires (n = 47, sired 99 subject foals) were graded the same as the progeny.

All data were analyzed using SAS. The GLM procedure was used to compare means across specified groups. Two-way classifications were analyzed using the FREQ procedure. Significance was set at p < 0.05.

3. Results

Subjects were graded an average of 10 times (range, 4 - 13 times), and the average age at last evaluation was 463 days (range, 122 - 628 days). The average birth weight for subjects was 52.3 kg (range, 38.6 - 65.9 kg), and there were 61 fillies and 58 colts in the study. During the perinatal period (first week of life), 67 foals had their forelimb conformation graded (mean age of 2 days). Carpal conformations were graded as 3% straight, 94% valgus, 57% outward rotation, and 7% offset. Combinations of carpal deviations (such as valgus and outward rotation) were apparent in 55% of foals. Fetlock conformations during the perinatal period were graded as 55% straight, 37% outward deviation, and 7% inward deviation.

Conformation was graded in 70 subjects at a mean age of 525 days (yearlings; range, 433 - 628 days) and a mean weight 428 kg. Carpal conformations at this time were graded as 7% straight, 21% valgus, 56% outward rotation, and 69% offset. Combinations of carpal deviations were seen in 53% of yearlings, especially outward rotation and offset. Yearling fetlock conformations were graded as 79% straight, 9% outward deviation, and 13% inward deviation.

Heavier birth weights were significantly associated with an offset carpal conformation at birth (p = 0.001) and 46 (p = 0.003), 176 (p = 0.0001), and 362 days (p = 0.0008), and had a strong trend of association (p = 0.1) at 525 days of age. Heavier birth weights were significantly associated with fetlock inward deviation at the following ages:46 (p = 0.005), 176 (p = 0.02), 362 (p = 0.003), and 525 days (p=0.002).

Sire carpal conformations were 19% straight, 19% valgus, 49% outward rotation, and 64% offset. Dam carpal conformations were 31% straight, 3% valgus, and 67% offset. Sire fetlock conformations were 68% straight, 23% outward deviation, and 8.5% inward deviation. Dam fetlock conformations were 46% straight, 27% outward, and 27% inward. There were associations between parental and progeny carpal conformation as yearlings. Sire carpal conformation was significantly associated with yearling outward carpal rotation (p = 0.03) and carpal offset (p = 0.003). Dam carpal valgus had a very strong trend (p = 0.06) of association with progeny yearling valgus. There were no associations for fetlock conformation between parents and progeny.

4. Discussion

Heavy birth weights are associated with offset carpal and fetlock inward conformation in yearlings. Although a casual relationship is not proven, it may be that early heavy loads have an impact on yearling conformation. Parental carpal phenotype was associated with the same conformation in yearlings, suggesting a genetic influence. The lack of the same relationship at the fetlock suggests other factors impact yearling fetlock conformation.

The authors thank the University of Wisconsin School of Veterinary Medicine Equine Research Fund.