Abstract
Linear description (LD) of conformation traits was introduced in horse breeding to minimise subjectivity in scoring. However, recent studies have shown that LD traits show essentially the same problems as traditionally scored traits, such as data converging around the mean value with very small standard deviations. To improve the assessment of conformation traits of horses, we investigated the application of the recently described horse shape space model based upon 403 digitised photographs of 243 Franches-Montagnes (FM) stallions and extracted joint angles based on specific landmark triplets. Repeatability, reproducibility and consistency of the resulting shape data and joint angles were assessed with Procrustes ANOVA (Rep) and intra-class correlation coefficients (ICC). Furthermore, we developed a subjective score to classify the posture of the horses on each photograph. We derived relative warp scores (PCs) based upon the digitised photos conducting a principal component analysis (PCA). The PCs of the shapes and joint angles were compared to the posture scores and to the linear description data using linear mixed effect models including significant posture scores as random factors. The digitisation process was highly repeatable and reproducible for the shape (Rep = 0.72–0.99, ICC = 0.99). The consistency of the shape was limited by the age and posture (p < 0.05). The angle measurements were highly repeatable within one digitiser. Between digitisers, we found a higher variability of ICC values (ICC = 0.054–0.92), indicating digitising error in specific landmarks (e.g. shoulder point). The posture scores were highly repeatable (Fleiss’ kappa = 0.713–0.857). We identified significant associations (p(X2) < 0.05) with traits describing the withers height, shoulder length and incline, overall leg conformation, walk and trot step length. The horse shape data and angles provide additional information to explore the morphology of horses and therefore can be applied to improve the knowledge of the genetic architecture of LD traits.
Highlights
Since the early stages of their domestication, horses have been selectively bred to produce offspring with adequate behavioural docility, overall health and a conformation adapted to their intended use [1]
Following a Generalised Procrustes analysis (GPA) of the consensus shapes, thereafter we evaluated the inter-repeatability between the two digitisers by comparing the consensus shapes for the 62 randomly sampled horse digitised by both digitisers
The intra-class correlation coefficients (ICC) based on the centroid size remained at 0.99 for all comparisons while repeatability based on the Procrustes distance (Rep) based on the Procrustes distance tended to be lower (0.72 to 0.99)
Summary
Since the early stages of their domestication, horses have been selectively bred to produce offspring with adequate behavioural docility, overall health and a conformation adapted to their intended use (e.g. agriculture, transport, sports, leisure) [1]. Selection decisions are commonly based on such preliminary traits assessed before the animals start being used competitively. This way, selection intensity can be increased and the generation interval shortened. The scoring scale, which represents a weighting scale from good to bad, is often not used efficiently (lower end not used at all) with bias towards the optimum. Such protocols are useful in a breeding competition because they allow for a final ranking. Compared to measurable traits (e.g. height at withers), judged conformation traits have shown lower heritabilities, which has often been attributed to the broad trait definition and the subjectivity of judging [5]
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