Can early training of show jumpers bias outcome of selection events?

Evaluation of free jumping at sub-maximal heights is common practice within selection procedures for young breeding stallions. Early training might cause an unjustified bias. To investigate this, data from a 5-year longitudinal study on 30 horses were used. Half of these horses (experimental group) had received early training between 6 months and 4 years, the other half (control group) had not. Between 4 and 5 years, all horses had received standard training under saddle. At the age of 5 years, the horses were tested in a puissance competition and the 7 best and 6 worst jumpers were used for the present study. Kinematic variables that were different at ages 5 years and 6 months, and had been shown to be predictive for performance in earlier studies, were analysed at the age of 4 years. It showed that early training had effaced the differences between potentially good and less good show jumpers in 3 of 3 predictive variables and had introduced a (false, because not related to performance) difference between the trained and untrained horses in one of them and a nearly significant trend in another. Early training may to a certain extent obscure differences in talent among individuals at the age at which selection events occur. Experienced judges may be able to account for this, but studbooks and judges should be aware of this possible pitfall.