What is the difference between starting a colt and starting a filly and does gender affect the training approach?

The practical differences between starting colts and fillies are less dramatic than popular belief suggests, but there are genuine tendencies that experienced trainers acknowledge — while being careful not to let gender expectations override what the individual horse is showing. Colts, particularly as they approach sexual maturity, may show more dominant and testing behavior than fillies of the same age — studying the environment for mares, becoming distracted, pushing boundaries that were previously accepted. Clinton Anderson addresses this directly in his colt starting program, noting that colts in the twelve-to-twenty-four-month range often require more leadership clarity and more consistent enforcement of boundaries than fillies, not because they are more difficult horses but because their developing hormonal state produces behavior that needs clear, consistent response. Fillies tend to be more sensitive — both more responsive to light communication and more reactive to pressure or negative experience. What produces a mild concerned response in a colt may produce a more significant reaction in a filly of the same age and background. This sensitivity is an asset when it means the filly learns quickly from light communication, and a liability when it means negative training experiences affect her more deeply and persist longer. Warwick Schiller's individual assessment approach is most applicable here: the colt and filly distinction provides a starting hypothesis about potential tendencies, not a prescription for how to handle a specific horse. An individual filly might be bolder and less sensitive than the average colt. An individual colt might be extremely sensitive and relationship-oriented. Reading the individual horse and adjusting the approach accordingly is always more accurate than applying gender-based generalizations. For breeding considerations, the intact colt has the additional management challenge of his interest in mares, which can be significantly distracting during training. Many trainers prefer to start colts before they reach full sexual maturity — around eighteen months to two years — or to have them gelded before starting if competition rather than breeding is the intended career.