How do I handle a horse that becomes increasingly difficult as a competition day progresses?

A horse that starts a competition day calm and manageable but becomes progressively more difficult — more reactive, harder to rate, more resistant to direction — as the day progresses is showing the cumulative effects of environmental stimulation that its management and training have not fully prepared it for. The stimulation of a competition environment — ongoing shooting sounds, other horses running past, crowd energy, and the rider's own competitive tension — builds through the day in ways that eventually exceed the horse's current tolerance level. The most effective management response to a horse that accumulates tension through a competition day is to find quiet, low-stimulation intervals between runs where the horse can genuinely decompress rather than remaining in the high-stimulation environment continuously. A horse that spends the waiting periods between runs standing near the trailer, grazing, and being handled quietly by a calm person has more opportunity to reduce its stress level than one that is kept saddled and circling in the warm-up area throughout the day. At home, deliberate conditioning to extended competition-like environments — taking the horse to multiple shows in a single season, spending full days at events rather than arriving only for a specific class, and exposing the horse to sustained shooting activity from a comfortable distance over several hours — builds the tolerance for extended competition exposure that prevents the progressive deterioration that appears in horses without that conditioning. A horse that is consistently difficult by mid-day regardless of management efforts may have a physical component to its stress response that warrants veterinary discussion. Gastric issues, magnesium levels, and other physical factors can contribute to stress reactivity that management and training alone cannot resolve, and addressing the physical component alongside the training component produces better results than either approach alone.