What type of metabolism is engaged during a slow trot or canter over level ground?

The correct answer, aerobic metabolism, is engaged during a slow trot or canter over level ground because this type of exercise occurs at a moderate intensity where the horse is able to take in enough oxygen to support its energy needs.

In aerobic metabolism, energy is produced with oxygen, allowing for longer durations of activity without fatigue and utilizing fat and carbohydrate stores efficiently. This is crucial during slower paces like a trot or a controlled canter, where the horse can maintain the effort for extended periods.

In contrast, the anaerobic pathways, such as anaerobic alactic and anaerobic lactic, typically take over during high-intensity efforts that exceed the capacity of the aerobic system. These processes do not rely on oxygen and are characterized by quicker bursts of energy, which is not the case during a leisurely trot or canter on flat ground. Creatine phosphate is part of the immediate energy system that provides a very short burst of energy, which is also not necessary during slower paces. Thus, aerobic metabolism is the primary energy source in this scenario.