Which of the following is not considered one of the five main nutrients for horses?

The selection of oxygen as the answer is accurate because oxygen is not classified as a nutrient in the context of equine nutrition. While oxygen is essential for horses as it is for all aerobic organisms, it is not a nutrient that is consumed through diet. Instead, horses obtain oxygen from the air during respiration, which is crucial for energy production and overall metabolic processes.

In contrast, carbohydrates, fats, and vitamins are fundamental components of a horse's diet. Carbohydrates are the primary energy source, fats provide concentrated energy and essential fatty acids, and vitamins play vital roles in metabolic functions and maintaining health. These three categories are essential nutrients that horses need for optimal growth, performance, and health. Thus, while oxygen is critical for life, it does not fit within the structured classification of nutrients that are typically considered when discussing a horse's diet.