Which of the following is not considered a nutrient?

The selection of dry matter as the correct answer highlights an important distinction in nutrition. Nutrients are specific substances that provide nourishment essential for the growth and maintenance of living organisms. These include proteins, vitamins, and minerals, which each play critical roles in various bodily functions.

Proteins, for example, are vital for building and repairing tissues, and they serve as a source of energy. Vitamins are necessary for numerous biochemical processes, including metabolism, and they support immune function. Minerals are also crucial, as they help in a variety of bodily functions, including nerve transmission and muscle function.

On the other hand, dry matter refers to the portion of forage that remains after all the water has been removed. While it's an important concept in assessing the nutritional value of feed (since it provides a basis for measuring the concentration of nutrients), it is not itself a nutrient. Instead, dry matter serves as a measure of the quantity of feed available, impacting the intake and digestion of actual nutrients contained within that feed.

Understanding these definitions and distinctions is essential in horse evaluation and nutrition, particularly when formulating a diet that meets the specific needs of horses based on their activities or life stages.