Nutrition is always a study of the individual.
There are basic requirements that will serve a population well, but each horse then has to be considered separately. Their needs change due to factors we may or may not recognize. Knowledge of the basic needs and added nutrition that may help support things like stress, growth, and training is helpful. Providing high quality forage and understanding its basic content is essential. Combining information with horsemanship and observation remains our best tool when it comes to formulating the optimal diet for our horses.
Our means of testing for deficiencies and toxicities remain quite limited even in this day and age.
Very few nutrients can accurately be evaluated in the blood. Vitamin E and Selenium are among the few that can be accurately evaluated. There is a mineral panel that laboratories offer for horses but the value of the information is limited. For example, copper is stored in the liver, so the levels found in the blood do not represent what is in the body. Hair mineral analysis is limited as well. The primary issue is hair represents the past. In most cases we are interested in the horses’ current status. Hair mineral analysis can be useful for showing heavy metal toxicity and detecting environmental toxins but has minimal value when it comes to the nutritional status of a horse. Selenium can be evaluated with hair analysis but blood samples are more accurate.