What stands out the most? All but four minerals are meeting requirements for your average 1,000lb horse at maintenance / light work, and two minerals are extremely high.
Let’s address the high levels of potassium and iron first. High levels of potassium are easily excreted by the kidneys; therefore, these levels will not affect most horses. An exception would be a horse with HYPP. HYPP is a hereditary disease in Quarter Horses that stems from the sire Impressive. This disease can cause severe muscle tremors and cramping. Excess potassium can make the condition worse. Iron, unlike potassium is not easily excreted. High levels of iron will impede absorption of other trace minerals such as copper and zinc (which are already low) creating secondary deficiencies. Your average horse needs around 400mg if iron per day. The average iron level of grass hay in VT and NH between 2016-2018 provided 1,893mg per day! That is almost 5x the amount your horse needs.
Next are the minerals that are deficient in the hay; magnesium, sodium, zinc and copper. Magnesium deficiencies can cause nervousness, muscle tremors, and ataxia (loss of muscle control). Horses with copper and zinc deficiencies may have poor hoof quality, prone to thrush or white line, dull coats that fade or a rusty colored mane and tails and compromised immune systems. Lastly; sodium, which could be argued to be the most important mineral in your horse’s diet. Salt is 40% sodium by weight. Most horses have access to a white salt block. To meet their daily sodium requirement, they would need to consume a two-pound salt block in one month. Adding salt directly to meals is a better way to guarantee your horses are getting what they need (1-2oz/day).
The bottom line is, hay alone will not provide your horses with all the nutrients they need. To balance the hay in the above graph, look for a mineral supplement or ration balancer that has magnesium, high levels of copper and zinc and little to no iron. Top it off with 1-2oz of salt.