Why do some horses get more hot than others when fed alfafa?

While this isn't directly related, here's an article which might give a little insight to diet and excitability, and what changes in diet can possibly help the 8yo. (or why changes in the diet may not help him).

http://ker.equinews.com/article/feeding-fat-calmer...

Why are they getting alfalfa? Not that it's a bad thing, but most horses just don't require it. It's a much richer hay, in vitamins, minerals, and calories, and is more or less unnecessary to the average horse in light to moderate work. It's low in starch and sugar, so horses who become hot on alfalfa aren't getting a "sugar rush" in a similar fashion to horses fed cereal grains. Protein also does not convert to energy as well as other nutrients, so protein levels are not to blame for their increased excitement. It's difficult for the body to convert protein into energy to use for work, as it's main purpose is to build and maintain structure within the body. Their excitability comes from excess nutrients (vitamins, minerals, calories) being converted to energy.

Young horses need more than mature horses, so chances are that your horse is using what he's getting in his feed to help him keep growing. He is a baby still, and his body will continue to grow and change throughout the next five or six years, though at a slower rate as he ages.

http://www.thehorse.com/articles/28965/proteins-ro...

Here is another article regarding protein and how it works. You may need to make an account, do so, as this site has a lot of very valuable information but it's completely free.

Alfalfa actually isn't hIgh in the soluble starches and sugars that are digested by enzymes in the fore gut and rapidly enter blood as glucose to produce hyperglycemia. This is what typically prompts massive release of insulin which transports the excessive glucose into target cells for storage or use as energy, and then stress hormones are released to stabilize blood glucose levels. These hormone fluctuations are associated with producing "hot" behavior.

However, although the soluble starch and sugar content in alfalfa is not high, the total digestible energy content (calories) in alfalfa is usually a good 20 to 30% higher than it is in grass hay, and for reasons that are not clear, some horses exhibit the same behavior changes when alfalfa hay is consumed as is commonly seen when high starch cereal grains are fed.

Hyperglycemia (high blood sugar) can result when the stress hormones stimulate breakdown of glycogen stores in the liver (glycogenolysis) or when amino acid precursors in the liver (gluconeogenesis) are used to produce glucose. It may be that in some horses, one of these mechanisms plays a role in the behavioral response seen when alfalfa is fed.

Some horses are just better at metabolizing glucose and volatile fatty acids from dietary carbohydrates consumed, and others have metabolic disturbances affecting the way they metabolize glucose or the way their muscles store glycogen so that their bodies react differently to the carbohydrates consumed in the diet. The endocrine system is responsible for the hormones secreted directly into the bloodstream, which gets very complicated and hormone responses can vary significantly among individuals as well.

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