Riboflavin (Vitamin B₂) is one of those very important B vitamins.
It is a precursor to two coenzymes. Coenzymes are molecules that carry chemical compounds between two enzymes -- very important to transport many substances in the body. Being a precursor simply means that the equine body uses B₂ to make the coenzymes.
Anyways, back to the coenzymes. The two coenzymes that B₂ is a precursor of are:
Both FAD and FMN are part of reduction-oxidation reactions. Reduction-oxidation reactions (also known as redox reactions) are simply reactions that take place to exchange electrons between molecules...
...like when carbon is oxidized to make carbon dioxide.
Redox reactions are essential for most life processes, including digestion of feed. The particular redox reactions that FAD and FMN are involved in are:
There's a few more reactions they play a part in, but these listed are the most important ones.
Legumes (alfalfa and clover) are the best sources of B₂ in the equine diet. They contain around 15 mg/kg of dry matter.
Grass hays come in next, with approximately 7-10 mg/kg of dry matter.
The cereal grains contain the lowest concentrations in the equine diet.
It also appears that the microbes in the large intestine can produce B₂ for the horse's use, as horses that were fed riboflavin deficient diets had increased B₂ concentrations in the cecum and colon.
When B₂ is found naturally in the diet, it is often in the form of those coenzymes, FAD and FMN.
Thankfully for us horse owners, B₂ deficiency has never been reported in the horse.
However, it has been reported in other species, and symptoms include:
B₂ toxicity appears to be of little concern in the horse. Toxicity by consuming B₂ has never been reported and appears to be of little concern.
In rats, B₂ had to be supplemented at 10 mg/kg body weight orally before adverse reactions were seen. Likewise, injections in the abdominal cavity or under the skin required doses of 0.56 mg/kg body weight and 5 mg/kg body weight respectively to produce a reaction.
So, unless injections of almost 300 mg or oral ingestion of 5000 mg occurs, toxicity is probably not a worry.
Despite its importance in the horse's every day life, riboflavin intake is of little concern for the average horse owner. Because of the concentrations found in various forages, the horse easily meets the daily requirement of 2 mg/kg of air dried feed (ie. hay).
Because toxicity and deficiency are of little concern in the horse, owners have even less reason to worry about this very important B-vitamin.