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August 2015 - Riboflavin (Vitamin B2)

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B-vitamins are a group of 8 water soluble vitamins that play important roles in cell metabolism. They can not be produced by the body itself, but can be produced by bacteria, fungi and plants. In the horse, microorganisms in the gut (mainly the hind gut) is capable of synthesizing B-vitamins and nutrient requirements have only been established for thiamin (B1) and riboflavin (B2). In this article we will look at the function, dietary sources and requirements for riboflavin (vitamin B2).


Riboflavin is a precursor to the coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). Both are involved in oxidation-reduction reactions used in energy metabolism (ATP synthesis), lipid metabolism and antioxidant defense mecanisms.

Dietary sources
Riboflavin is found in relatively high concentrations in legumes like lucerne and clover and slightly less in grass hays (see table).

Legumes 13-17 mg/kg DM
Grass hays 7-10 mg/kg DM
Cereal grains 1.4-1.7 mg/kg DM

These concentrations of riboflavin in feeds will be reduced to about 70% during 1 year storage.

Riboflavin is included in most concentrate mixtures for horses (5 – 15 mg/kg) and in vitamin supplements.

Riboflavin is absorbed with decreasing efficiency with increasing amounts ingested. Excess is rapidly excreted in the urine, and there is little storage in the body.

The NRC 2007 gives requirements for riboflavin for horses (see table).

Maintenance 0.04 mg/kg body weight
Light exercise 0.04 mg/kg body weight
Moderate exercise 0.04 mg/kg body weight
Heavy exercise 0.05 mg/kg body weight
Very heavy exercise 0.05 mg/kg body weight
Pregnant mares 0.04 mg/kg body weight
Lactating mares 0.05 mg/kg body weight
Growing horses 0.05 mg/kg body weight

Riboflavin is produced by microorganisms in the intestine of the horse and deficiency has not been described.

As riboflavin is a water-soluble vitamin, excess intake will be excreted in the urine. Riboflavin toxicity in horses has never been reported.

PC-Horse calculates requirements for riboflavin individually for each horse according to the NRC recommendations. It has also included values for riboflavin (vitamin B2 ) in all feed tables. When composing rations for your horse using PC-Horse, you will always be able to control that your horse’s riboflavin intake is in accordance with its requirements.


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This article was originally written by Dr. Day Austbø.
Copyright: PC-Horse International - Norway 2015
Feel free to use and publish the material. Please indicate the source and author.