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<td><strong>B-vitamins are a group of 8 water soluble vitamins that play important roles in cell metabolism. They cannot 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, are capable of synthesizing B-vitamins. Nutrient requirements have so far only been well established for thiamin (B<sub>1</sub>) and riboflavin (B<sub>2</sub>). The requirements included in PC-Horse are consequently to be regarded as guidelines.</strong></td>
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<p><strong>Function<br /></strong>Vitamin <span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span> performs a wide variety of functions in the body. It is a component of several enzyme systems, most of which are concerned with protein synthesis, carbohydrate and lipid metabolism and the synthesis of purine and pyrimidine bases that are crucial parts of DNA and RNA. Purines may also function as neurotransmitters.</p>
<p><strong>Dietary sources<br /></strong>Vitamine B12 encompasses several chemical forms that all have similar vitamin B<sub>12</sub> activity. The synthetic form of the vitamin used to supplement feeds or be given to animals is most often cyanocobalamin. The amount of vitamin&nbsp;<span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span> is low in most grains and also in brewers yeast. However, in most forages it is relatively high compared to the animal’s requirements. Average values for grass hays are about 10 µg per kg dry matter (or 0,01 mg/kg DM). Most of the vitamin B<sub>12</sub> in hays originates from bacterial contamination as plants cannot synthesize the vitamin.</p>
<p>Although Vitamin B<sub>12</sub> is not synthesized by plants, it is produced by the microbes in the intestines. To synthesize the vitamin, the microbes need cobalt. It is therefore necessary to ensure that the horse receives sufficient cobalt in its ration. The requirement for cobalt is set to 0,05 mg per kg dry matter in the ration for all categories of horses (NRC, 2007)</p>
<p>Compound feeds are often supplemented with 0.10-0.15 mg (100-150 µg) vitamin&nbsp;<span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span> per kg dry matter. However, not all compound feeds for horses are supplemented with the vitamin.</p>
<p><strong>Requirements<br /></strong>Based on available literature and requirements given for other domestic animals, PC-Horse provides the following guidelines for vitamin <span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span>in horse rations:</p>
<p><em>For all types of adult horses: 0.0002 mg (0.2 µg) per kg body weight. For young growing horses, the calculation is done according to expected adult body weight.</em></p>
<p><strong>Deficiency<br /></strong>Vitamin B<sub>12</sub> can be stored in the body, mainly in the liver. Deficiency symptoms have not been reported in horses.</p>
<p><strong>Toxicity<br /></strong>As vitamin&nbsp;<span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span> is a water-soluble vitamin, excess intake will be excreted in the urine and in the bile. Vitamin&nbsp;<span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span> toxicity in horses has never been reported.</p>
<p><strong>PC-Horse<br /></strong>PC-Horse calculates guidelines for Vitamin&nbsp;<span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span> requirement individually for each horse. Where values can be obtained,&nbsp;<span style="font-size: 14pt; line-height: 107%;"><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span></span> content is included in the feed tables. When composing rations using PC-Horse, you will always be able to control that the intake of vitamin&nbsp;<span style="font-size: 14pt; line-height: 107%;"><sub><span style="font-size: 10pt; line-height: 107%;">B<sub>12</sub></span> </sub></span>(and cobalt) is at least sufficient to comply with the proposed guidelines. However, as requirements for the vitamin do not exist, you will not be warned of any deficiency or excess.</p>
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<p style="margin-top: 0.1pt; margin-right: 0cm; margin-bottom: 0.1pt;"><em>This article was originally written by Dr. Day Austbø.</em><br /> <em>Copyright: PC-Horse International - Norway 2015</em><br /> <em>Feel free to use and publish the material. Please indicate the source and author.</em></p>