Too little Vitamin D
Vitamin D is a precursor for hormones which regulate calcium metabolism in the body. Vitamin D deficiency leads to abnormal development of the skeleton (rickets), poorly mineralized bones and reduced appetite and muscle strength.
Solutions: Vitamin D is produced in the skin when horses are exposed to sunlight. Sun-cured hay, cod liver oil and most concentrate and vitamin/mineral mixes are good sources of vitamin D.
Too much Vitamin D
Over feeding for prolonged periods vitamin D will lead to intoxication characterized by abnormal bone development (hypermineralization), calcium deposition in blood vessels and heart and kidney, and other organs damage.
Stiff gait, loss of appetite and a generally poor condition may be symptoms. Blood samples show abnormally high calcium levels in plasma. Some plants contain vitamin D analogs which may lead to vitamin D toxicity.
Solutions: Provide less vitamin D rich vitamin/mineral mixtures. Substitute commercial (vitamin-fortified) concentrates with grain. Be careful of giving more than one vitamin/mineral mix as many will contain vitamin D.
Do not give cod liver oil if the ration already contains a lot of vitamin D.