Vitamin B1 (Thiamine) is under normal healthy conditions produced in sufficient quantities by the large intestine of the horse. When stressed or when workloads are high B vitamin production can be reduced and supplementation is recommended. The ratio of grains to forage also influences B vitamin production with high grain and low forage diets negatively impacting on production. Always feed adequate forage.
Concentrates and most vitamin/mineral mixtures for horses are supplemented with vitamin B1. Bakers yeast and brewers grain are good sources of B1.