Thiamin is found in most feedstuffs, but in widely differing concentrations. The thiamin content of most common feeds should be three to four times greater than requirements for most species (Brent, 1985). For poultry consuming typical diets (e.g., corn-soybean meal), thiamin is one of the vitamins least likely to be deficient. Although thiamin levels supplied by feedstuffs in the ration are generally considered adequate for poultry, thiamin deficiency and inadequacy have been observed in poultry under commercial production conditions. Field experience and research showed that although the amounts of thiamin supplied by feedstuffs in practical corn-soybean rations met the minimum requirement for broilers, adding supplemental thiamin and (or) biotin to these rations improved growth rate and feed conversion (Wagstaff, 1978). Increasing supplemental thiamin to levels that exceed NRC requirements and current industry averages has been reported to decrease mortality in turkeys (Cook, 1992).
Drying and processing can lower the concentrations of available thiamin in feedstuffs because thiamin is heat labile. Utilization of available thiamin in feedstuffs may be limited and may also be impaired by thiamin antagonists; therefore it is common practice to add supplemental thiamin to poultry feeds to replace thiamin loss during processing and storage. Thiamin supplementation is greatly modified if diets contain anti-thiamin substances, such as thiaminases from fish or moldy feed. In addition, of the several water-soluble vitamins analyzed, two cultivars of corn were found to experience substantial declines only in thiamin (Kao and Robinson, 1972).
Anti-thiamin substances present in some feedstuffs and weeds, such as oxythiamin, should be considered. Non-nutrient substances intentionally added to diets are sometimes of concern, such as the coccidiostat amprolium, a thiamin antimetabolite. A mild thiamin deficiency from amprolium added to a standard commercial hen feed caused a reduction in the feed intake and in egg laying performance and an increase in the mortality of embryos and chicks. These phenomena could be prevented or effectively counteracted by high thiamin doses in the feed. At recommended levels, apparently, amprolium does not interfere with the thiamin metabolism of the chicken (Scott et al., 1982). Other thiamin antagonists, such as the free bisulfite, may be present in feeds and reduce free thiamin activity. It has been reported that use of high-moisture barley treated with sulfur dioxide resulted in destruction of 61% of dietary thiamin (Gibson et al., 1987). Treatment of feed ingredients with sulfur dioxide inactivates thiamin.
Thiamin sources available for addition to feed are the hydrochloride and mononitrate forms. Because of its lower solubility in water, the mononitrate is preferred for addition to dry premixes. The mononitrate form has somewhat better stability characteristics in dry products than the hydrochloride (Bauernfeind, 1969).
Stability of thiamin in feed premixes can be a problem. More than 50% of the thiamin was destroyed in premixes after one month at room temperature (Verbeeck, 1975). When thiamin was in premixes without minerals, no losses were encountered when kept at room temperature for six months. However, vitamin premixes that contain choline and trace minerals will experience problems with instability.
A discussion has arisen on removal of vitamin and trace mineral supplementation from poultry and other species' diets some time prior to slaughter. Skinner et al. (1992) reported that removal of vitamins and trace minerals from broiler diets did not impact performance. In contrast, Teeter and Deyhim (1993) detected reduced performance and carcass variables when the same period was examined. Deyhim et al. (1996) withdrew vitamin and trace minerals for 21 days in broiler diets during heat stress and found 23% less thiamin in the Pectoralis major muscles. Such effects have the potential to affect consumer perception of poultry meat as wholesome and should be considered when vitamin withdrawal is being contemplated.