Riboflavin requirements vary with heredity, growth, environment, age, activity, health, other dietary components and synthesis by host. Poultry species have a requirement between 1.8 and 4 mg per kg (0.45 and 1.8 mg per lb) of diet (NRC 1994). The NRC (1994) requirement for broiler chicks is reported as 3.6 mg per kg (1.6 mg per lb) of feed. However, Ruiz and Harms (1988) suggest that to prevent signs of leg paralysis in broilers fed a corn-soybean diet, the minimum requirement of 4.6 mg riboflavin per kg (2.0 mg per lb) of feed is needed. Olkowski and Classen (1998) likewise suggest a higher than NRC requirement for broilers at a level of 5 mg per kg (2.27 mg per lb). They suggest that the current recommended allowance of 3.6 mg per kg (1.6 mg per lb) is not sufficient for modern breeds of broiler chickens. Ruiz and Harms (1989) also reported that 3.5 mg per kg of added riboflavin was needed to optimize the growth of turkey poults fed a corn-soybean meal diet. However, it took more added riboflavin to eliminate signs of deficiency—poor feathering and paralysis of one or both legs.
Where sufficient data are available, studies indicate that riboflavin requirements decline with animal maturity and increase for reproductive activity. Chicks receiving diets only partially deficient in riboflavin may recover spontaneously, indicating that the requirement rapidly decreased with age (Scott et al., 1982). However, Deyhim et al. (1992a) reported that 3.6 ppm of dietary riboflavin for growing broilers was satisfactory through four weeks, but that benefits were obtained by exceeding the 3.6 ppm recommendation through eight weeks.
Increased dietary fat or protein increases requirements for riboflavin in rats and chickens. It was assumed that high urinary riboflavin excretion during periods of negative nitrogen balance for a number of species was a reflection of impaired riboflavin utilization or retention. However, Turkki and Holtzapple (1982) suggested, in studies with rats, that the effect of protein on riboflavin requirement is related to rate of growth and not to protein intake per se.
Microbial synthesis of riboflavin has been shown to occur in the gastrointestinal tract of a number of animal species and thus affects requirements. However, utilization of this endogenously synthesized riboflavin varies from species to species. Within a single species, utilization depends on diet composition and incidence of coprophagy. Carbohydrates such as starch, cellulose, or lactose are absorbed slowly and therefore exposed for longer times to the intestinal bacteria, resulting in an increased riboflavin synthesis. Dextrose, fat or protein as chief dietary constituents decreases intestinal production, thereby increasing dietary riboflavin requirements. Antibiotics, such as tetracycline, penicillin, and streptomycin, reduce the requirements of several animal species for riboflavin or might stimulate microorganisms that synthesize riboflavin. They may inhibit microorganisms in the gut that compete for riboflavin. Alkaline dietary pH appears to increase the level of riboflavin required in chick diets (Donaldson, 1986).
Temperature extremes apparently have an effect on riboflavin requirement. According to studies with pigs, riboflavin requirement is substantially higher at low than at high environmental temperatures (Seymour et al., 1968). At environmental temperatures below 11ƒC (52ƒF), feed required per unit of gain increased, and rate of gain decreased with decreasing temperature in pigs fed minimum levels of riboflavin. On the contrary, Onwudike and Adegbola (1984) report riboflavin requirements to be higher for chickens in a tropical environment. A dietary level of 4.1 mg per kg (1.9 mg per lb) riboflavin was adequate for egg laying with 5.7 mg per kg (2.6 mg per lb) for hatchability, compared to 2.1 and 3.6 mg per kg (1.0 and 1.7 mg per lb), respectively, for the NRC (1994) requirement. When exposed to chronic heat stress, the riboflavin requirement of broilers was in excess of 7 mg per kg (3.2 mg per lb) to prevent deficiency symptoms (Whitehead, 2000). Other factors which can influence riboflavin requirements have been reviewed (Ward, 1993). When requirement comparisons between animals residing at different environmental temperatures are calculated, total feed intakes must be considered.
