Riboflavin requirements vary with heredity, growth, environment, age, activity, health, other dietary components and synthesis by the host. Hughes (1940) proposed that the daily minimum requirement of riboflavin for young growing pigs ranged between 1 and 3 mg per 100 lbs of pig. Krider et al. (1949) stated that 1.4 mg of riboflavin per pound of ration seemed to represent the practical minimum requirement for weanling pigs fed in drylot. Mitchell et al. (1950) found the riboflavin requirement of growing pigs to be approximately 1.2 and 2.3 ppm when pigs were fed at environmental temperatures of 85°F and 42°F, respectively. These values were reported to be equivalent to 1.4 and 4.2 mg riboflavin per 100 lbs of body weight, respectively. Forbes and Haines (1952) determined that baby pigs fed a "synthetic" milk diet and kept at an environmental temperature of 85°C with a relative humidity of 70% had a riboflavin requirement of between 1.5 and 2.0 mg per gram of dry matter. Miller et al. (1954) reported that for optimum growth and feed efficiency the riboflavin requirement for the baby pig is approximately 3.0 mg per kg of solids. Terrill et al. (1955) indicated that for the growing pig kept at an environmental temperature of approximately 53°F, the riboflavin requirement is between 0.4 and 0.65 mg per pound of diet. Seymour et al. (1968) did not observe consistent interactions between riboflavin and temperatures ranging from -4° to 32°C and indicated that between 3.0 and 4.0 mg of riboflavin per kg of diet was required for baby pigs.
Swine have a riboflavin requirement between 2 and 4 mg per kg (0.91 and 1.8 mg per lb) of diet (NRC, 1998). The NRC (1998) requirement declines as the pig grows, from 4 mg per kg (1.8 mg per lb) of feed for pigs 1 to 5 kg (2.2 to 11 lbs) in body weight to 2 mg per kg (0.91 mg per lb) for growing-finishing hogs weighing 50 to 100 kg (110 to 220 lbs).
Based upon sow farrowing performance and erythrocyte glutathione reductase (an indicator of riboflavin status), Frank et al. (1984) estimated the available riboflavin requirement for pregnancy to be about 6.5 mg daily. Using the same criteria, the suggested lactational requirement was about 16 mg daily (Frank et al., 1985; 1988). Frank et al. (1988) suggested that first-litter gilts have a higher requirement for riboflavin than the second-litter sow based on needs for both maternal growth and reproduction. With regard to erythrocyte glutathione reductase, Pettigrew et al. (1996) confirmed that its activity coefficient (EGRAC) is a sensitive biochemical indicator of riboflavin deficiency. After activity of erthrocyte glutathione reductase is measured with and without flavin adenine dinucleotide in the assay medium, the ratio of the activities is expressed as EGRAC. A high coefficient is indicative of a riboflavin deficiency. As expected, sows fed a low-riboflavin diet had higher EGRAC values than those consuming diets supplemented with higher levels of riboflavin. However, these authors determined that EGRAC values cannot be used to distinguish the period when sows are most likely to be deficient in riboflavin. In their experiment, there were only minor fluctuations in EGRAC values based on the stage of the reproductive cycle.
Early work (Warkany and Schraffenberger, 1944) indicated that riboflavin-deficient pregnant female rats produced offspring with congenital malformations, especially defects associated with skeletal development as embryonic tissue differentiation was abnormal. Subsequent studies (Miller et al., 1962) noted that female rats that were riboflavin deficient had embryos with depressed total riboflavin and FAD as well as abnormal skeletal development. Additionally, these authors observed that a 60% reduction in total riboflavin and FAD in the rat embryos resulted in sterility and embryonic and fetal death.
Murray et al. (1980) and Moffatt et al. (1980) reported that free riboflavin increases markedly in uterine secretions of pigs between days six and nine of both the estrus cycle and pregnancy; during this period, uterine flushing appears to have a distinct yellow color. These concentrations of riboflavin are much greater than those found in blood or colostrum of sows. The significance of these increased concentrations of riboflavin in uterine secretions of sows is unknown; however, they occur when blastocyst development is occurring, a stage critical to embryonic development and survival.
Bazer and Zavy (1988) reported that 100 mg supplemental riboflavin per day provided on days four to 10 after the onset of estrus resulted in higher litter size, embryonic survival and allantoic fluid volume at day 30 of gestation in gilts and increased rates of conception and more live pigs at birth, day 21 and day 42 of lactation in primiparous sows. This is in contrast to Luce et al. (1990) who reported no benefit of 100 mg per day riboflavin supplementation provided on days four to 10 postbreeding in sows consuming 1.8 kg (4.0 lbs) of feed supplemented with 6.6 mg riboflavin per kg (3.0 mg per lb). Pettigrew et al. (1996) fed 10 (control), 60, 110 or 160 mg of riboflavin per day from breeding through 21 days after breeding. Although the riboflavin supplementation tended to increase the percentage of sows farrowing (66.7%, 85.7%, 93.3% and 86.7% for 10, 60, 110 and 160 mg riboflavin per day, respectively), no effect on litter size was detected.
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. Lutz and Stahly (1998) evaluated the dietary riboflavin requirement for protein versus fat accretion in pigs. These authors indicated that protein accretion but not fat accretion substantially increased the required dietary riboflavin levels. It was calculated that riboflavin needs for body protein accretion are six times higher than the requirement for body fat accretion. In addition, for high-lean, high-health status pigs, the requirement for riboflavin was found to be greater than the 1998 NRC estimate. Lutz and Stahly (1998) reported that as dietary riboflavin concentration increased (0, 3.7 or 7.4 mg per kg), efficiency of feed utilization and protein accretion increased linearly in both moderate- and high-lean gain strains of pigs.
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. Antibiotics, such as tetracycline, penicillin and streptomycin, reduce the requirements of several animal species for riboflavin via stimulation of microorganisms that synthesize riboflavin or inhibition of microorganisms in the gut that compete for riboflavin.
