Vitamins were discovered at the beginning of the twentieth century and birds have served as an experimental model for the discovery and characterisation of some of them. They are organic substances which are not chemically related like other groups of nutrients (carbohydrates, proteins or lipids) and they are active at low levels, which is to say that they are required in trace amounts (mg or microg). They are vital to the normal working of the body and therefore are required for physiological functions and to maintain an optimum level of health.

Traditionally, studies carried out on vitamins have been principally directed towards establishing the minimum amounts needed to avoid symptoms of deficiency. However, in the last few years there has been more interest in broadening our knowledge, given the important metabolic functions in which this group of nutrients is involved. Vitamin requirements of birds have been established in experimental conditions that are different from practical and real conditions. This is particularly evident in the case of layers since data are, in general, limited, and arise from old studies which do not reflect the type of animals, rations and production systems used today.

It must be pointed out that commercial egg-laying breeds have changed a great deal in a few years. The bird has changed in size, food consumption has decreased and, above all, egg production has improved in quantity and size. Logically, these changes in themselves should call for increased nutritional requirements in general, and in particular, increased vitamin requirements. In these hens with high productive yields, the vitamins involved in energy and protein metabolism as well as the immune system are doubly essential. Furthermore, the reduced ingestion of these modern strains means it is essential to concentrate the nutrients and increase their content per kilo of feed. In spite of the technical nature of poultry installations today, practical operating conditions are usually poorer than those in experiments, implying the need for greater vitamin allowances to obtain the same results.

Stressful conditions and pathological processes should be borne in mind, as these can provoke reduced absorption efficiency through the intestinal wall, greater metabolic rhythm or reduction in microbial synthesis of vitamins. One of the most important problems is the high temperature. During the summer months, hens are subjected to heat stress which leads to a reduction in productivity, egg quality and capacity for immune response.

A new concept of optimum vitamin nutrition is essential today. Its object is to develop a new standard for vitamin supplementation in the diet of animals, aimed not only at preventing the initial phases of some diseases, but also improving an animal's productive growth rate, given a healthier environment. That will permit an improvement in the animal's state of health and well-being, thus optimising its productive potential at the same time as permitting the production of high-quality, nutritionally balanced foods.

This paper will review more in depth some benefits of different levels of vitamin B2 for layers, what the industry is using as well as some recommendations given by experts in poultry breeding and vitamin nutrition .

Riboflavin is named for its yellow colour (flavin) and because it contains a simple sugar, D-ribose. It forms an essential part of the enzyme system which carries out the tricarboxylic acid cycle, a group of reactions necessary to obtain energy from nutrients. Riboflavin in the form of a coenzyme is involved in oxidation-reduction reactions, contributing to the maintenance of cellular metabolism and more specifically of carbohydrate, protein and lipid metabolism.

Water-soluble vitamins (C and B group) are, in general, absorbed passively through the mucous of the small intestine, and are transported in tissues either freely or bound to protein complexes. Apart from vitamin B12 and choline, they do not accumulate in the body in significant quantities so, to avoid deficiencies, a daily amount in food is necessary. They are excreted in urine and therefore it is difficult to reach levels of toxicity. Although the specific functions are different, symptoms of deficiency are similar. Deficiency has an effect on rapid growth tissues, such as bone, skin, feathers and blood.

The sources of riboflavin are very varied, and it could be said that all biological material contains riboflavin. Among the main sources of this vitamin are milk, offal, wholegrain cereals and various vegetables. . Corn-soya diets usually contain 2-2.6 mg/kg of riboflavin, 60% of which is bioavailable (Chung and Baker, 1990). But the increase in granulation temperatures and the use of expanders to control contamination by Salmonella has increased its susceptibility to degradation (Ibrahim, 1998). Furthermore, microbial synthesis of riboflavin takes place in the large intestine. Maintaining the high metabolic demand necessary for optimum egg production depends on the presence of sufficient riboflavin. Riboflavin provides the energy necessary for the maintenance of various physiological functions, including reproductive functions.

Levels recommended by the NRC in 1994 have increased by 13.7% in comparison with levels recommended in 1984, the levels being 2.5 mg/kg and 2.2 mg/kg, respectively. In a study carried out on breeding hens in conditions of a humid tropical environment, supplements from 2.5 to 12.5 mg/kg were administered. Of the various productive parameters analysed, the only one in which there was a response was egg production, which increased significantly when the riboflavin level was increased to 8.5 mg/kg (Arijeniwa et al ., 1996). Kirichenko (1991) agreed that egg production may increase when riboflavin is supplemented at a level 25% higher than that recommended, while in a study carried out by Flores and Scholtyssek (1992), where it was supplemented at levels from 1.7 to 9.7 mg/kg, no significant changes in any production parameter were noted. Squires and Naber (1993) obtained increased production, egg weight and body weight in breeding hens when increasing NRC recommendations from 1984 by 2 and 4 times, but no significant differences between these levels of supplementation were observed. Squires and Naber (1993b) studied changes produced in the riboflavin content of the egg when supplementing breeding hens with 1.55, 2.20, 4.4 and 8.8 mg/kg for 27 weeks. In the first week of treatment significant differences were already being observed between the two lower levels of supplementation and the two higher ones. This reduction in egg riboflavin content related to deficient levels of supplementation becomes even more important as the hens get older. In another study Naber and Squires (1993) found that when multiplying the requirements by one or two, transfer efficiency of riboflavin from the diet to the egg was 45%, while when four times the requirements was given, transfer efficiency decreased markedly. In the previously mentioned study by Squires and Naber (1993b), it could also be seen that the incidence of blood spots was lower in the eggs of hens which had received supplements of 4.4 and 8.8 mg/kg compared with lower levels. On the other hand these levels of supplementation apparently impaired the shell thickness for several weeks although this unexpected effect in shell quality is probably linked more to the increase in egg production and weight, than to the riboflavin level per se. Furthermore, it has been shown that riboflavin deficiency reduces the tolerance of the hens in dealing with heat stress.

Onwudike and Adegbola (1984) have documented that in situations of heat stress riboflavin requirements for layers increase. It would also be advisable to increase the dosage of riboflavin in the ration in cases of immunological stress (vaccinations, infections), because of its involvement in antibody synthesis. From these studies we may conclude that 2.5 mg/kg riboflavin in layer rations may be limiting. It has been seen that supplementation with 4.4 and 8.8 mg/kg may be beneficial for certain production parameters and egg quality.

González (1987) points out that the range of riboflavin used commercially in Spain in layer feed is from 2 to 6.5 mg/kg, and Villamide and Fraga (1999) speak of an average level of supplementation in Spain of 3.94 mg/kg (35%CV).