Feeding for Breeding -
The Nutrition of the Avian Embryo
B. C. Stockdale, BVM&S, MRCVS, AAV
The success of the avian reproductive cycle from the induction of follicle formation through ovulation, fertilization, egg production, and the subsequent rearing of the young, is dependent on adequate and appropriate nutrition. Because all the nutrients required for embryonic development and survival must be placed within the egg prior to shell formation, a heavy onus is placed on the nutritional status of the female bird.
Avian embryonic development takes place within the semi-closed environment of the egg, where only exchanges of gas and water take place. The egg composition is designed in such a way that all nutrients necessary for the development of the future embryo are accumulated within the egg yolk, white, and shell. Poor maternal state at the start of breeding affects the female bird's ability to produce and incubate eggs, and recycle. It also influences their willingness to provide parental care and ability to rear chicks up to fledging. Poor egg quality affects chick hatch size, vigour, early feeding behaviour, and the immune status of the chick. The future fecundity of female nestlings has also been linked to the nutrition received both from the egg and immediately post-hatch. Maternal nutrition can therefore be considered as the major determinant in the development and health of the avian embryo and viability in early post hatch life.
During reproduction, the nutritional needs of laying birds include the nutrients required for general maintenance, a rapid increase in tissue mass of the female reproductive tract, and those required for egg synthesis. The additional nutritional needs may be met either directly from the diet; from nutrients stored in anticipation of egg synthesis; from nutrient reserves, which may include stores or any tissue component that can be drawn upon to reallocate endogenous nutrients when daily intake falls below needs; or any combination of the above.
Among different nutrients that can significantly affect chick embryo development and subsequent viability, proteins, lipids, and antioxidants are specifically indicated and the following sections highlight their respective roles in the production of the 'optimal egg'.
Protein as a limiting factor
Egg production is a protein-demanding process. Birds laying eggs require protein for maintenance, development of the oviduct and accretion of egg proteins. The growth of the oviduct and the synthesis of several yolks are mostly complete before the first egg is laid. Consequently, the female’s requirements increase at least a week prior to her first oviposition. In most species, egg albumen is synthesised in the oviduct during a 24h period before ovulation, thus dietary amino acid requirements are especially high on the day preceding each oviposition. The quality of these proteins is reflected by the quality of the daily dietary protein level; eestimates of the additional protein requirements of several avian groups range from 86-232% of their minimum maintenance requirements. These estimates assume that the dietary protein is of satisfactory quality, i.e. that the pattern of the amino acids in the dietary protein is supplied more or less, in proportion to the birds’ needs. The requirements for specific essential amino acids for egg production, however, are often far in excess of those required for maintenance. Lysine, methionine, and cysteine are commonly cited as limiting amino acids, with, for example, levels of lysine 4.1 times greater than maintenance being required for egg production in the budgerigar.
Where dietary protein levels fall below the required levels for reproductive performance, body plundering occurs and use is made of endogenous reserves (not to be confused with stores). A significant loss of muscle weight at the time of egg formation is documented in 21 of 29 species studied, suggesting that the use of tissue protein to assist with egg production may be widespread in birds. Overnight, all birds rely at least partially on amino acids derived from tissue proteins to synthesize egg proteins, even if they restore tissue proteins the next day. During egg laying, protein removal from the sarcoplasm occurs over a range of different proteins. All tissues need to be considered as potential protein sources, but the pectoral muscle mass provides the main supply. This is a costly process for the female bird as the balance of amino acids in skeletal muscle is sufficiently different from that in egg protein. The individual amino acids are released more or less in proportion to their occurrence in proteins breaking down, not in proportion to the needs for synthesis. Eggs are enriched with methionine and cysteine relative to skeletal muscle and so greater volumes of endogenous proteins must be used to provide for those proteins in the egg. This can have a serious knock-on effect on the overall health of the female, on the levels of amino acids she is able to fund for the next clutch of eggs and her ability to both incubate and rear her nestlings.
Lipids and polyunsaturated fatty acids (PUFAs)
The production of a clutch of eggs requires the deposition of large amounts of yolk lipids, mostly during the several days prior to ovulation. Yolk lipids and proteins are synthesised in the liver under the influence of oestrogen and progesterone and are transferred through the blood to the ovarian follicles. Lipids in the yolk are of two main types: specific yolk-targeted very low-density lipoproteins (VLDLy) and vitellogenins (VTN). Lipids comprise about 10% of the total weight of an egg and 33% of the yolk weight. Almost all the lipids in the yolk are present as lipoprotein complexes with an overall lipid: protein ratio of about 2:1. Deposition of egg yolk is completed 24h before its ovulation.
About 90% of the energy required for the developing chick is supplied from egg yolk by fatty acid oxidation. Approximately 80% of the entire lipid content of the yolk is mobilised and absorbed into the embryonic tissues over the last few days of development and the remainder, in the form of residual yolk, is sufficient for the adequate maintenance of the chick for several days post hatch. During embryonic development, lipids, as well as providing an energy supply, play a crucial role in providing the developing embryo with a source of important constituents for the production of biological membranes and biologically active substances.
Dietary requirements of essential fatty acids
The fatty acid composition of the maternal diet and subsequent inappropriate fatty acid provision within the yolk, has been shown to affect embryonic development, viability, and hatchability, and is recognised as a cause of increased chick mortality.
The basic building block fatty acids that are deemed to be essential dietary requirements for birds are linoleic (LA) and -linolenic (LNA) acids. Natural foods contain differing proportions of these fatty acids. Green leafy vegetables, linseed and rape oils are good sources of LNA and grains and plant oils such as corn, and sunflower are rich sources of LA. As well as having sufficient levels of essential fatty acids in the maternal diet, it is important that they are supplied in approximately equal proportions. LNA is the parent for n-3 fatty acids and LA the parent for the n-6 series of fatty acids and they are not inter-convertible. Both of these groups compete for the same modifying enzyme systems and a preponderance of one affects the levels of the other.
Antioxidant systems and avian embryo development
The animal body is under constant attack from chemicals called 'free radicals' formed as a natural consequence of the body’s normal metabolic activity within its tissues and antioxidants provide protection against their damaging effects. The most important effect of free radicals on the cellular metabolism is in their participation in lipid peroxidation, (the process of turning fat rancid). Prime targets for peroxidation are lipids known as polyunsaturated fatty acids (PUFA). PUFAs are integral in the structure of cellular membranes, and in the preservation of their integrity and function. Failure to prevent lipid peroxidation within the egg will potentially result in excessive tissue damage and embryonic death. At hatching, the embryonic liver contains high levels of PUFAs derived from the yolk that require a considerable degree of antioxidant defence against peroxidation. The antioxidant system of the newly hatched chick includes the fat-soluble antioxidants, vitamin E and carotenoids; the water soluble antioxidants ascorbic acid (vitamin C) and glutathione; as well as antioxidant enzymes. The main fat-soluble antioxidants, vitamin E and carotenoids cannot be synthesised de novo by animals, so must be derived from the diet.
Vitamin E is a generic term which encompasses eight similar chemicals. It is considered to play a central role in antioxidant production during embryogenesis and low levels have been associated with decreased hatchability and slow embryonic development. Vitamin E is transported from the maternal diet to the hen bird’s liver and further to the developing oocyte as a component of VLDLy and is subsequently deposited within the lipid rich fraction of the yolk. In the chicken it has been shown that producing each egg requires the release of twice as much vitamin E as is stored in the liver indicating that the liver is not a good reservoir of vitamin E in the laying hen. Exclusion of vitamin E from the diet can therefore cause a rapid decrease in vitamin E concentration in the egg yolk.
The main site of vitamin E accumulation in the avian embryo is the liver. Yolk lipid droplets containing vitamin E are taken up by the yolk sac membrane and processed into lipoprotein particles that are released into the embryo circulation.. Vitamin E is particularly important at hatching, when the onset of pulmonary respiration results in increased oxygen tension and increased energy expenditure and free radical production. This can result in high levels of 'pipping death'.
Carotenoids play a specific role in avian embryonic development and there are some indications that carotenoids are extremely important elements in maintaining the chick immune system. They also function as important visual indicators of offspring health. The bright gape flanges of nestlings act as a temporal stimulus eliciting parental care and feeding. The intensity of the colour is thought to be a true reflection of the quality of the nestling's health - pale gapes being a consequence of insufficient provision or diversion of carotenoids to combat disease.
The most common carotenoids in seeds and plants are lutein, zeaxanthin and beta-carotene, which occur in varying levels. Plasma levels vary significantly depending on dietary provision and assimilation. During the rapid growth of the ova, carotenoids are deposited in a dose dependant manner, as opposed to a set level deposition, and the levels in eggs respond very quickly to dietary level changes. The main storage sites within the body are the liver and, surprisingly, the toe web. The gradual loss of pigment from the shank and beak of certain birds with egg production, can be explained by the mobilization of carotenoid stores for incorporation into the yolk.
The production of eggs is a nutritionally demanding process for many birds, particularly small passerines, some of which lay a clutch of eggs weighing more than the female's own body weight. As reproduction is a life-compromising activity there is inevitably a conflict between female and egg. Conservative diets aimed at providing maintenance levels of nutrients, with no additional provision for the nutritional demands of egg laying, may be contributing to reduced reproductive potential in captive birds. Reviews of captive bird diets focus principally on maintenance levels of nutrients and generally fail to address the issue of additional reproductive nutritional needs. Feeding levels for essential nutrients (as far as these are known) are usually linked to energy based formulae. These tend to be misleading, as energy expenditure and thus demand, in captive birds is much lower than wild birds. Protein levels geared to energy requirements of wild birds will grossly under-fund captive birds protein needs. Reproduction puts an even greater strain on a limited amino acid pool. Japanese quail (Coturnix coturnix japonica), for example, require a maintenance level of protein around 5.5%. This increases to over 23% at breeding without a substantial increase in energy requirements.
Whilst the proximate factor in initiating breeding in birds from temperate regions is photoperiodic change, for opportunistic breeders from neotropical regions (where a large number of our avicultural species originate), food availability is a much stronger cue. The protein component of ripe cereal seeds has long been recognized as having the potential to be nutritionally limiting for birds, the results of a poor match between the amino acid balance of seed and the requirements for synthesis and renewal of tissue proteins. Biological mechanisms exist within wild birds to time breeding to coincide with the production of ripening seeds. These have a much higher essential amino acid percentage than dried seeds. When protein is the limiting factor in cereal-based diets, the deficiency is in quality rather than quantity. Nevertheless, dry seeds still make up the main ingredients of many captive bird diets.
The provision of a range of foods whilst offering a wider amino acid profile may not necessarily correct the problem. Foraging birds can find foods containing protein of various quality to satisfy their amino acid needs by either choosing only those foods that contain a suitable array of essential amino acids – protein shift, (a move in breeding granivorous birds to also consume high protein legume seeds and invertebrates), or by choosing foods in amounts that permit complementation of constituent amino acids. Small birds appear, however, to have limited ability to actively exploit dietary amino acid complementation. Their dietary habits are fixed more by their mechanical ability both to husk and consume a seed type in the most expedient time, regardless of content. With psittacine birds familiarity of their food stuffs tends to drive their eating habits. Whilst mobilization of body tissues occurs in many species to aid with the amino acid needs of reproduction, the relative costs are high. Whether by preferential selection or chance consumption, wild birds are able, to a greater extent, balance their amino acid intake. Captive birds are limited by the very nature of their captive diets, in their ability to manipulate their protein intake. More onus is placed, therefore, on endogenous reserves. Supplementation of diets with foods high in limiting amino acids or the use of formulated diets may achieve a solution.
Adequate provision of antioxidants is essential for successful embryonic development and hatching. To date there is little information available relating to the vitamin E content of the egg yolk of different avian species. Species-specific differences in the metabolism of vitamin E may influence yolk levels but perhaps more importantly for captive birds, the level of vitamin E in the diet will have a direct influence. Independent of later nutrition, individuals experiencing a short period of low quality nutrition during the nestling period have a two-fold reduction of these antioxidants at adulthood. The deposition of these antioxidants declines with laying sequence, suggesting that the decline in provisioning may be a consequence of resource limitation. It is suggested that low-quality neonatal nutrition results in a long-term impairment in the capacity to assimilate dietary antioxidants, resulting in a reduction of both reproductive and actual longevity.
As vitamin E is poorly stored, sufficient carry over from diet to egg on a daily basis must be made in breeding birds. Listed maintenance levels for vitamin E need to be increased, therefore, to accommodate the provisioning of the egg. With many avicultural species, especially psittacine birds, vitamin A levels may also be marginal. Low dietary vitamin A can have a direct adverse effect on reproduction and a knock-on effect on the levels of carotenoids availability. Artificially high dietary vitamin A levels can equally have an adverse effect on vitamin E levels. A lot of the
attention has been focused on vitamin A levels in avian diets but little on the levels of carotenoids.
As more evidence accumulates showing that the maternal diet can have a profound effect on the health status, growth and development of the avian embryo and newly hatched chick, perhaps it is time that we reassessed the role of nutrition in reproductive management of captive birds. Failure to breed, dead in shell, failure to incubate, nestling mortality, and nestling desertion may all have a greater nutritional basis than currently appreciated. Diets for breeding birds need to better reflect their reproductive needs. Food quantity should never be an issue in captive bird management, whereas food quality, expressed in terms of available nutrients, can be.