Forecasting is fraught with difficulties associated with poor data and unknown unknowns. Fortunately, in the case of laying hens, this is not true. We have an excellent history of how our birds have improved over the past decade (Table 1), and a fair idea of what can be expected in future breeding programmes used by the primary breeding companies. Our understanding is that the requirements for energy and protein per unit of output have remained unchanged and that different genotypes utilise dietary components in a similar manner (Morris and Njuru, 1990; Lopez and Leeson, 2005; Kimiaeitala et al., 2017). The real challenge will be how to feed birds that are widely called ‘long-life’ layers. These hens are expected to lay 500 eggs by 100 weeks of age and are likely to be on farms by 2020 (de Keijze, 2014). Indeed, some reports of this target being achieved in practice are already filtering through. These birds will be in production before we can take the opportunity to determine their peculiarities and specific nutrient shortfalls. Trials carried out in the past may no longer be applicable as the genotypes used are not representative of the modern bird. This paper will deal briefly with the matter of genetic change and how this will alter the nutrient requirements of hens. More importantly, some speculation about how we should feed the long-life layer will be raised.
II. GENOTYPE AND NUTRIENT REQUIREMENT
Continuous selection for improved egg production is the pre-eminent selection criterion applied to laying hens, with an annual increase of two to three eggs expected. The key parameter is improved persistency, which entails selecting birds that lay longer clutches of eggs (Preisinger, 2018; Rubinoff, 2018). Egg size has decreased, which is a conscious decision made to ensure egg shell quality late in lay. Although egg size increases with age, this is not accompanied by a proportional increase in shell weight, leading to decreased shell thickness. Egg mass output has increased during the entire laying period, accompanied by lower feed intakes that lead to improved feed efficiency. Feed usage has dropped by 22% per egg produced over the past 20 years. Alternative production systems add a new dimension, and diets will need to be formulated to ensure nutrient intake under different conditions. Hens can cope well under alternative systems, provided that they are permitted to consume adequate feed (Pottgüter, 2013).
At first glance it would be easy to assume that the nutrient requirements of laying hens have increased as flock egg output improves. However, bear in mind that although the modern bird has been selected for increased persistency, it still lays a single egg daily. The hen’s nutrient requirements should be considered on a daily basis. Since our hens are at least the same size, if not even a little smaller, with a reduced daily egg output (see Table 1), it is probable that daily nutrient requirements may have decreased rather than increased. Rubinoff (2018) reports that the birds we will see on our farms by 2020 will be slightly heavier.
The modern bird purportedly consumes less feed, but it is difficult to be sure of this from published data. Practically, for example, a large commercial operation in South Africa, using the Hy-Line Brown, is achieving an average feed intake of 116 g/bird/d, so feed intakes may not have declined significantly. Birds coming in to peak production are sometimes not capable of consuming enough feed, forcing them to draw on body fat reserves as an energy source. If this is inadequate, flocks exhibit a typical ‘post-peak’ dip. This represents a major problem for modern layer genotypes (Pottgüter, 2016) and impacts on the lifetime performance of the bird. Feeding low-density, high-fibre rearing feeds helps to train young hens to achieve higher feed intakes and ensures adequate carcass fat deposition. Practical experience has taught us that little can be done to the diets offered in the layer house to overcome this consumption problem as its origin is during rearing. In the case of broiler breeders, high protein diets during rearing result in reduced carcass fat (Van Emous et al., 2014; Soumeh et al., 2018). It is likely that a similar pattern will exist in laying hens, which means that the use of high protein diets during rear may be the incorrect strategy to follow.
III. ENERGY REQUIREMENTS
The energy requirements of laying hens will continue to be driven by the need for maintenance (determined by body weight), egg output and feather cover. Glatz (1998) demonstrated that poorly feathered brown egg layers consumed 19% more feed than birds with good feather cover. It has long been understood that laying hens are able to meet their energy requirements by simply adjusting their feed intake (Morris, 1967; Kleyn and Gous, 1988). Recent research indicates that this adjustment may not be perfect, and that energy intake tends to increase slightly with higher dietary energy levels. This may or may not have a link with social interaction or competition within the cages. To all intents and purposes, however, the layer of the future is likely to be able to consume adequate feed to meet its energy requirements. Considerable interest has been shown in the use of split feeding, where diets fed in the morning are rich in energy, and those fed in the afternoon are rich in protein and Ca. Despite this, hens seem capable of regulating their feed intake from the various diets to achieve the same energy and nutrient intake as birds on a continuous diet. Little advantage is to be gained from split feeding as birds do not appear to be able to adjust their feed intake based on a fraction of a day (Traineau et al., 2015).
IV. PROTEIN REQUIREMENTS
The requirements for protein and amino acids are less well understood, which subjects the inexperienced nutritionist to many different opinions on the requisite quantities. It is perhaps worth starting this discussion by dealing with those aspects that are known:
- The provision of the correct level of essential amino acids in the diet is of concern. The inclusion of a crude protein minimum is unlikely to lead to increased egg numbers, but it will increase egg size. The rule of thumb suggests that, for each additional gram of protein a bird consumes, the egg size will increase by 1.4 g.
- We have a reasonable idea of what a correct amino acid profile should look like.
- We should always be profit driven when deciding on dietary levels of amino acid. It is unlikely that a single recommendation will ever be correct for all circumstance. Differences in the cost of ingredients and the value of the egg produced preclude this outcome.
Achieving adequate protein intake will be less of a challenge than might be expected, as the birds are likely to have a reduced requirement on a daily basis, coupled with an ability to simply consume more feed. The danger lies in overfeeding protein, which will cause the egg size to increase with the concomitant shell quality problems. Paradoxically, the production of big eggs will become more difficult because the innate ability of hens to produce large eggs has decreased.
V. CALCIUM AND PHOSPHORUS
During their lifetime, hens will deposit 30 to 40 times as much Ca in egg shells as is present in their own skeleton. The shell is formed in the uterus as an extracellular process, governed by the proteins responsible for Ca transport and by the establishment of the pH gradient needed for crystal formation. Some proteins are secreted and integrated into the shell where they regulate the calcification process and become part of the organic shell matrix. Approximately 5.5 g of Ca carbonate is deposited into each eggshell in a 17–20-hour period, making it one of the fastest bio-mineralisation processes known. The P content of the eggshell is small (20 mg) when compared to the egg content (120 mg).
During the later stages of rearing, Ca and P are deposited into the medullary bone (as crystalline hydroxyapatite (Ca10(PO4)6(OH)2) while, during the laying cycle, Ca is ‘withdrawn’ from the skeleton. Early interventions stimulate bone development during the rearing stage. Neijat et al. (2018) found that birds reared in an aviary system (as opposed to cages) had a heavier total bone weight. Dietary nutrient levels may be less important than management. Jing et al. (2018) demonstrated that a low P feeding regime had no effect on growth parameters or bone characteristics. Notably, the diets used in this study did not contain phytase.
Maturity of the medullary bone begins when the oestrogen levels rise at the onset of sexual maturity (when the wattles colour up) and is completed by about 30 weeks of age. The magnitude of daily Ca requirements cannot be met by dietary sources alone, forcing the hen into a daily ritual of bone remodelling. Regardless of dietary Ca supply, hens use this reserve to supply up to 1 g of Ca per day (Leeson, 2017). During shell formation, 60–75% of Ca in the shell is of dietary origin and the remainder is drawn from skeletal stores. If there is insufficient bone reserve, egg shell quality declines fairly quickly. Issues of Ca depletion in the bird may have more to do with metabolic Ca insufficiency rather than dietary deficiency (Fosnaught, 2009).
Mineral nutrition is complicated because hens are able to utilise minerals differentially, depending on their dietary level. Clunies et al. (1993) were able to measure Ca retention in laying hens in the range 36–62%, depending on the Ca level in the diet. It is often erroneously believed that only dietary Ca has any bearing on shell quality. Therefore, when shell quality problems arise, it is tempting to increase dietary Ca and, thus, limestone in the diet. Improvement may be observed, but more often there is none. Increasing dietary Ca in late-lay may increase shell strength and bone strength if the bird retains adequate levels of Ca in the medullary bone. Pongmanee et al. (2018) demonstrated that supplementing layer diets with 600 FTU of phytase (twice the normal level) increased bone strength and prevented bone loss throughout the laying cycle.
Broiler chickens can adapt to the dietary challenges brought about by low dietary Ca by improving Ca uptake at a later stage. This improvement is achieved through modulation of certain genes that encode intestinal Ca and P transporters (Rousseau et al., 2016). Leeson (2017) suggests that the same phenomenon may exist in laying hens. This was supported by the finding of Ieda et al. (1999) that feeding low Ca diets doubled the levels of Ca binding protein (CaBPD28k) mRNA, in the intestine. If the young hen is fed too much Ca (no more than about 3.5 g/day is required), the Ca uptake system appears to lose its ability to become more efficient in the face of increased demand. This aspect is of particular importance when feeding hens during extended lay, when hens are required to become more efficient utilisers of Ca.
The strong interaction between Ca and P cannot be ignored. Diets low in available phosphorus lead to reduced blood P concentrations which stimulate the synthesis of the active vitamin D3 metabolite, 1.25(OH)2. This in turn stimulates intestinal P absorption, as well as intestinal Ca absorption, even when blood Ca levels are normal (Wideman, 2015). Substantially lower levels of dietary P than are commonly applied in commercial diets have been researched (Snow et al., 2005). Lambert et al. (2014) found that a retained P intake of 2.6 g/kg was adequate until 65 weeks of age; thereafter, this intake needed to increase to 2.8 g/kg. Both values are far lower than recommendations by Hy-Line (2014). Ahmadi and Rodehutscord (2012) reported that the addition of phytase (300 FTU/kg) lowered the laying bird’s requirement for dietary P.
VI. FEEDING THE LONG-LIFE LAYER
By 80 weeks of age, it is estimated that the bird will have produced 24.1 kg of eggs, comprising 7.1 kg of yolk and 2.1 kg of shell (after Hy-Line, 2016). At this point, we would expect the birds to produce an egg a day (the best birds will achieve this) for a further 20 weeks. Clearly, feeding the long-life layer will be a challenge. The greatest hurdle will be maintaining egg quality with regard to both its internal characteristics and shell quality. Using genomic tools, breeders will be able to address the issues of declining albumen quality, egg shell colour, shell breaking strength, bone quality (although this is difficult to measure) and, importantly, the weight and efficiency of the oviduct (De Keijze, 2014; Bain et al., 2016).
Managing the long-life layer begins before the hen is ‘old’. Nutritional support will be required throughout the hen’s life, preventing post-peak dips for example. It is difficult to know when birds can be considered old but, for the purposes of this paper, it will be assumed to be 60 weeks of age. From a physiological perspective, the areas that will require attention are the maintenance of good plumage, the maintenance of skeletal integrity including Ca reserves, the maintenance of a fully functional oviduct, and the management of the health and function of the bird’s liver. Liver management will be the key aspect in supporting production and egg quality (Rutten, 2018). Nutritionists will be required to focus on providing the correct levels of nutrients throughout the production cycle. Although the requirements of the individual hen will not have changed much, feed specifications and feeding programmes may need to be adapted as eating patterns change and flock output (persistence, in particular) increases.
As old age approaches, the cells in the hypothalamus that control the levels of oestrogen produced by the hen become less efficient. Oestrogen plays a crucial role in the maintenance and performance of the medullary bone, as well as the growth and maintenance of the oviduct. The net effect of the decline in oestrogen production is a depletion of medullary Ca and an oviduct that is damage-prone and less well developed (Bain et al., 2016). Differences between individuals in this regard show that phenotype extends to the physiological level, which is what the geneticists will be able to focus on in order to improve both aspects (Dunn, 2013).
During rearing, the pullet is required to develop robust organs and a healthy, wellfunctioning gastro-intestinal tract. Structurally, the bird will have an ideal frame size and body weight, with the correct body composition (sufficient fat reserve). Importantly, the Ca deposits in the medullary bone should be maximised. Once the bird begins to produce eggs, it is difficult to replenish the Ca reserves in the medullary bone and, once the circulating oestrogen levels begin to decline, replenishment of these reserves becomes more difficult. Thus, Ca reserves decline steadily as the bird ages. At the point where they become depleted, osteoporosis occurs, shell quality declines and production ultimately ceases, regardless of dietary Ca levels. Any insult to the bird’s Ca supply during early lay will deplete lifetime skeletal reserves. If dietary Ca levels are too high in early life, there is a possibility that the bird will not respond to increased Ca levels later, although this needs to be confirmed experimentally.
Maintaining good plumage and the reduction of feather pecking are further issues that require our attention throughout the life span of the hen. Feather pecking is as much a behavioural problem as a nutritional one. The addition of some fibrous material to the diet is known to reduce feather pecking, as is the rearing of birds at a low light intensity. In addition, slightly elevated dietary methionine, tryptophan and glycine have helped to maintain plumage in laying hens, or at least to reduce injurious pecking (Prescilla et al., 2018). A project in the Netherlands aims to unravel the underlying physiological mechanisms of feather pecking. This will include such aspects as satiety and the activation of the gut-brain axis, as related to nutrient supply and density. Eating and foraging behaviour impact on the motivation for feather pecking which will also be investigated. The intention of the project is to develop practical dietary strategies during rearing to reduce feather-pecking behaviour in later life (Mens et al., 2018).
The liver provides most of the nutrients for the development of the yolk and albumen. It plays a support role in shell formation through its provision of the lipoprotein component of the shell, which contributes to shell tensile strength and elasticity (Pottgüter, 2016). The maintenance of a healthy, functional liver is a challenge. Fatty liver haemorrhagic syndrome (FLHS) is a major issue, particularly in the late-layer. The balance between hepatic synthesis and secretion of lipids is of vital concern in the regulation of hepatic and extrahepatic fat deposition in hens. Liver fat accumulation can be increased by many factors, including nutrition, housing conditions and inflammatory challenges (Shini, 2014).
This subject has received little attention and our understanding is far from complete. Knowledge of liver function is clouded because we draw conclusions from the examination of dead hens – not their live, healthy counterparts. ln order to avoid FLHS and relieve pressure on the liver, at least a proportion of the dietary energy should be supplied as lipid throughout the laying production cycle. Nutrients that promote liver function and the export of fat from the liver are termed lipotropic nutrients. These include methionine, choline, inositol (elevated by the use of phytase), Vitamin B12, biotin, tryptophan, carnitine and selenium. Supplementation with these nutrients has been used as a treatment for FLHS with variable success (Hy-Line, 2017). Betaine is also used to relieve liver metabolism (Pottgüter, 2016). Dietary 25(OH)D3 and vitamin E have also been shown to limit FLHS in layers (Bouvarel and Nys, 2013). Prevention of FLHS will be more successful than treatment, as is the case with most nutritional or metabolic conditions. This will require a focus on liver health throughout the bird’s life, which is difficult to measure as it requires bird sacrifice. Perhaps the biomarker technology used in human medicine could be applied in this case?
From a nutritional perspective, it seems logical that increasing dietary Ca levels may be beneficial. The recommendation is to increase dietary Ca levels during late-lay to achieve a consumption of about 4.5 or 5 g/hen/day, but research results in this regard are conflicted. Kershavarz and Nakajima (1993) found no benefit from feeding birds a ‘step-up’ Ca regime, while Star et al. (2013) perceived no difference between birds fed diets with 3.5 g/kg or 3.9 g/kg. However, An et al. (2016) found that the percentage of cracked eggs in aged laying hens (70 weeks old) declined as Ca levels were increased, while Nascimento et al. (2014) showed that a high Ca level can have significant detrimental effects on feed conversion and production. It is suggested that the results are confounded by the level of the Ca reserve contained in the medullary bone of the hen.
Practical experience has taught us that the Ca source is more important than dietary levels per se. The use of larger, less soluble particles creates a supply of Ca during the hours of darkness, which is when the bird needs it most for shell formation. Rather than increasing dietary Ca levels, the daily feeding of coarse grit may be more effective. Another alternative is to implement a split feeding regime, with diets high in Ca being fed in the afternoon. However, Molnár et al. (2018) found that split feeding did not maintain shell quality, although it did improve relative shell weight. Al-Zahrani and Roberts (2015) showed that the addition of 1 g of 25(OH)D3 resulted in the highest shell weight, percentage of shell, shell thickness and lowest shell deformation in Hy-Line Brown layers (aged 19–80 weeks), but 0.5 g/ton did not have a significant outcome. Most commercial diets probably contain more than enough phosphorus. In the late-layer, the real danger lies in feeding too much P since high levels in the blood inhibit the mobilisation of Ca from bone.
The next generation of laying hens will have reduced daily nutrient requirements but may have different feed intake patterns. Under normal circumstances – even using disparate farming systems - hens will be able to achieve their genetic potential provided that they are able to consume adequate levels of nutrients and energy. While we have a fair idea of the essential energy requirements of a laying bird, there is still much debate about the level of amino acid provision. Any decision should be based on the economics of egg production, but we need to bear in mind that it is probably pointless to feed for large eggs when the genetics of the bird have been modified to produce smaller eggs. Remarkably, the requirements for the key minerals Ca and P have not yet been resolved. It appears that we are overfeeding P and that, until such time as we have a better understanding of the dynamics of the Ca reserves in the medullary bones of laying hens, Ca provision will be more guesswork than science.
The real challenge will be in feeding the long-life layer. Current recommendations are based on hypothesis and conjecture and have yet to be tested. Many of the proposals put forward may simply be too expensive for commercial systems. Some issues, namely the degeneration of the oviduct and the way in which Ca is deposited in the medullary bone, will be addressed by the geneticists. The prevention of FLHS will continue to be an issue for as long as we fail to track liver health and function throughout the bird’s life. The management of feather cover involves the establishment of good plumage, which is largely determined by nutrient provision and addressing the behavioural aspects of feather pecking. This may be feed related, but it also has much to do with farm management and the bird’s innate behaviour. The modern laying hen is an efficient and robust bird, with an almost unbelievable propensity to produce eggs. The challenges we face in designing feeding programmes for the bird will be overshadowed by the expected improvements in performance.
Abstract presented at the 30th Annual Australian Poultry Science Symposium 2019. For information on the latest edition and future events, check out https://www.apss2021.com.au/.