Extending Layer Hen Lifespan: Studies in the Australian Context

I. INTRODUCTION

With ongoing genetic selection for improved persistency of lay and feed efficiency (FE) in laying hens, the egg industry is pursuing the opportunity to extend layer hen lifespan until they are 100 weeks of age (WOA), with an aim for each hen to produce up to 500 eggs. This presents some challenges for the hen as during this longer laying cycle she will be producing more than 31 kg of egg product (Muir et al., 2022c) and using approximately 1 kg of calcium (Ca) to produce up to 3 kg of eggshell (Nys, 2017). Continuous egg production places high demand on the organs and tissues involved in producing eggs. This includes the liver, which generates yolk lipid and, the oviduct which produces the egg white, shell membranes and eggshell. Further, the continuous demand for Ca for eggshell formation may impact bone integrity, especially the risk of bone fractures and osteoporosis (Whitehead and Fleming, 2000). However, recent reports have not found a direct relationship between continuous egg production and bone quality (Dunn et al., 2021). Eggshell quality is central to the production of first grade eggs and is a key determinant for the flock to continue through a longer laying cycle. Optimal liver and skeletal health are also critical for the successful extension of flock life. Recent Australian studies that have explored hen characteristics, productivity, FE, egg quality, liver health and bone health in longer laying cycles will also be discussed.

II. EGG PRODUCTION AND FEED EFFICIENCY

The transition of a pullet from rearing into egg production presents many challenges. This includes the stressor of transportation from the rearing to the laying facility (Kolnik, 2021), and acclimatisation to the layer shed. During this transition it is ideal that the hen continues to grow and then starts to lay eggs (Bain et al., 2016). In Australia, layer pullets are typically reared to above breed standard weight (BSW) at point of lay (Parkinson et al., 2015). This is practiced as larger pullets appear to manage the transition to the laying shed more readily than lighter sized hens and they start to lay eggs of a larger size at an earlier age (Summers and Leeson, 1991). However, heavier hens have higher nutritional needs and hence higher feed intake (FI).

There is also a greater risk of a drop in egg production during peak lay if their nutritional needs are not met (Pottguter, 2016). The ongoing measurement of FI, egg production and egg mass throughout the laying cycle allows hen FE to be calculated (O’Shea et al., 2020).

III. EGG WEIGHT AND EGG QUALITY

Egg characteristics, including the appearance of the yolk, albumen and eggshell, are a priority for the consumer. Eggs classified as Extra-Large, where a carton of 12 eggs weighs 700g, with average egg weight (EW) 60 g, is the target for most Australian producers. Maintaining EW below 70 g avoids increased egg losses due to cracked eggshells (Parkinson et al., 2015). An EW of 60-65g (Parkinson et al., 2008) accommodates market demand and eggshell quality.

Egg size typically increases with hen age (Leeson and Summers, 1987; dePersio et al., 2015). Larger eggs contain a larger yolk and less albumen (Jiang and Sim, 1991). In Australia, a more golden coloured yolk, i.e. ≥11 on the Roche colour scale, is preferred (Roberts 2004). Albumen viscosity and quality calculated as Haugh unit (HU) also tends to decrease with hen age (Marzec et al., 2019). A HU of 82 is recommended throughout an 18–100 week egg laying cycle (ISA Brown Product Guide, Cage Production System, 2017).

The shell of each egg requires approximately 2.2 g of Ca (Bouvarel et al., 2010). To meet this requirement, Ca is sourced from both the hen’s diet and her skeletal system. There are several comprehensive reports on nutritional management for eggshell quality (Roberts, 2004; Nys, 2017) and specifically within the longer laying cycle (Pottguter, 2016; Korver, 2020). As hen age and EW both increase, eggshell quality tends to decline, becoming thinner and more liable to cracks and fractures (An et al., 2016). Concurrently there is a reduction in intestinal uptake of Ca by older laying hens (An et al., 2016). Eggs with ≤9.5% eggshell weight are more susceptible to breakage (Abdallah et al., 1993) whereas ≥10% eggshell weight will minimise cracks. Typically, increased variability in eggshell quality with flock ages will determine when the flock will be terminated (Dunn, 2013).

V. LIVER AND BONE HEALTH

The liver processes starch, carbohydrate, and fat to form yolk lipid, which also involves the production of hepatic fat (Squires and Leeson, 1988). When this process is disrupted or experiences an imbalance, fatty liver haemorrhagic syndrome (FLHS) may ensue (Yang et al., 2017). Hens housed in cages and receiving high energy diets are most susceptible to FLHS (Shini et al., 2019). Environmental and bird genetics may also predispose birds to FLHS (Squires and Leeson, 1988). In addition to abdominal and hepatic fat accumulation, severe or extensive acute liver haemorrhage can cause sudden death (Shini et al., 2020). The impact of less severe chronic FLHS is more poorly understood (Bryden et al., 2021).

Korver (2020) concisely described the continual recruitment and re-deposition of labile Ca in the medullary bone deposits of the skeletal system for the formation of eggshell. At night, when low dietary Ca intake coincides with eggshell formation, Ca may also be mobilised from structural bone. This can occur even when there is ample medullary bone but, unlike medullary bone, structural bone is not replaced while the hen is in lay (Korver, 2020). Loss of structural bone increases the risk of bone fractures and osteoporosis (Whitehead and Fleming, 2000). As hens age, bone density declines and bone porosity increases (Yamada et al., 2021), which may be exacerbated with longer laying cycles. Interestingly, recent studies have not found a direct relationship between high egg production, eggshell quality, and bone integrity (Alfonso-Carrillo et al., 2021, Dunn et al., 2021) suggesting that eggshell and bone quality may be managed independently through optimum nutrition and genetic selection.

Continuous high egg production has also been attributed to keel bone fractures (KBF) (Toscano et al., 2020). These are most frequently seen in cage free systems but also occur in caged flocks (Baker et al., 2020). The incidence of KFB has been reported to peak around 50 WOA (Petrik et al., 2015; Toscano et al., 2020), but KBF have been observed in more than 50% of hens in older flocks (Käppeli et al., 2011). In addition to high egg production, age of first egg and hen inactivity have also been implicated with KBF (Toscano et al., 2020).

VI.AUSTRALIAN STUDIES

As Australian egg farmers are interested in extending layer hen lifespan, there is a need to identify ways of extending the egg laying cycle together with the production of saleable eggs. This has been the objective of some recent studies funded by Australian Eggs.

a)Study 1

This study was designed to compare the performance of pullets of either above or below BSW when 18 weeks of age (WOA) (former heavier weight; HW and latter lighter weight; LW) in a laying cycle that extended from 18 to 89 WOA (Muir et al., 2022a,b,c). As lighter sized hens have innately lower FI (Harms et al., 1982), the experiment also included two early-lay dietary treatments. This entailed a diet of higher nutrient density (HND), as a potential mechanism for the LW birds of lower FI to receive adequate nutrition as they entered lay, and a more common diet of lower nutrient density (LND). The HND diet may also be a primer for hens destined for an extended laying cycle (de Persio et al., 2015). Given the higher cost of HND diets, it was provided for 7 weeks only, rather than the more common duration of production (dePersio et al., 2015; Perez-Bonilla et al., 2012; Scappaticcio et al., 2021).

Individually housed LW and HW ISA Brown hens were allocated to one of the two early-lay dietary treatments. These were a LND diet, formulated to 110g FI/day, (2726 kcal/kg, 16.4% crude protein (CP) and 0.74% standardised ilea digestible Lysine (SID), 2.54% crude fat (CF)) or the HND formulated to 90 g FI/day (2901 kcal/kg, 17.63 % CP, 0.83% SID and 4.92% CF) which birds received from 18–24 WOA inclusive. From 25-39 WOA all birds were fed the early-lay LND diet, followed by a mid-lay LND diet formulated to > 110g FI/day (2724 kcal/kg, 16.0% CP, 0.70% SID, 2.53% CF) from 40-77 WOA. Finally, when 78 WOA all birds were moved onto a late-lay LND diet formulated to 110g FI/day (2753 kcal/kg, 16.2 CP, 0.73% SID, 2.5% CF). This was fed until hens were 90 WOA, when the experiment concluded. Observations when hens were 69-70 WOA and 89-90 WOA are presented.

The HW pullets (at 18 WOA) remained comparatively heavier with higher FI throughout the study (Table 1). All birds gained weight between 18–70 WOA (Muir et al., 2022b) with comparatively smaller increases in BW until 90 WOA. Despite this, HW birds continued to be notably heavier than BSW for age. Lighter weight hens achieved BSWfor age around 62 WOA and BW remained relatively stable to 90 WOA. There is agreement that small increments of weight gain for LW hens are beneficial (Perez-Bonilla et al., 2012, O’Shea et al., 2020), with the latter recommending LW hens attain BSW for age during mid-lay. Further, the gradual increase in weight of smaller sized hens allows them to reach full maturity, with sustained egg production and FE.

Hen-day egg production (EP) was similar for birds of both BW groups, averaging 89% at 69 and 81% at 89 WOA, above breed standard rate for age of 84% and 74.4% respectively (ISA Brown Product Guide, Cage Production System, 2017). The total number of eggs produced were similar for all birds, averaging approximately 465 eggs/hen at 89 WOA (Table 1).

Cumulatively, LW hens had lower FI from 18-89 WOA, generating lower egg mass and lower, but not statistically significant cumulative FCR compared to HW hens (Table 1; Muir et al., 2022c). This is in contrast with the significantly lower cumulative FCR of LW hens earlier in production at 18-24 and 18-69 WOA (Table 1; Muir et al., 2022b) and 18-50 (Muir et al., 2022a), Hence characteristics of FE with hen size observed during early lay were maintained until 69 WOA but became more variable later in lay.

Table 1 - ISA Brown hen bodyweight at 70 and 90 weeks of age and, cumulative feed intake, number of eggs laid/hen and feed conversion ratio from 18-69 and 18-89 WOA (Muir et al., 2022b,c).

O’Shea et al., (2020) also identified that, compared to LW hens, heavier ISA Brown hens had higher FI and higher FE at peak and late lay. Earlier studies in White layers also calculated superior FE in LW hens through to 84 WOA (Lacin et al., 2008). Comparisons of hen BW with FI by Leeson and Summers, (1987) and Parkinson et al. (2015) drew similar conclusions, determining that for each additional 100 g BW a further 3.5 g FI/day was needed. The former also estimated a concurrent increase of 1.2 g EW.

As continuous FI and egg production data was collected (Table 1; Muir et al., 2022c), a simple cost-benefit comparison of cumulative FI with cumulative egg production across this extended 18-89 WOA laying period was possible. Compared to LW hens, HW hens consumed an extra 4.85 kg feed to produce an additional 7 eggs. Feed costs and returns on eggs will vary but at estimated cost of $512 AUD /ton layer feed and $0.14 AUD return/first grade egg, the extra cost is approximately $1.48/HW hen. In a 50,000 flock this is an additional $74,000 from 18-89 WOA. Alternatively, to break even, each HW hen needed to produce an additional 11 eggs, or to consume only 1.91 kg extra feed.

Diet nutrient density did not affect EW in older hens. At 69 WOA, EW for all hens was above 60 g and HW hens produced the largest eggs (Muir et al, 2022b). At 89 WOA, the heaviest eggs were being produced by HW hens that had received the LND (average 63.4 g) and lightest eggs were from LW hens on early-lay LND diet (average 60.8 g). LW hens on HND early-lay diet generated an intermediate EW of 62.3g (Muir et al., 2022c).

Egg quality was assessed on a focal group of hens at 66-70 and 86-90 WOA (Muir et al., 2022b,c). Yolk colour score decreased with age from 11 at 70 WOA to 9 by 90 WOA (Muir et al., 2022b,c). Haugh units were generally high, including average 90 HU at 90 WOA (Muir et al., 2022c). Hens that had received the LND diet during early-lay had higher HU between 86-90 WOA. Several studies have assessed internal egg quality in relation to hen BW and diet nutrient density, with varying results (Muir et al., 2022b). Interestingly hens of higher FE and lower BW produced eggs with higher HU and higher amino acid concentration in the albumen, compared to less efficient, heavier hens (Akter et al., 2019; Anene et al., 2021).

Neither hen size nor diet nutrient density altered eggshell weight. At 66-70 WOA, shell weight was > 10% and at 86-90 WOA > 9.5% EW. However, at both 70 and 90 WOA, hens fed the early-lay HND diet produced thicker and stronger eggshells than hens fed the LND diet (Muir et al., 2022b,c). Neither shell weight %, shell ash nor shell mineral levels provided an insight into the reason for the thicker and stronger shells.

At 70 and 90 WOA, FLHS scores were similar for all treatment groups (Muir et al., 2022b,c). However, at 50 WOA, FLHS scores and hepatic lipid peroxidase were lower in LW hens and hens that had received the HND diet during early-lay (Muir et al., 2022a). O’Shea et al. (2020) also reported lower FLHS scores in LW, more FE hens at 45 WOA. Liver lipid peroxidase did not differ at 70 WOA, but at 90 WOA it was lowest in HW hens that received the LND diet and in LW birds that received the HND diet during early lay. Keel curvature and bone breaking strength were similar across all treatments at 70 and 90 WOA (Muir et al., 2022b,c). However, higher levels of zinc (Zn) and manganese (Mn) were found in the bones of 90-week-old LW compared to HW hens (Muir et al., 2022c). Lower serum levels of both Zn (Mutlu et al., 2007) and Mn (Rondanelli et al., 2021) have been observed in osteoporotic female patients, and hence their higher levels in LW hens are indicative of a lower likelihood of developing osteoporosis.

Overview of study 1: Lighter weight hens demonstrated persistency of lay comparable to HW hens, together with more favourable liver health in mid-lay and bone mineral composition in very late lay. They also maintained a lower FCR until late lay. Feeding a HND diet in early lay increased eggshell strength in late and very late lay for all hens.

b) Study 2

As Study 1 illustrated that hen size trajectory is established by point of lay, Study 2 was designed to grow pullets to three different BW at 16 WOA. Their egg production and egg quality are to be followed through to 100 WOA. Using two lighting and three feeding programs during rearing either BSW or BW above and BW below BSW were attained.

Hy-line Brown chicks were grown in floorpens under two lighting and three feeding programs. Lighting was either standard (SL) lighting of 10h/day from 7–16 WOA, or rapid step-down (RSD) lighting of 9 h light/day from 4-16 WOA. From 4 WOA pullets were either fed ab libitum, or to achieve either BSW or 88% BSW, identified as Managed feeding, at 16 WOA. At 16 WOA the pullets were transferred to the Poultry Research Unit Camden and housed in individual pens in the layer shed. Here they received a pre-lay, early, mid and late lay diet ad lib. Lighting was stepped up from 11h /day at 16 WOA to 16 h/day at 32 WOA, where it remained until hens were 100 WOA and the study concluded. Pullet FI, BW at 16 WOA, age of first egg and weight of first three eggs was measured. Hen performance, egg quality and hen health will be assessed through to 100 WOA, with data to 92 WOA being reported here.

The outcomes of the rearing phase are presented in these proceedings (Muir et al., 2023). In brief, ad lib feeding under SL lighting resulted in the heaviest pullets at 16 WOA. There was an interaction between feeding and lighting on age of first egg. Ad lib fed pullets under RSD lighting were the first to lay, and pullets on Managed feeding under SL lighting were the last to start producing eggs. Weight of the first three eggs was independently affected by lighting and feeding where heaviest eggs were from SL lighting, BSW and Managed feeding.

At 92 WOA, birds reared under SL lighting were heavier than those reared with RSD lighting. Pullets fed both ad lib and to achieve BSW at 16 WOA, were also heavier than Managed fed pullets. The average BW of all treatment groups at 92 WOA was > 2.2kg, above 1.92-2.04 kg BSW for age (Hy-Line Brown Commercial Layers Management Guide, 2019). This may be due to the individual hen housing with ready access to feed and water. Average daily FI at 92 WOA ranged from 110-115g/d, whereas breed recommendation is 105-111 g/d. At 92 WOA there were no differences in rate of lay (ROL), EW, FI, egg mass nor FCR due to treatments during rearing. When reviewing the data, a range of hen BW within each treatment group was noted. This indicates that managing feeding during rearing may not have an ongoing impact on FI and BW once the hens have ad libitum feeding. Based on individual hen BW at 92 WOA the flock was divided into quartiles (Q), ranging from lightest to heaviest BW (Q1–4 respectively). Production data of the Quartiles is presented in Table 2.

Table 2 - Hy-Line Brown hen body weight, feed intake, rate of lay, egg weight and feed conversion ratio at 92 weeks of age and, cumulative feed intake, cumulative eggs per hen surviving and cumulative feed conversion ratio from 17.4 to 92 weeks of age.

Quartile 1 had the lowest BW which corresponded with Hy-Line Brown BSW for age. Average BW increased with each quartile groups (Table 2). Daily FI was lowest in Q1, and birds in Q3 and Q4 consumed significantly more feed/day. Quartile 2 had the highest ROL (83%) which was significantly higher than Q4 (70%) and above 92 WOA breed recommended 71-73% ROL. Quartile 4 birds produced the heaviest eggs (66.6 g) (Table 2), above recommended 60-65 g EW range. Quartiles 1,2 and 3 had significantly lower 92 WOA FCR than Q4. Cumulatively 17.4-92 WOA FI was highest in Q4, lowest in Q1, while Q4 hens produced the least number of eggs. Quartiles 1 and 2 had the lowest cumulative FCR, compared to Q4 (Table 2).

As in Study 1, a simple cost-benefit analysis based on BW quartile rankings in Study 2 was completed using the same estimated feed costs and return/first grade egg. Comparing Q1 (lightest hens) with Q4 (heaviest hens) the latter consumed an additional 6.4 kg feed to produce 20 fewer eggs to 92 WOA. Quartile 4 hens had additional feed costs of $3.28/hen and lower return on eggs (-$2.80/hen) totalling an extra cost of $6.08 /hen compared to Q1. Quartiles 1 and 2 had similar and the lowest FCR. Compared to Q1, Q2 hens produced 6 more eggs, an additional return of 0.84c/hen, for an extra 2.3 kg feed, an additional cost of $1.18/hen. Therefore, each Q2 hen cost an extra 0.34c to 92 WOA when compared to Q1 hens. In a 50,000- hen flock this is an additional cost of $17,000 from 17.4-92 WOA.

These findings to 92 WOA generally concur with the outcomes of Study 1 (to 89 WOA), and O’Shea et al. (2020) between 70-75 WOA, in that LW hens are capable of sustained egg production but with a lower FCR compared to HW hens.

VII. CONCLUSION

In Australian studies, lighter sized hens at the start of lay have demonstrated strong persistency of lay, with lower FI and FCR through an extended production lifespan compared to larger sized hens. Gradual weight gain to mid-lay allows LW hens to reach mature body size, without being overly fat and with more favourable liver function and bone integrity. Further, an early-lay diet of HND can improve eggshell quality, especially shell thickness and breaking strength in older hens. These findings require further evaluation in cage free systems in Australia.

ACKNOWLEDGEMENTS: Thank you to Australian Eggs for funding Studies 1 and 2.

Presented at the 34th Annual Australian Poultry Science Symposium 2023. For information on the next edition, click here.