The influence of hen size and diet nutrient density in early lay on hen performance, egg quality, and hen health in late lay

INTRODUCTION

Our previous research identified that pullet body weight (BW) at point of lay (POL) or, the feeding of diets of different nutrient density during the early laying period, generate differences in egg production (EP) and liver health at 50 wk of age (WOA) (Muir et al., 2022). The heavier (HW) birds at POL consumed more feed and remained significantly heavier than the lighter weight (LW) birds to 50 WOA. During that time HW birds also produced more eggs and a greater egg mass (EM) but with poorer cumulative feed conversion ratio (g feed / g egg) (FCR) than LW hens. In contrast the LW hens had lower fatty liver hemorrhagic syndrome (FLHS) scores and liver lipid peroxidase compared to HW birds. But the question remained as to whether BW would have a continuing effect on EP, feed efficiency and hen health throughout a longer laying period.

Identifying the ideal pullet weight at the start of lay is important for the management of hen sexual maturity, egg productivity, and flock uniformity (Lacin et al., 2008). Comparisons between hens of different size and BW are insightful as the average BW in Australian laying flocks is frequently above breed standard weight for age (Parkinson et al., 2015). This is in part to achieve heavier egg weight (EW) earlier in lay. But as egg size generally increases with bird age, eggshell quality may become compromised and especially if eggs become very large (Joyner et al., 1987) with ongoing egg production. Bish et al. (1985) and more recently Parkinson et al. (2015), identified that LW hens may be able to sustain a viable rate of lay through to 70 WOA. In this regard the latter research group recommended greater focus on aligning layer hen BW more closely with breed standard weight for age, if not slightly lighter. Additionally, the smaller sized hen has more favorable feed efficiency when compared to larger hens (Akter et al., 2019) associated with differences in ADFI and EM (Anene et al., 2021).

However, there is some caution around the ability of LW hens to consume sufficient nutrients throughout the early to peak lay period, to meet their nutritional needs for sustained EP when lay extends to at least 70 WOA. In this regard feeding a diet of higher nutrient density (HND) may allow for adequate nutrient intake in smaller sized hens with naturally lower ADFI (Harms et al., 1982). Providing a HND diet during early lay may achieve both immediate and longer term benefits. This research group have previously reported that ISA Brown hens that received a HND diet between 18 and 24 WOA produced greater EM and lower cumulative FCR at 24 WOA compared to birds fed the lower nutrient density (LND) diet (Muir et al., 2022). Furthermore, the lower cumulative FCR achieved with the HND diet was sustained through to 50 WOA. Additionally, birds that had been fed the HND had thicker femur cortical bone at 50 WOA, indicating a diet related benefit for bone integrity at that age.

As the world’s layer hen industry is progressing the extension of the laying period (Bain et al., 2016), it is imperative that the impact of bird size and diet nutrient density during the early laying period be assessed in production cycles beyond 50 WOA. The aim of an extended laying cycle is for a hen to produce 500 eggs in a single 100-wk production period (Korver, 2020) which, if achievable would improve the sustainability of the poultry industry and food security (Dunn, 2013). However, a decline in rate of lay, deterioration in shell quality and reducing hen health are factors likely to limit these longer laying cycles (Bain et al., 2016) and frequently result in the decision to replace flocks in late lay, when hens are around 70 WOA.

Bone integrity and liver health are central to hen health and bird welfare, both tending to decline with ongoing EP and bird age. Throughout lay, medullary bone experiences remodeling as bone derived Ca is sourced for eggshell formation (Korver, 2020). As EP continues structural bone may be eroded (Yamada et al., 2021) increasing susceptibility to osteoporosis (Whitehead and Fleming, 2000). While the incidence of keel bone fractures may not increase markedly after 50 WOA (Petrik et al., 2015; Toscana et al., 2020) flock prevalence of greater than 50% keel bone fractures has been reported (Käppeli et al., 2011) and is a concern for hen welfare. Liver health, and in particular the characteristic accumulation of fat in the liver and abdominal cavity with FLHS is most common in highly productive caged hens (Shini et al., 2019) and can cause sudden mortality. Further, as a chronic condition FLHS can be difficult to identify and its implication for hen health may not be immediately apparent (Bryden et al., 2021).

Given the impact bird size and the nutrient density of the early lay diet on layer hen health and production to 50 WOA (Muir et al., 2022), this investigation followed the experimental flock through to late lay at 70 WOA. Observations during late lay may also be indicative of the suitability of hens of different BW or fed different diets in early lay, for a further 20 to 30 wk of production in a longer laying cycle.

Many studies exploring the effect of diet nutrient density and bird performance have involved white egg layers (Latshaw et al., 1990; Leeson et al., 2001; Ribeiro et al., 2014; dePersio et al., 2015) or both brown and white shell layers (Harms et al., 2000). Few studies have evaluated HND diets in brown shell layer hens. Of these studies the timing of the dietary treatment varies, for example, during mid lay (Harms et al., 2000) or throughout the entire laying period (Perez-Bonilla et al., 2012b; Scappaticcio et al., 2021). Diets of HND are more costly than LND diets and hence an economically practical option may be to provide a HND diet for a relatively shorter period during early lay.

This study compared ISA Brown pullets of either heavier or lighter weight at 18 WOA, that received either a HND or LND diet during early lay in terms of their performance and health in late lay. Specifically, the impact of these treatments during early lay were evaluated on BW, feed consumption, egg production, egg mass, feed conversion ratio, internal egg quality, eggshell quality, liver health, and bone parameters through to late lay.

MATERIALS AND METHODS

RESULTS

DISCUSSION

This study explored the effect of BW at POL and the nutrient density of the diet fed during early lay on ISA Brown laying hen ADFI, hen-day EP, EM, and FCR at 24 and 69 WOA and BW at 24 and 70 WOA. Additionally, egg quality from 66 to 70 WOA, hen carcass characteristics, liver health, and bone characteristics were assessed when the hens were 70-wk old. The 2 BW groups were established when the birds were 18 WOA and form a central component of the study design, assessing the effect of BW at the start of lay on EP, egg quality and hen health. Irrespective of diet, BW continued to be differentiated throughout the study with HW birds remaining significantly heavier than LW birds at 70 WOA. The differential in initial BW continuing throughout lay has also been reported by Harms et al. (1982), Bish et al. (1985), Lacin et al. (2008), and Muir et al. (2022).

At 18 WOA the recommended weight of ISA Brown hens is 1.576 kg (ISA Brown breed standard guide, 2018). The LW birds weighed 1.49 kg at 18 WOA, gained an average of 0.5 kg by 70 WOA to weigh 1.99 kg, matching the ISA Brown breed standard recommended weight for age of 1.988 kg (ISA Brown breed standard guide, 2018). The HW birds also gained weight, on average 0.55 kg, between 18 and 70 WOA, but at 2.20 kg at 70 WOA they were noticeably heavier than the breed standard weight for age. The ISA Brown standard guide recommends a gain of approximately 0.412 kg BW between 18 and 70 WOA. Both the HW and LW birds gained more weight than recommended, which may be due in part to being housed in individual cages with no competition for feed and water.

Perez-Bonilla et al. (2012a,b) are two of the few relatively recent reports on the production of brown egg-laying hens of different initial BW, but with Lohmann Brown hens in the former, and Hy-Line Brown hens in the latter. Lohmann Brown birds of lighter initial BW gained more weight than HW hens across a 22–50 WOA production period (Perez-Bonilla et al., 2012a) but HW Hy-Line Brown hens did not experience differences in BW gain during a 24–59 WOA production period (Perez-Bonilla et al., 2012b). While these differences are likely due to different breed of hen and diet composition, a critical point is that some weight gain in LW birds during the laying phase is likely to be more beneficial for those hens than weight gain in HW hens (Perez-Bonilla et al., 2012b). This may be especially so when the LW hens achieve their recommended weight for age by late lay, as observed in the current study. O’Shea et al., 2020 also identified benefits of small weight gains in LW hens, in that case the LW hens reached close to breed standard recommended weight during mid lay, compared to the typically larger weight gain of HW hens that then remain above breed standard weight. This growth pattern in LW hens is indicative of them reaching full maturity and it could be expected that failure to do this may negatively impact their performance. Specifically, LW hens that matched breed standard weight around mid lay demonstrated better feed efficiency, egg quality and liver health than heavier birds.

Our observed higher hen-day egg production in LW hens at 24 WOA but not at 69 WOA culminated in no difference in total number of eggs produced by birds of different initial BW between 18 and 69 WOA. Heavier Lohmann White hens had a higher rate of lay between 24 and 40 WOA but as the birds aged the LW hens had the higher rate of lay, so that overall BW did not affect total EP to 84 WOA (Lacin et al., 2008). Similarly, no difference in EP was observed with Dekalb XL hens from 28 WOA for 16 wk (Harms et al., 1982). However, conflicting effects of BW on rate of lay are seen between the two studies of Perez-Bonilla et al. (2012a,b). Lohmann Brown hens of heavier initial BW had a higher rate lay across a 22 to 50-wk production period (Perez-Bonilla et al., 2012a) while Hy-Line Brown hens demonstrated no difference in rate of lay due to initial BW across a 24 to 59 WOA production period (Perez-Bonilla et al., 2012b). In the current study the rate of lay across the 18 to 69-wk production period is most accurately reflected in cumulative EP during that time. Numerically (P = 0.07) the HW birds produced more eggs, but as this finding was only approaching significance it also indicates some inconsistent effect of BW on EP.

Birds fed the early-lay HND diet were heavier at 24 WOA, the final week of the dietary treatments, than the birds that had been receiving the LND diet. Compared to the hens on the LND diet, birds receiving the HND diet reduced their ADFI for 1 wk only that is, when 24 WOA (Table 2), of the 7-wk dietary treatment. Overall, there was no difference in the cumulative FI due to diet nutrient density from 18 to 24 WOA (Muir et al., 2022). As the birds receiving the HND diet were consuming approximately 10 g feed / day more than the expected ADFI for this formulation the dietary treatments ceased at the end of wk 24. Whether the reduction in ADFI due to the HND diet during wk 24 would have continued is unknown. Lohmann Brown Classic hens did not adjust their ADFI for the energy content of the diet and therefore the hens on the higher energy diet had higher energy intake, but this did not generate differences in BW at the end of their 19 to 59 WOA EP cycle (Scappaticcio et al., 2021). Hy-Line W-36 hens that continuously received diets of different nutrient density from 19 WOA for 52 wk (dePersio et al., 2015) reduced their ADFI with increasing diet density during the early laying period (19–26 wk) only. This reduction in ADFI was not sustained across the entire study (19–70 WOA) and hens that had received the more nutrient dense diet experienced a linear increase in BW at 27, 33, and 70 WOA compared to birds on the LND diets. Interestingly Perez-Bonilla et al. (2012b) observed reductions in ADFI across the 24 to 59 WOA study, but BW gain remained significantly higher for birds on the diet of highest energy concentration. In our study there was no difference in BW at 70 WOA due to diet density which is most likely due to the cessation of dietary treatments from 25 WOA. Similarly, no differences in cumulative FI from 18 to 69 WOA were observed due to the dietary density treatments of early lay.

At 24 WOA eggs produced by hens on the HND diet were heavier than eggs produced of LND diet treated birds. Heavier eggs have been identified with diets of HND (Sohail et al., 2003; Wu et al., 2005; dePersio et al., 2015; Scappaticcio et al., 2021), but it is not always a consistent response (Ribeiro et al., 2014; Perez-Bonilla et al. 2012a,b). In most cases HW birds have been found to produce heavier eggs (Harms et al., 1982; Lacin et al., 2008; Perez-Bonilla et al. 2012a,b; Muir et al., 2022), which was also observed at 69 WOA in this study. However, at a younger age (24 WOA) the EW of HW birds was not different to EW of LW birds. Further, while being numerically higher, there was no difference in 66 to 70 WOA EW of the HW and LW egg quality focal birds. The lower number of replicates for assessment of EW with the focal birds, that is 24 birds / BW group, as opposed to 110 birds / BW group for EW measured at 69 WOA, likely contributed to this difference in observations of EW.

The significant improvement in EM and FCR (g feed / g EM) at 24 WOA with HND diet treatment from 18 to 24 WOA, corresponds with the linear improvement with increasing diet density across the 19 to 70 WOA dietary treatment period reported by dePersio et al. (2015) and improved FCR throughout a 23 to 40 WOA production phase observed by Ribeiro et al. (2014). Diets of higher energy concentration generated higher EM and improved FCR (g/g) across 24 to 59 WOA (Perez-Bonilla et al., 2012b) but differences in dietary crude protein and fat content (Perez-Bonilla et al., 2012a) did not affect EM and FCR when evaluated between 22 and 50 WOA. The provision of diets of different nutrient density across varying production periods have typically resulted in improved FCR but with varying effects on FI, EP, and EM (Pell and Polkinghorne, 1986; Wu et al., 2005; Khatibi et al., 2021; Scappaticcio et al 2021; Muir et al., 2022). In the current study the relatively short dietary treatment period during early lay (18–24 WOA inclusive) did not generate any change in EM nor FCR later in lay at 69 WOA (Table 3). Similarly, the cumulative EM and FCR across 18 to 69 WOA (Table 4) was not altered by early-lay dietary treatment.

While initial BW did not alter EM at 24 and 69 WOA, LW birds had improved FCR at 24 WOA which was still evident (P = 0.054) at 69 WOA. Similarly, the cumulative (18–69 WOA) FI and EM was higher in HW birds but their cumulative FCR across that same time was notably poorer than the LW birds (P = 0.053). Between 35 and 41 WOA, improved FCR was also observed in LW ISA Brown hens, compared to HW hens (Akter et al., 2019). Higher EM generated by HW hens did not follow through to impact FCR (g/g) (Perez-Bonilla et al., 2012a,b) across the previously mentioned production periods for Lohman Brown and Hy-Line Brown birds, respectively. But in both studies an improvement in FCR based on kg feed/dozen eggs were achieved by the LW hens. In overall agreement with the current study Harms et al. (1982) and Lacin et al. (2008) found LW hens produced lighter eggs, consumed less feed / day, and achieved lower FCR than HW birds.

The shell quality, assessed as eggshell thickness and eggshell breaking strength, was superior in hens that had received the early-lay HND compared to the LND diet (Table 5). Interestingly other studies have not identified a positive association between the eggshell breaking strength and a more nutrient dense diet (dePersio et al., 2015; Scappaticcio et al., 2021). Diet density was not seen to affect shell thickness (Khatibi et al., 2021) and similarly shell thickness and shell density did not differ due to dietary crude protein and fat content in isocaloric diets (Perez-Bonilla et al., 2012a). Other studies have assessed shell quality through percent shell weight. This did not differ with dietary treatments in our current study despite the differences in shell thickness and eggshell breaking strength. Both Ribeiro et al. (2014) and Wu et al. (2005) found similar percent shell weight with diets of a variety of energy levels. A decrease in relative shell weight with increasing diet nutrient density has been reported (Perez-Bonilla et al., 2012b) as has an increase in relative shell weight with increasing nutrient density in the diet (Khatibi et al., 2021). Considering these variable effects of dietary content on eggshell quality the improved shell strength and shell thickness due to diet density in our study is difficult to explain. Furthermore, mechanisms behind the observed higher shell thickness and breaking strength are not elucidated through the percent shell ash, and eggshell quantity of Ca, P, and other minerals Na, K, Mg, S (data not shown) which were similar in the two dietary treatments. However, given the improved shell thickness and breaking strength achieved with the HND diet used in this study a reduction in the loss of eggs through shell breakages and cracks (Mertens et al., 2006) would be expected.

Initial BW did not alter eggshell features of shell thickness, strength, percent weight, ash, Ca, or P content in this study. In a similar vein shell thickness did not differ with initial BW (Lacin et al., 2008; Perez-Bonilla et al., 2012a): nor eggshell density (Perez-Bonilla et al., 2012a) or shell strength (Lacin et al., 2008). However, a decrease in relative shell weight with higher initial BW was identified by Perez-Bonilla et al. (2012b).

The internal features of eggs produced by the focal birds including EW, HU, yolk color, and percent albumen weight, percent yolk weight did not differ due to hen BW nor diet nutrient density during early lay (Table 5). There is considerable variety in the effects of BW and diet nutrient density on internal egg quality. For example, Lacin et al. (2008) observed higher HU in LW hens, but Bish et al. (1985) identified that differences in the effect of BW on HU changed with the stage of production, with HW birds generating higher HU at 36 WOA but not at 48, 60, or 72 WOA. Perez-Bonilla et al. (2012b) found no differences in HU with BW but decreased HU with increasing dietary energy. Similarly, Wu et al. (2005) reported lower HU with higher dietary energy content, while Ribeiro et al. (2014), Khatibi et al. (2021) and Scappaticcio et al. (2021) found no effect of nutrient density on HU. Similar contradictions have been reported for yolk color. Diets of lower nutrient density have tended to generate higher yolk color (Muir et al., 2022) but Perez-Bonilla et al. (2012b) found the opposite. Both Muir et al. (2022) and Perez-Bonilla et al. (2012b) reported no differences in yolk color due to BW in ISA Brown and Hy-Line Brown hens respectively, but Lacin et al. (2008) identified an increase in yolk pigmentation in heavier, compared to LW Lohmann White hens. The absence of an effect of BW on the percent egg albumen and yolk in the current study likely reflects the similar weight of eggs produced by the focal birds, rather than the more typical differences of lower percent albumen and higher percent yolk weight, in heavier eggs produced by HW hens (Perez-Bonilla et al., 2012b). However, it should be noted that in the current study the percent weight of the albumen and yolk followed this pattern with both approaching significance due to BW. Further, as with the current study Wu et al. (2005), Perez-Bonilla et al. (2012b), and Scappaticcio et al. (2021) found no effect of the nutrient density of the diet on percent egg albumen and yolk.

Despite BW differences breast scores, % weight of the fat pad and % liver weight did not differ. However, relative liver weight was highest (P = 0.051) in LW birds on LND and lowest in LW birds on the HND diet. In contrast, Akter et al. (2019) identified higher % fat pad and liver weight in HW compared to LW hens. But it should be noted that this was observed when hens were 45 WOA as opposed to the 70 WOA observations in the current study. Earlier in the production period (50 WOA), the HW birds and birds that had received the LND diet during early lay had higher FLHS and liver lipid peroxidase (Muir et al., 2022). At 70 WOA, the FLHS scores and liver lipid peroxidase levels were generally higher than at 50 WOA, but similar for all treatment groups.

Keel curvature was also not different between treatment groups at 70 WOA. The femur of the HW birds was heavier and longer with higher total ash, but not percent bone ash than that of the LW birds. As mechanical load stimulates bone formation, (Fleming et al., 2006; Rodrigues-Navarro et al., 2018), it follows that higher hen BW generates heavier bones (Alfonso-Carrillo et al., 2021). In the current study BW did not impact cortical thickness, medullary bone diameter nor breaking strength, but was tending to higher femur diameter in HW birds, at 70 WOA. The HW birds had higher femur weight to length ratio, indicating higher bone density. A comparison of layer strains of different BW also found a tendency for bones of higher density in heavier strains (Skomorucha and Sosnówka-Czajka, 2021). In older hens (105 WOA), tibial breaking strength correlated with percent bone ash and cortical thickness (Alfonso-Carrillo et al., 2021). The absence of significant differences in these parameters due to either BW or diet nutrient density in the current study aligns with the similar femur breaking strength reported for all treatment groups. Overall, at 70 WOA the integrity of the femur and keel were similar for LW and HW birds and for birds receiving HND and LND diets during early lay.

From this study it is evident that events of early lay can have a notable influence on fresh egg production and hen health through to late lay. Bird weight at 18 WOA influenced ADFI and EW, both being higher in HW birds than LW birds at 69 WOA but did not alter hen-day EP nor EM. The LW hens at POL remained LW throughout the study and had the lower FCR at 69 WOA. Across the 18 to 69 WOA egg-production period LW hens had lower cumulative FI and EM and more favorable FCR compared to the HW hens. Further LW hens that received the HND diet from 18 to 24 WOA had the numerically lowest cumulative FCR to 69 WOA. Notably eggshell quality was superior in hens fed the HND diet early in the laying period compared to those that received the LND. With no differences in bone breaking strength and liver health, the production characteristics of the LW hens illustrate their suitability for egg production up to 69 WOA. Additionally, the features of the LW hens indicate a solid base from which they could continue through an extended laying cycle to 100 WOA. Based on these findings an evaluation of the egg production, egg quality, and health of LW hens in a longer production cycle is worthy of investigation.

CONCLUSIONS

The heavier birds at 18 WOA remained heavier than the 18 WOA LW birds, to 70 WOA. The heavier birds also had higher ADFI, higher EW but poorer FCR at 69 WOA. Between 18 and 69 WOA the HW bird cumulative FI and EM were higher than LW birds, but the LW hens had the lower cumulative FCR while producing a similar number of eggs across this time. At 24 WOA (final week of the dietary treatment period), birds receiving the HND diet had higher BW, EW, and EM, with reduced ADFI and lower FCR compared to birds on the LND diet. During late lay hen-day production, EW or EM were not altered by the provision of a HND diet during early lay but the HND diet generated thicker eggshells with higher shell breaking strength. No differences were found in bone strength, measured through keel curvature and femur breaking strength, nor in liver health determined by fatty liver hemorrhagic syndrome and liver lipid peroxidase when birds were 70 WOA. These findings support the hypothesis that LW hens can sustain efficient productivity to late lay. Furthermore, the cumulative FCR of LW hens was improved by feeding a HND diet during early lay, which also improved eggshell quality.