Introduction

A better understanding on the fundamentals of poultry nutrition could lead to sustainable feeding strategies that meet the nutritional requirements of pullets and laying hens while reducing nutrition waste through excretion and keeping feed at a reasonable cost. Edible oil is commonly incorporated as an ingredient in the diets of laying hens, primarily as an energy source (Izuddin et al. 2022). Other than providing energy, dietary oil has the potential to improve diet palatability, feed intake and animal immunity, which contribute to better growth and production performance of laying hens (Saminathan et al. 2020). The main constraints on the use of edible oil in poultry diets are serious consumer concerns about the environmental impact of conventional plant-based oil production and the fact that its price can be prohibitive (Makkar 2018). Significant efforts have been made to find alternative vegetable oil sources for laying hens’ diets due to the increasing prices of conventional oil, which serves as an energy-rich feedstuff (Gao et al. 2022). Palm oil (Izuddin et al. 2022), rapeseed oil (Yuan et al. 2019), soybean oil (Ekine et al. 2020) and sunflower oil (Aguillón-Páez et al. 2020) are common vegetable oils included in laying hen diets.

Previous studies have emphasised the incorporation of crude palm oil (CPO) as a more cost-effective alternative source of energy than other types of vegetable oils in laying hen diets, with an optimum recommended inclusion level of 40 g/kg in the total ration (Areerob et al. 2019; Palomar et al. 2020; Saminathan et al. 2020). However, Choo et al. (1996) discovered that the residual oil (5-6% on a dry basis) derived from palm-pressed fibre, known as palm-pressed fibre oil (PPFO), which is a by-product of CPO extraction from oil palm mesocarp, contains more vitamin E (tocotrienols and tocopherols) and carotenoids than CPO. PPFO also contains a minimal amount of other phytonutrients, such as phytosterols, squalene, co-enzyme Q10, polyphenols and phospholipids (Hasliyanti et al. 2021). Blending PPFO with CPO, as palm oil millers commonly do to increase the oil extraction rate, can damage the quality of certain properties of CPO (Hasliyanti et al. 2021), which consequently discourages the use of these blended oils for human consumption. PPFO contains impurities such as gums, waxes, trace metals and free fatty acids, so blending it with CPO will deteriorate the original CPO quality. However, the use of PPFO in non-food applications, such as livestock feed, is worth exploring because of its high concentration of phospholipids (633 ppm) and free fatty acids (5.30%) (Teh et al. 2019; Sulihatimarsyila et al. 2020).

The fatty acid composition of PPFO is comparable to that of CPO, with the major fatty acids being palmitic and oleic acid (Teh et al. 2019), except for the linoleic acid content, which is slightly higher in CPO at 6% (Noorshamsiana et al. 2017). Nevertheless, a significant amount of lauric acid (7.12%) was expected to be detected in PPFO, which was absent from CPO, as suggested by Neoh et al. (2011). PPFO is also rich in vitamin E and carotenes, which are essential for laying hens and have the potential to improve their health, increase egg production rates and enhance egg nutrition. The vitamin E content of PPFO differs from that of CPO, and PPFO has higher levels of α-tocopherol, while CPO contains more tocotrienols (Norshamsiana et al. 2017). In addition, the price of PPFO is lower than that of CPO because the former oil is classified as sludge oil (Hasliyanti et al. 2021), which makes PPFO a cheaper energy source and a viable alternative to CPO in laying hen feed. Thus, our hypothesis was that the inclusion of nutrient-rich PPFO as a potentially cost-effective dietary lipid to substitute CPO in the layer diet could improve laying hen egg production and quality traits without negatively affecting the hens’ performance. To date, no study has been carried out on the utilisation of PPFO in livestock feed formulation, including laying hens. Thus, the aim of this study was to examine the effects of substituting CPO in the diets of laying hens with different concentrations of PPFO on their performance, egg production and egg quality parameters.

Materials and methods

Birds, dietary treatments and management

A total of one hundred fifty 16 weeks old of beak-trimmed commercial Hisex Brown layers with a mean body weight (BW) of 1484.60 ± 96.14 g (previously fed a commercial layer diet) were obtained from the local layer poultry farm and used in the experiment. The birds were reared in the Climate Control House (CCH), Feed Research Group, MPOB Keratong, using a battery cage under a photoperiodic lighting period of 16 h of total light (11 h of natural light with an additional 5 h of LED lighting) and 8 h of total darkness. The temperature in the CCH was in the range of 25 to 28 ºC, and the mean humidity was 70 ± 5% during the entire feeding trial. The experimental protocol for feeding, handling and care of hens had been approved by the Animal Ethics Committee of Universiti Kebangsaan Malaysia (UKM.PPI.AEC.800-4/3/1).

The birds were assigned to five treatments using a completely randomised approach, comprising five levels of PPFO with five replicates of six hens each (two birds per cage, three cages per replicate). All dietary treatments were isonitrogenous and isocaloric and formulated to meet the nutrient recommendations for Hisex Brown layers, according to the NRC (1994). The dietary composition and calculated nutrient analysis, as described by AOAC (2005), of the dietary treatments are shown in Table 1. The experimental diet, which contained 4.0% CPO (w/w), was considered a control (basal diet) dietary treatment. Treatment diets included: (T1) control, (T2) 25% of CPO replaced by PPFO, (T3) 50% of CPO replaced by PPFO, (T4) 75% of CPO replaced by PPFO and (T5) 100% of CPO replaced by PPFO. Thus, the dietary treatments contained 0, 1.0, 2.0, 3.0 and 4.0% PPFO (w/w) of diet. Throughout the experimental period, birds were supplied with fresh water and fed on an ad libitum basis. The feed used in this feeding trial was a complete diet in mash form. The laying hens were subjected to a four-week adaptation period from 16 to 19 weeks of age, followed by a 16-week experimental period from 20 to 36 weeks of age. The PPFO used in this study was obtained from Lekir Palm Oil Mill, Perak, Malaysia, which was extracted from fresh PPF with hexane by using soxhlet methods. Whereas the CPO was purchased from Selancar Palm Oil Mill, Pahang, Malaysia. The fatty acid composition profiles of PPFO and CPO used in this study are shown on Electronic Supplementary Material (Table S1). The major components of fatty acids found in both dietary oils are palmitic acid, oleic acid, stearic acid and linoleic acid.

Table 1 Dietary composition and nutrient content of the experimental diet
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Sampling and data collection

Each individual laying hen was weighed at 20 weeks of age (beginning of the experiment after the adaptation periods) and at 36 weeks of age (end of the experimental period) to determine the BW gain. During the experimental period, all eggs per cage were collected at 10 a.m. daily to determine the number and weight of eggs per replicate to calculate the average egg production for every 4-week interval (20–24, 25–28, 29–32, 33–36 weeks). The egg mass was measured by multiplying hen-day egg production with the average egg weight in the same replicate. Tolal feed intake (FI) of birds was measured on a weekly basis and was used to calculate the average FI at every 4-week interval (20 to 36 weeks of age). Then, the feed conversion ratio (FCR) was determined by dividing the total feed intake (g) by the total egg mass (g). A total of 30 eggs/treatment (six eggs/replicate) were randomly collected from laying hens throughout three consecutive days at the end of 28 and 36 weeks of age to measure egg quality traits.

Fifteen laying hens were randomly selected from each treatment (three hens/replicate) at the end of the experimental period, weighed and slaughtered after 16 h of feed withdrawal. The slaughter weight was determined after the laying hens were defeathered. Immediately, the skin colour of the upper left breast was measured at three locations using a chromameter (Model CR-400 Minolta, Ramsey, New Jersey, USA) to determine the L* a* b* colour indices, where L* represents lightness, a* denotes redness and b* denotes yellowness. Then, birds were dissected and internal organs of laying hens were removed and the weight of the hot carcass was measured. The weight of internal organs such as the heart, gizzard, spleen, liver and pancreas were determined immediately after the postmortem. Relative (to final body weight) organ weight was calculated as follows:

$$\:Relative\:organ\:weight\:\left(\%\right)=\frac{Weight\:of\:organ\:\left(g\right)}{Final\:body\:weight\:\left(g\right)}\times\:100$$

The hot carcasses were chilled at 4 ºC for 24 h and then reweighted to measure the cold carcasses weight. The cold carcasses were portioned into three parts: wings, breast meat (without adherent fat and skin) and whole legs (drumstick and thigh) and then weighed. The colour traits of breast and leg meat were determined directly using a similar technique to that used for measuring skin colour. The hue angle (hab) (°) and chroma values (C*) of skin, breast and leg meat were calculated using the a* and b* values (AMSA, 2012):

$$\begin{array}{cc}h_{ab}=\tan^{-1}\begin{Bmatrix}b^\ast\\a^\ast\end{Bmatrix}&C^\ast=\sqrt{{(a^\ast)}^2+{(b^\ast)}^2}\end{array}$$

Egg physical quality analysis

The specific gravity of eggs collected for quality traits was measured immediately after collection by using the standard gradational saline solution test method. Saline solutions with concentrations ranging from 1.060 to 1.100 were prepared at nine incremental concentrations of 0.005 (Holder and Bradford 1979). Other parameters, such as Haugh unit and yolk colour, were measured by using an EggAnalyzer® (ORKA Food Technology Ltd.). Eggshell thickness, typically with the membrane, was determined using a thickness measurement gauge (ORKA Food Technology Ltd.). The egg components, including eggshell, albumen and yolk were weighed separately. The weight of the yolk was measured by removing the albumen, whereas the eggshell weight was determined after the eggshell was oven-dried at temperature 80 °C until it reached a constant weight. The weight of the albumen was determined by subtracting the weight of the eggshell and yolk from the total egg weight. The relative yolk and albumen weights are expressed as a percentage of the total egg weight.

Statistical analyses

All data in this experimental study were subjected to a one-way ANOVA (analysis of variance) analysis as a complete randomised design using a general linear model procedure of the Statistical Analysis System (SAS) software package version 9.4, University Edition (SAS Institute Inc., Cary, NC). Duncan’s multiple range test was used to compare the significant mean differences among the experimental diets. The linear and quadratic effects of PPFO inclusion at different levels as a substitution for CPO were determined using regression analysis in SAS. The data were expressed as the least squares mean value with a pooled SEM (standard error of the mean). Mean differences were considered significant with a probability of P < 0.05.

Results

Growth performance

Effects of substituting CPO with different levels of PPFO on the initial and final BW, BW gain and feed intake of laying hens from 20 to 36 weeks of age are presented in Table 2. There were no significant differences (P > 0.05) in initial BW (at 20 weeks of age), final BW (at 36 weeks of age) or BW gain among the experimental diets. During the 16-week feeding trial, laying hens fed the dietary control (T1) showed a significantly higher (P < 0.05) feed intake compared with those fed diet T5 but no significant difference compared to diets T2, T3 and T4.

Table 2 Effects of crude palm oil substitution with different levels of palm-pressed fibre oil in laying hen diets on performance traits from 20 to 36 weeks of age
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Laying performance

The results for egg production performance parameters are presented in Table 3. At 20–24, 25–28 and 33–36 weeks of age, the egg number was not significantly (P > 0.05) affected by PPFO level in the treatment diets. However, egg numbers per hen were linearly affected (P < 0.05) by the PPFO levels in diets from 29 to 32 weeks of age. During the entire experimental period, laying hens fed diets containing 25% (T2), 50% (T3) and 75% (T4) PPFO substitutions showed significantly higher (P < 0.05) egg numbers per hen than those fed diets containing 100% (T5) PPFO substitution, but not significantly different than T1 (0% PPFO substitution). During the first four weeks, there were no significant egg production differences among the experimental diets (P > 0.05). However, the diet containing 100% PPFO substitution (T5) significantly decreased (P < 0.05) the egg production of laying hens from 25 to 28 and 29–32 weeks of age compared with the other treatments. Over the whole 16 weeks of the experiment, laying hens fed diets T2, T3 and T4 showed significantly (P < 0.05) higher egg production compared with T5, but it was not significantly different than those fed the T1 diet.

Table 3 Effects of crude palm oil substitution with different levels of palm-pressed fibre oil in laying hen diets on laying performance from 20 to 36 weeks of age
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During the periods from 20 to 24, 25–28, 29–32, 33–36 and 20–36 weeks of age, the substitution of CPO with PPFO at any ratio in the layer diet did not significantly affect (P > 0.05) the egg weight trait (Table 3). From 20 to 24 weeks of age, different levels of PPFO substitution had no significant (P > 0.05) effect on egg mass, but they exhibited linear effects (P < 0.05) during the subsequent 4-week and entire feeding trial period. In comparison to other treatments, the egg mass of birds fed 100% PPFO substitution (T5) significantly decreased (P < 0.05) over the entire experiment. The substitution of CPO with PPFO in the diets had no significant effect (P > 0.05) on the FCR of laying hens over any specific 4-week period or during the entire period of the feeding trial (Table 3).

Carcass quality traits

The effects of dietary treatments on their carcasses’ characteristics and relative weights of internal organs are shown in Table 4. The slaughter, hot and chilled carcass weights, breast meat, leg and wing weights and percentage of birds fed different concentrations of PPFO were similar (P > 0.05) among the treatments over the entire experiment. The relative weights of the liver, gizzard and pancreas were not significantly different (P > 0.05) among the dietary treatment groups. Nevertheless, the relative organ weight results indicate the significant effects (linearly, P < 0.05) of different PPFO levels on heart weight. Treatments T4 and T5 showed significantly lower (P < 0.05) relative heart weight compared to T2 and T3, but not significantly different (P > 0.05) than T1 (control). Significantly heavier (P < 0.05) spleen organs were observed in laying hens fed with diet T5 than in the T1 and T4 treatments, but it was not significantly different (P > 0.05) compared to that of the T2 and T3.

Table 4 Effects of different levels of palm-pressed fibre oil in laying hen diets on carcass quality traits at the end of the experiment (36 weeks of age)
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Laying hens skin and meat colour

Table 5 shows the effects of substituting CPO with different levels of PPFO on the skin, breast and leg meat colour of laying hens at 36 weeks of age. The skin colour of carcasses fed diets T3 and T5 recorded significantly higher (P < 0.05) lightness (L*) values than the T1 (control) and T2 groups, but they were not significantly different (P > 0.05) compared to T4. Higher lightness indicates a lighter skin colour in laying hens. Meanwhile, no significant differences (P > 0.05) were identified among dietary treatments for the redness (a*) of the carcasses’ skin. The skin of laying hens receiving diet T4 showed significantly more (P < 0.05) yellowness (b*) compared with that of the T1 and T2 groups, but not significantly different compared to groups T3 and T5. Laying hens fed T5 had higher (P < 0.05) hue angle values than those fed T2, T3 and T4, but they were not significantly different than the T1 group. Meanwhile, the chroma value of skin increased with PPFO substitution but exhibited linear effects (P < 0.05).

Table 5 Effects of different levels of palm-pressed fibre oil in laying hen diets on skin and meat colour at the end of the experiment (36 weeks of age)
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In this study, no significant differences (P > 0.05) were identified among dietary treatments for the lightness or yellowness of the breast meat. However, the redness of the breast meat increased linearly (P < 0.05) with the substitution of CPO with PPFO. In contrast, the hue angle value of breast meat significantly decreased (P < 0.05) with the substitution of CPO with PPFO. The inclusion of PPFO had no effect (P > 0.05) on chroma values of breast meat. Similar to the breast meat, the substitution of CPO with PPFO did not significantly affect (P > 0.05) the lightness or yellowness of the leg meat. T2 showed a significantly higher (P < 0.05) redness score of leg meat than T3 but no significant difference compared to the T1, T4 and T5 groups. The hue angle value of leg meat was significantly decreased (P < 0.05) with increased PPFO levels. Nevertheless, T2 exhibited significantly greater (P < 0.05) chroma values of leg meat in comparison to the hens fed diets T1 (control), T3, T4 and T5.

Egg quality traits

Shell thickness, Haugh unit, specific gravity, egg yolk weight or egg albumen weight of hens fed various levels of PPFO were similar (P > 0.05) among the treatments at the end of the 28th and 36th weeks of age (Table 6). In this study, significantly higher (P < 0.05) yolk colour scores were observed in experimental diets T4 and T5 when compared with T1, T2 and T3 treatment groups at the end of the 8th and 16th weeks of the experimental period.

Table 6 Effects of different levels of palm-pressed fibre oil in laying hen diets on egg physical quality at 28 and 36 weeks of age
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Discussion

The results of the present study demonstrate that feed formulation based on the substitution of CPO by PPFO, did not affect the growth performance of laying hens. However, the increase in PPFO levels in dietary treatments indicated a lower feed intake over the entire experimental period. The reduced feed intake may be associated with low palatability or appetite depression because of intolerance to the high inclusion levels of PPFO in the laying hen diet. It is likely that the efficiency of the dietary PPFO treatments was directly associated with the suppression of total feed intake by laying hens. However, the replacement of CPO with PPFO up to 75% yielded a similar feed intake to that of 100% CPO, indicating that PPFO could be used as a partial substitute for CPO. Similarly, a study conducted by Muangkeow (2011) using a different mixture of oils found no significant differences in feed intake between laying hens receiving diets containing a partial replacement of soybean with palm blended oil up to 75%, which is a relatively cheaper and more versatile edible oil compared to conventional oil.

Although the feed intake parameter indicated a statistically significant difference between the control (T1) and T5 treatment diets, substituting CPO with PPFO either partially or totally did not significantly modulate the final BW and BW gain of the hens (Table 2). The comparable growth performance observed in this study among the groups receiving different balanced dietary treatments might be explained by sufficient nutrients being obtained from the feed intake to meet the energy requirements for optimal growth, maintenance needs and egg production performance. Several studies of laying hens fed a diet containing an increasing amount of CPO have found improved growth performance (Oluyemi and Okunuga 1975; Areerob et al. 2019; Elsayed and El-Afifi 2021), even though the energy level was similar among the feed treatments. Nevertheless, this study has shown that PPFO – which is richer in valuable components, namely carotenes and vitamin E, despite being higher in phospholipids and free fatty acid content than CPO – can be used as an alternative to CPO without affecting the laying hens growth performance.

In this study, substitution of CPO with up to 75% PPFO (total of 3% PPFO in the experimental diet) demonstrated no effect on egg production or egg number per hen, but substitution with 100% PPFO (total of 4% PPFO) adversely affected egg production (Table 3). Hence, the impacts of adding PPFO to the diet of laying hens on egg production are not clear but presumably depend on the nutritional properties. The lower feed intake among laying hens fed more than 75% PPFO substitution seemed to reduce the efficiency of egg numbers and hen-day egg production. This study may not be able to entirely explain the observed unfavourable effects of adding 4% PPFO on laying hens’ performance. However, the birds in treatment group T5 may have received sufficient energy and nutrients from the feed for their body maintenance, but not enough for egg production. A significant reduction in feed intake leads to inadequate nutrient absorption, which is directly associated with lower egg productivity in layers (Khatibi et al. 2021).

In the present study, the egg weight trait was not significantly affected by including PPFO up to 4% in the diet (Table 3). The average egg weight was between 51 and 56 g. Iqbal et al. (2016) demonstrated that the egg weight produced is directly related to laying hens’ BW and egg size. Moreover, Ekinci et al. (2023) reported that as the BW of the hen increases, the egg weight increases linearly. Hence, in this study, we considered that the similar egg weights observed among the dietary treatments may have been due to the similar BWs (Table 2) and ages of the laying hens used. On the other hand, the lack of a significant difference in albumen and yolk weight, as shown in the egg quality trait analysis (Table 6), may also explain the insignificant differences among the dietary treatments in the egg weight parameter. Even though a numerically higher egg weight was recorded in T5 (Table 3) during 20–36 weeks of age, the lower egg production rate contributed to the lower egg mass.

During the entire feeding trial, the substitution of CPO with 100% PPFO had no significant effect on laying hens’ FCR (Table 3). A study carried out by Omidi et al. (2015) observed that different sources of oil did not show a significant effect (P > 0.05) on the FCR of 23-week-old laying hens (Tetra-SL). In the current study, the insignificant differences in feed intake and egg weight (except in the T5 group) could explain the similar FCR among the groups. In contrast, the reduced feed intake and higher egg weight in response to treatment T5 yielded a lower FCR, which makes this treatment similar in significance to the other diets. The cumulative FCR observed in the current study is lower when compared with that in the commercial Hisex Brown management guide’s 2.27 at 36 weeks of age (Hisex 2021). The lower FCR indicates that the laying hens more efficiently convert the feed into eggs (Macit et al. 2021; Anene et al. 2023).

There has been a growing interest in incorporating spent layer meat into the food chain; thus, providing more information about the meat quality or colour of spent hens can boost customer demand and producer profits (Petek and Çavuşoglu 2021). Throughout this experiment, the inclusion of different levels of PPFO did not affect the carcass composition traits of laying hens (Table 4). These findings indicate the possibility of supplementing optimal PPFO with up to 4% of the layer diets, as this does not adversely affect the yield of carcass parts or meat quality. This result might be expected because there were no significant differences in the final BW of laying hens fed different levels of PPFO (P > 0.05). Enlargement of organ weight can serve as an indication of the general health conditions of livestock, including laying hens (Semwogerere et al. 2019). Kurniawan et al. (2023) suggested that birds may react to feeds that contain exogenous antinutrients by interfering with nutrient absorption and metabolic processes, which may result in abnormal organ growth and relative organ weight. The present study indicates that a high level of dietary PPFO can affect laying hens’ relative weight of spleen and heart (Table 4). The avian spleen is an important immune organ that plays a vital role in immune responses and acts as the site for lymphocytes production and storage (Lewis et al. 2019). The presence of high concentrations of heavy metals (impurities) in PPFO, such as iron and copper, may have resulted in increased spleen weight, which is detrimental to the health of laying hens. This indicates that laying hens could tolerate up to 3% PPFO in their rations without exhibiting any adverse effects on their internal organs. Further research to elucidate the effects of substituting CPO with high levels of PPFO in laying hens’ diets on specific organs is warranted, as a lack of studies has been reported on PPFO utilisation in poultry diets.

The present finding showed that the yellowness of the skin increased linearly (P < 0.05) as the inclusion concentration of PPFO increased (Table 5), which could be attributed to the high carotenoid content in the PPFO being absorbed by the laying hens and deposited into the skin, as suggested by Wu et al. (2021). Poultry species, including laying hens, acquire carotenoid pigments through the type of feed matrix ingested. Typically, PPFO contains between 4000 and 6000 ppm of carotenoids, which is about six times higher than CPO (Sulihatimarsyila et al. 2020). The carotenoid pigments in PPFO, which are naturally red, orange and yellow, are known to be biologically active compounds that affect skin colour in laying hens. In a study of shelf-life evaluation, Nogareda et al. (2016) noted that the intensity of the yellow hue in broiler chickens’ skin is retained during storage, which is likely due to the strong antioxidant activity produced by a carotenoid-rich diet.

Substituting CPO with different levels of PPFO in the laying hen diets had no significant effect on lightness or the yellowness of breast meat. Nevertheless, the increase in the chroma value with a decreased hue angle value is interpreted to signify that feeding laying hens with PPFO produced meat with a more reddish hue (colour) and less vibrant skin colour (Semwogerere et al. 2019). Although the meat qualities of laying hens are a little-studied area, poultry meat colour is mainly affected by factors such as diet, bird age, strain, myoglobin content and pre-slaughter conditions (Albrecht et al. 2019). In this study, the redness of breast and leg meats increased significantly as the PPFO concentration increased (Table 5). This might be due to the high levels of antioxidants (carotenoids, tocotrienols and tocopherols) and phytonutrients (squalene and sterols) in PPFO. This characteristic of PPFO makes it possible to influence the pigmentation of laying hens’ meat. The changes observed in the skin and meat colours of the laying hens may not only be influenced by the increasing levels of PPFO in the diet but possibly also by other factors, such as the hens’ genetics (Wu et al. 2021).

Several studies have shown that optimal dietary lipid inclusion level at 40 g/kg in layers diets had no significant effect on egg quality traits such as albumen height and weight, Haugh units or shell thickness (Akter et al. 2014; Areerob et al. 2019). In line with previous research, the findings of this study revealed that hens fed various levels of PPFO had no significant effects on shell thickness, Haugh unit, specific gravity, egg yolk weight or egg albumen weight, except for yolk colour at the end of the 28th and 36th weeks of age (Table 6). This is because all dietary treatments provide similar amounts of key nutrients that meet the nutritional requirements of laying hens. Typically, specific gravity is a vital functional trait used to determine eggshell quality relative to other egg components and acts as an indicator of eggshell percentage (Ketta and Tůmová 2016). The higher amount of phytonutrients – namely vitamin E – in PPFO, which can act as antioxidants, has no influence on the specific gravity of the eggs produced. Similarly, Laganá et al. (2019) found that egg-specific gravity was not affected by supplements enriched with vitamin E in Hisex White Line diets. In the current study, the proportions of yolk and albumen were not affected by substituting CPO with different levels of PPFO, indicating that inclusion of different types of dietary lipids with various ratios in diets did not influence the relative weight of egg yolk and albumen, as noted by Pérez-Bonilla et al. (2011). On the other hand, egg shell thickness was not measurably affected by substituting PPFO for CPO, indicating that egg shell thickness most likely depends on supplementation of dietary minerals such calcium rather than just the inclusion of a lipid source. These findings are in agreement with those of previous studies (MacIntyre et al. 1963; Selim et al. 2018; Laganá et al. 2019).

Yolk colour is another key egg quality trait that determines consumer acceptance of the product, as people associate intense yolk colour with healthy, nutrient-rich food (Saleh et al. 2021). It was found that the yolk colour score generally increased as the substitution of PPFO increased but exhibited linear (P < 0.05) effects. The yolk’s natural colour is determined by the accumulation of carotenoids, which are deposited in the egg yolk by the hens. The quantity of carotenoids in the diet directly impacts the colour of egg yolks due to the inability of laying hens to produce the necessary pigments for egg yolk colouring (Ping and Gwendolin, 2006; Al-Harthi 2014). Increasing inclusion levels of PPFO, which are naturally rich in carotenoid pigments (approximately ten times higher than CPO), may be linked to the higher yolk score achievement in eggs from laying hens fed varying diets in this study. Moreover, PPFO contains a higher amount of pigmenting agents, such as lycopene and xanthophyll, which are lipid-like bioactive compounds derived from carotenoids (Sundrasegaran and Mah 2020) that are efficiently absorbed from the gut, transported to the ovarian follicles and deposited into the egg yolk, resulting in more intense egg yolk pigmentation. Synthetic colourants are commonly included in poultry feeds to improve the attractiveness of the meat and egg yolk colour. The current study showed that PPFO can be used as a natural colourant to intensify the colour of the yolk. Natural colourants are preferred in poultry diets over synthetic colourants, which are more expensive and may be harmful to the health of consumers (Downham and Collins 2000).

Conclusions

In conclusion, the substitution of PPFO up to 75% of CPO (at the level of 3% of the diet as a substitution for 4% CPO in the layer diet) served as a valuable natural, cheaper energy source for laying hens (20–36 weeks of age) but exerted no detrimental effects on their growth performance (BW gain and feed intake), egg production (egg number and egg weight) or egg quality traits with a significant elevation in egg yolk colour. However, the complete substitution of CPO with 4% PPFO in diets may have detrimental effects on laying performance. Overall, the findings of the present study suggest that the use of PPFO in up to 3% of laying hen diets as a novel, cost-effective, energy-rich and phytonutrient-enriched feed ingredient could produce beneficial effects on their performance. However, further research should be conducted to identify the limiting factors that prevent the application of more than 3% PPFO in laying hen rations.