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. 2024 Nov 26;104(1):104596. doi: 10.1016/j.psj.2024.104596

Quality assessment of spent laying hens and analysis of influencing factors

Runzhe Wang a, Jiahui Lai a, Guiyun Xu a, Chuanwei Zheng a,b, Zhiqiong Mao a,b, Jiangxia Zheng a,⁎
PMCID: PMC11665342  PMID: 39631281

Abstract

In China, a considerable number of spent laying hens are disposed annually, with the majority being processed into various food products. The sale of spent laying hens has become a significant revenue stream within the poultry industry. We selected spent laying hens at 100 weeks of age as the research subjects to explore the factors influencing their market price. Information was gathered through product and enterprise investigations. Thereafter, 90 Rhode Island Red laying hens, also at 100 weeks old, were acquired for slaughter. The carcass evaluation was conducted utilizing the methodology prescribed by the United States Department of Agriculture (USDA). Tibial specimens were procured for the purpose of quantifying their length, mass, diameter, maximum breaking strength, maximum breaking distance, as well as for determining the concentrations of calcium, phosphorus, and ash content. Through comprehensive market research and industry surveys, this study found that the primary factor affecting the selling price of spent laying hens is body weight, with 1.5 to 2 kg of chickens being preferred. The USDA quality assessment standards for post-slaughter evaluation, it was found that the incidence of dislocation was 11.1 %, which emerged as a primary factor affecting carcass quality, directly resulting in a decline in quality. The results of the correlation analysis showed that there was no significant relationship between the bone health status and dislocation in laying hens. Dislocations occur mainly due to direct external forces during the culling process caused by the injuries during the slaughter process. Surveys show that the main factor affecting the selling price of spent laying hens is body weight, and the optimal weight range is where farmers gain price advantages during the culling process. The principal factor impacting the carcass quality of spent laying hens is dislocation, which are mainly caused by excessive culling and have no obvious correlation with bone health.

Keywords: Spent laying hen, Economic research, Dislocation, Bone parameter, Animal welfare

Introduction

In recent years, with the continuous improvement of people's living standards and consumption concepts, eggs are rich in nutrition, easy to cook, highly digestible, and inexpensive, and have gradually become one of the highest-quality sources of animal protein for food (Pan and Yan, 2022). With the rapid development of China's economy, poultry production reached 35.46 million tons, with an average annual growth of 1.4 % (National Bureau of Statistics of China, 2023; Zhu, 2023). In 2023, the inventory of commercial laying hens in China exceeded 1.6 billion, representing a year-on-year increase of 13.82 %. The total number of broilers marketed was 13.022 billion, with spent layer hens accounting for approximately 11 % of the total (Ling, 2023; National Bureau of Statistics of China, 2023). Concurrently, the global stock of laying hens has surpassed the milestone of 5 billion birds (Egg, 2022). However, the rapid growth in production was accompanied by a decline in profits, and the average price of eggs in China fell to USD 1.29 per kilogram, a year-on-year decrease of 9.3 % (Mysteel, 2024). At the same time, feed prices have risen, and the profitability of laying hen farming needs to be solved urgently (Sun et al., 2022). Economic accounting research related to poultry farming shows that the sale of spent laying hens is the second-largest source of income for the egg industry (Bo, 2023). Therefore, ensuring the quality of spent laying hens after the laying period and further exploring their potential value are of great practical significance for stabilizing industrial development and creating new sources of income.

Spent laying hens are used in various ways in different countries and regions around the world. In the United States and the European Union, most spent laying hens are euthanized on-farm or in processing plants and then buried, composted, incinerated, or processed into oil and protein meal for use in animal feed or pet food (Kersey and Waldroup, 1998; Newberry et al., 1999). In countries such as South Korea, India, Thailand, and Brazil, spent laying hens are processed into a variety of products, including chicken soup, snacks, and processed meat products. Particularly, spent hens in India are further processed into a variety of products or snacks, such as sausage, cutlet, jerky, kachori, chicken nuggets, patties, and tikka, among others (Souza et al., 2011; Kumar et al., 2015; Sabikun et al., 2021). In China, old laying hens have long been considered a nutritious traditional food source. For example, stewed old hen soup is a dish prepared specifically for pregnant women, the elderly, and the sick, highlighting its status as a nutritious traditional food (Chen, 2011). The most common destination for spent laying hens is a variety of cooked food products, including roasted chicken, braised chicken, and fried chicken.

Due to the long rearing cycle, spent laying hens have tough meat, rich protein content, low fat content, and are rich inω-3 polyunsaturated fatty acids that are beneficial to health. These characteristics give them unique nutritional value, high-value utilization, development potential (Barido and Lee, 2021). However, due to the small weight and poorer tenderness of spent laying hens, they are difficult to cook. They often have to be processed before they can be put on the market. They are usually sold as whole chickens or processed into foods such as roasted chicken, braised chicken, and fried chicken (Egg, 2022). Therefore, the integrity and carcass quality of live chickens when they are slaughtered have a great impact on the selling price. In addition, due to the high awareness of the ice-chilled chicken market in China, a small number of spent laying hens still enter the market in the form of ice-chilled chicken. However, due to the vast territory of China and the different eating habits in different regions, consumers in different regions also have different preferences for chicken. In southern markets, consumers generally have certain preferences for comb size, eye color, shank length, shank color, and skin integrity. These preferences often show regional characteristics, and the abovementioned specific traits will have a certain impact on the sales price in the terminal market. Therefore, the appearance and carcass quality of spent laying hens directly affect their economic value, and it necessitates research in this area. Hence, the objective of this study is to identify the primary carcass quality factors that influence the market price of spent layer hens through market research. Additionally, the study aims to determine the key factors affecting carcass quality during the slaughter and processing stages. Furthermore, it seeks to establish whether there is a correlation between the tibial bone quality parameters and overall carcass quality.

Materials and methods

Survey and analysis of the current quality status of spent laying hens

In terms of products, to understand the types of whole chicken products sold to consumers, we collected information on different net content weights of whole cooked chicken products from 50 stores on major Chinese e-commerce platforms such as Taobao, JD.com, Tmall Supermarket, and Pinduoduo. This data was gathered to gain insights into the main types of whole chicken products currently on the market, as well as their weight and other parameters.

In the context of live poultry, we conducted a survey by selecting one representative entity each from the categories of “breeding chicken farms, commercial egg sales companies, and live poultry slaughterhouses.” The selected entities are as follows: Beijing Zhongnong Bangyang Egg Chicken Breeding Co., Ltd., located in Beijing, which houses over 100,000 laying hens, was chosen as a representative of the northern market; Hubei Shendan Healthy Food Co., Ltd., situated in Hubei, China, primarily engages in the processing and sales of egg products. Its processed products are distributed in more than 10,000 supermarkets across China. This company was selected as a representative of the southern market; Hebei Dingxing Shuangxi Agricultural Development Co., Ltd., which specializes in the slaughter of spent layer hens for the regions of Tianjin, Beijing, and Hebei, with an annual production capacity of approximately one million birds.

Cost and profit analysis of laying hens

We selected a poultry farm in Beijing to collect data on the rearing of layer hens up to 100 weeks of age. In terms of costs, we primarily gathered data on chicklets, feed, vaccines, utility expenses (water and electricity), depreciation of equipment and cages, and employee wages. The profitability of layer hen farming in China is mainly derived from the sale of eggs and the disposal of spent hens. For the statistical analysis of egg production, we estimated the daily egg output during the rearing period. The egg production value was calculated as follows: (daily egg production * average selling price) / average egg weight. Additionally, the paper collated market data for spent laying hens starting in January 2023, using the examples of two agricultural economic internet platforms: the Spent Laying Hens Quotation Website and Zhuochuang Information. Refer to Karaman et al.'s method to collect the above data for the cost accounting of the elimination of laying hens (Karaman et al., 2023).

Ethical statement on animal experimentation

All research involving animals is conducted in accordance with the Regulations on the Administration of Laboratory Animal Affairs (Ministry of Science and Technology of the People's Republic of China; revised in June 2004). Samples were collected in accordance with the experimental animal guidelines formulated by the Animal Conservation and Utilization Committee of China Agricultural University (License No. AW51404202-1-3).

Animal management and living environment

In the present study, we selected 90 healthy Rhode Island Red hens at 100 weeks of age to undergo carcass scoring (the specific scoring method is detailed in the following section). The laying hens were reared in a well-ventilated, hygienic chicken house, with each bird housed individually within a cage (40 × 35 × 35 cm). The housing facility incorporated a natural ventilation system and a lighting program that combined natural and artificial illumination. The artificial lighting regimen entailed 16 h of light followed by 8 h of darkness, with a light intensity ranging from 18 to 22 lux. The temperature within the layer house was maintained at 18-21 °C, and the relative humidity was kept between 55-70 %. The diet provided to laying hens consists of a conventional complete formula feed.

Methods for evaluating carcass quality

Based on the findings from the survey in Section Survey and analysis of the current quality status of spent laying hens, it was discovered that the integrity of the spent laying hen's carcass is the primary factor determining the selling price of spent laying hens. Consequently, subsequent research was focused on this aspect. The evaluation criteria for carcass integrity referred to the standards outlined in the United States Department of Agriculture Poultry-Grading Manual.

Catching chickens and slaughter

To simulate the actual conditions of culling, the birds were captured by their legs and inverted before being placed into transport cages (122 × 56 × 104 cm), with each cage holding 8 chickens, the stocking density during transport was maintained at 11.7 individuals per square meter, followed by the slaughter process. The slaughter method involved electrical stunning to induce unconsciousness via water bath (with a voltage of 15 volts, a frequency of 650 Hz, and a stunning duration of 9 seconds), followed by exsanguination via cervical cut. The bleeding of the chickens was maintained for a period of 5 min. Following this, the chickens were scalded with water at a temperature of 60 degrees Celsius for 80 seconds before undergoing depilation, which lasted for 40 seconds. Thereafter, the carcasses were eviscerated to remove the viscera, decapitated, and the carcasses were subsequently washed. The pre-cooling of the carcasses was achieved using water at a temperature range of 0-2 degrees Celsius. Subsequent to these processes, a comprehensive carcass quality evaluation was conducted.

Analysis of influencing factors of dislocation

Based on the results of the chicken capture and slaughter procedures, it was observed that all instances of dislocation occurred exclusively in the wing region. Consequently, it was determined that dislocation is a primary factor affecting the integrity of the carcass and leads to a decline in quality. No issues of carcass contamination, skin damage, or missing parts were observed. Following these findings, the 90 chickens were divided into a dislocation group (Group D, n = 80) and a non-dislocation group (Group C, n = 10). Post-slaughter, a keel bone assessment was conducted, categorizing the curvature of the keel bone into four grades: normal, slightly curved, moderately curved, and severely curved. These grades were assigned scores ranging from 1 to 4, respectively, with 1 indicating normal and 4 indicating severe curvature (Zhang, 2022). Tibial measurements were conducted to determine the weight, length, major diameter, minor diameter, relative diameter of the tibia, and the breaking strength of the tibia (Toscano et al., 2013; Kolakshyapati et al.), 2019. Prior to measurement, leg muscles, cartilage, and the fibula were removed. Fresh, moist tibias were weighed using an YP5001 electronic balance (Shanghai Youke Instrument Co., Ltd., Shanghai, China). The length and diameters of the tibia were measured with a BK-302 digital caliper (Dangshan County Tonghe Measuring Tools Co., Ltd., Shanghai, China). The length of the tibia was measured from the intercondylar eminence to the most proximal point of the lateral malleolus, and the major and minor diameters were recorded at the midpoint of the line connecting the endpoints of the tibia. The coefficient of relative tibial diameter was calculated as (major diameter/minor diameter) * 100 %. The breaking strength of the tibia was measured using a food texture analyzer (Texture Analyser TA-XT Plus, Stable Micro Systems, UK). The left tibia was subjected to a three-point bending test until failure, following the method described by Toscanno et al (Toscano et al., 2013). Following the completion of the aforementioned measurements, 10 samples each from the normal and dislocation groups were selected to determine the contents of calcium, phosphorus, and ash in the tibia. These analyses were conducted by the Feed Valuation and Safety Testing Center of the Ministry of Agriculture and Rural Affairs (Beijing). The measurements were referenced against the national standards for the determination of calcium, phosphorus, and ash contents in feeds, which are GB/T 6436-2018 for calcium, GB/T 6437-2018 for phosphorus, and GB/T 6438-2007 for ash.

Statistical analysis

The data collected from the survey was analyzed using Rstudio version 4.3.2. The “tisyr” package was employed to clean up the data on the sales prices of spent laying hens across different regions and the average sales prices over various time periods. The monthly sales volumes of products with different net contents, gathered from e-commerce platforms, were categorized by net content. Subsequently, the “ggplot2” package was used to visualize the results of the data analysis.

The sample data collected were analyzed with the statistical software IBM SPSS Statistics 26 (SPSS 23.0; SPSS Inc., Chicago, IL, USA). A one-way analysis of variance (ANOVA) was conducted to explore the relationship between dislocation and the physicochemical properties of the tibia. Spearman's rank correlation analysis was further performed to investigate the interrelation between dislocation and each of the properties examined. The level of significance (α) was set at P < 0.05.

Results and discussion

Investigation and analysis of quality status of spent laying hens

This study compiled data from 50 stores on large Chinese e-commerce platforms such as Taobao, JD.com, Tmall Supermarket, and Pinduoduo in March 2024, revealing that the sales of four whole chicken processed foods were the highest: braised chicken, roasted chicken, pre-fried chicken, and old hen soup ready-meals. Consumers exhibited distinct weight preferences for different products (Fig. 1, A–D). Consumers of roasted chicken and ready-made old hen dishes prefer the highest net content, whereas those who purchase grilled and fried chicken show a preference for products with a net content of 0.5 Kg–0.75 Kg. Based on the net content of these products and the slaughter rate, the estimated live weight range of the poultry is between 0.8 Kg and 1.5 Kg (Kang et al., 2021; Park et al., 2021). Typically, the market weight for white-feather broiler chickens ranges from 2.2 kg to 3.5 kg (Qiu et al., 2021). It is hypothesized, therefore, that the products of roast chicken, grilled chicken, fried chicken, and old hen should be processed from the raw material of spent laying hens. According to a report by NetEase News, the sales volume of roast chicken dominated among the aforementioned products, with the annual sales of Dezhou braised chicken reaching 1 billion units, which is approximately in line with the number of spent laying hens in China (Er, 2020). For the common white broiler meat chickens, which have a high body weight (Lasekan et al., 2013), their rapid growth rate results in meat with a high moisture content. This leads to a loose texture and low toughness in the processed meat, making it unsuitable for the preparation of roasted chicken, grilled chicken, fried chicken, and old hen products. Consequently, spent laying hens in China has emerged as the optimal raw material for the processing of such food products (Abougabal, 2020; Wang et al., 2023).

Fig. 1.

Fig 1

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Market research on spent laying hens. Figures A-D show the monthly sales volume proportions of different full eviscerated weight (W) of braised chicken (A), roasted chicken (B), pre-fried chicken (C), and old hen soup ready-meals (D), respectively. (Braised chicken is typically prepared by marinating the whole chicken carcass with spices and simmering it in soup; the taste is predominantly salty and aromatic. Roasted chicken is prepared by marinating the chicken with spices and then roasting it in an oven or over charcoal; the taste is mainly spicy and sweet. Fried chicken is made by coating the marinated chicken with breadcrumbs and deep-frying it. Old hen soup is primarily consumed for its soup, which is made by stewing the chicken for a long time at high temperature until it becomes soft and falls off the bone, resulting in a richly nutritious soup with a savory taste, suitable for nourishment.) Due to the variations in packaging specifications for different products, the current figure was plotted based on the converted full eviscerated weight of laying hens for each product.

Secondly, the sales price of spent laying hens exhibits certain fluctuations over time (Fig. 2). The overall pattern of price volatility for spent laying hens throughout the year shows an increase in the first and third quarters followed by a decrease in the second and fourth quarters, which is consistent with the pricing patterns observed in previous studies (Liu, 2024). This is primarily due to changes in the market supply and demand for spent laying hens. Since major Chinese holidays such as Qingming Festival, Mid-Autumn Festival, Spring Festival, and National Day are predominantly distributed in the first and third quarters, these holidays somewhat influence the market price of spent laying hens.

Fig. 2.

Fig 2

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The average selling price of spent laying hens in China from 2023 to 2024. This table collects the prices of spent laying hens in the Chinese market from January 2023 to April 2024, with a weekly record. (These prices are the sales prices of farmers and not the final market prices), and they are converted to the exchange rate of RMB to USD as of April 9, 2024.

Taking an egg farm in Beijing as an example, the calculation of feeding cost and profit for a production cycle (100 weeks) of laying hens is shown in Table 1. It can be observed from Table 1 that feed costs constitute the majority of the overall farming expenses. Due to the relationship between feeding patterns and costs, the feeding costs in China are approximately 36 % higher than those in the United States (Li, 2018). Selling spent laying hens can partially increase the profit margin of the layer farming industry, with the profit contribution ranging from 50 % to 55 %. Currently, some large-scale production companies in China process and package spent laying hens for direct sale, allowing them to cater directly to the end market and gain a competitive price advantage. Conversely, some small-scale producers opt to sell live poultry directly, which is convenient and quick and also reduces processing costs.

Table 1.

Cost and profit statement of laying hens1 (100wks).

Parameters Per Birds ($) %
Chick 0.7 2.66
Feed2 22.78 86.48
Vaccine 0.7 2.66
Total variable cost (1) 24.18
Housing rental and management 0.2 0.76
Depreciation of hydropower and equipment 1.96 7.44
Total fixed cost (2) 2.16
Total cost (1 + 2) 26.34 100
Egg production value (3)3 31.41
Egg profit (3-1-2) 5.07
Spent laying hens production value4 2.5
Total profit 7.57
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1

The table shows the cost and profit of keeping a hen for 100 weeks. The format refers to the cost study of Turkey published by Karaman et al (Karaman et al., 2023).

2

The feed consumption during the extended laying phase is calculated based on a feed-to-egg ratio of 2:1. The total number of eggs produced is quantified at 450 units. the feed price is US $0.48 USD / kg based on the daily feed intake of 100g per chicken.

3

Egg production value = (Average egg production/Average eggs per kilogram) * Average egg price (2023).

4

The market price of laying hens is about 0.84-1.12 USD/kg, and the average weight of hens is 1.5-2 kg.

Carcass scoring results

To further simulate the impact of the handling and slaughter process on the quality of poultry carcasses for spent laying hens, we randomly selected 90 chickens to conduct a simulated culling and slaughter experiment. After slaughter, the carcasses were scored according to the United States Department of Agriculture (USDA) grading standards. The scoring results are presented in Table 2, with a total of 10 chickens exhibiting dislocation (all occurring in the wing region), including 2 with two or more instances of dislocation (one with unilateral dislocations at both the mid-wing and wing root joints, and the other with bilateral dislocations at the wing root joints), resulting in a dislocation rate of 11.1 %. In the process of broiler chicken slaughter, injuries, fractures, dislocations, epidermal damage, and intramuscular hemorrhages are the primary factors affecting the quality of chicken carcasses (Kong et al., 2022). Fractures and dislocations occur most frequently in the wing areas (Wu et al., 2022). This is primarily due to the trauma or stress-induced pain during the handling of chickens, which leads to struggles and subsequent dislocations (Werner et al., 2023). No issues of carcass contamination, skin damage, or missing parts were observed.

Table 2.

USDA score.

Factors A Quality B Quality C Quality Disjointed bone
Exposed Flesh 90 0 0 0
Disjointed and broken bones 88 2 0 0
Missing parts 0 0 0 0
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According to the USDA standards for dislocation scoring, a single dislocation without a fracture should be graded as A; two dislocations or one dislocation with one non-critical area fracture or one non-critical area fracture should be graded as B. Other than dislocations, no issues of carcass contamination, epidermal damage, or missing parts were detected.

The effect of bone mass on dislocation

In addition to body weight, carcass quality also influences the marketability of spent laying hens. This experiment found that dislocation was the primary factor leading to a decline in quality (with a dislocation rate of 11.1 %), while issues such as carcass contamination, skin damage, and missing parts were essentially absent. Therefore, the 90 tibial samples collected were divided into a dislocation group (Group D) and a non-dislocation group (Group C) for further analysis.

Univariate ANOVA experimental measurements of tibial traits of different carcass integrity (Table 3) revealed that in the traits of keel bone score, maximum breaking force, maximum breaking distance, and tibial weight, the control group (Group C) had higher values than the experimental group (Group D). Conversely, in the traits of major diameter (LD), minor diameter (SD), relative diameter (RD), calcium content, phosphorus content, and crude ash content, the control group (Group C) had lower values than the experimental group (Group D). However, the differences in the above results were not significant (P > 0.05).

Table 3.

Descriptive statistics of tibial characteristics of different carcass integrity.

trait group
 P Value
C D
KS 1.83 ± 0.4 1.67 ± 0.33 0.756
MBF 207.31 ± 17.01 189.44 ± 8.37 0.368
BD 2.11 ± 0.17 1.93 ± 0.08 0.368
TL 115.57 ± 0.65 113.7 ± 1.17 0.194
TW 14.52 ± 0.38 12.43 ± 1 0.080
LD 7.82 ± 0.31 8.03 ± 0.23 0.620
SD 6.37 ± 0.15 6.39 ± 0.17 0.950
RD 1.11 ± 0.01 1.13 ± 0.02 0.510
Ca(%) 23.58 ± 1.71 24.93 ± 1.04 0.515
P(%) 7.46 ± 0.53 7.71 ± 0.41 0.724
CA(%) 47.28 ± 3.35 49.72 ± 2.7 0.583
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C(Group C), D(Group D), KS(Keel score), MBF(Maximum breaking force), BD(Breaking distance), TL(Tibial length),TW(Tibial weight), LD(longest diameter), SD(shortest diameter), RD(Relative diameter), Ca(%)(Tibia calcium content), P(%)(Tibia phosphorus content), CA(%)(Crude ash content in tibia)

In the present study, the tibial fracture strength, calcium content, and phosphorus content in 100-week-old hens were found to be lower compared to the previously reported skeletal parameters of 70-week-old Rhode Island Red hens in the literature (Fleming et al., 1998). This discrepancy may be associated with the extended laying period, which could result in excessive mobilization of the hens’ calcium and phosphorus reserves (Kolakshyapati et al., 2019; Maidin et al., 2021; Yamada et al., 2021; Liu et al., 2022). Due to the prolonged laying period, the intestinal absorption of calcium and phosphorus in aged laying hens gradually decreases, leading to an increased reliance on skeletal-derived calcium (Gregory and Wilkins, 1989; Diana et al., 2021).

The results of the correlation analysis between the traits of the tibia in the dislocation group and the control group are presented as follows (Fig. 3). Using Pearson's method for rank correlation analysis, it was found that there was no significant correlation between dislocation and the various traits of the tibia (P > 0.05). Maximum breaking force (MBF), breaking distance (BD) and tibial length (TL) showed a significant positive correlation with tibial weight (r = 0.68, P < 0.05). MBF and BD also exhibited a significant negative correlation with the minor diameter of the tibia (SD) (r = 0.65, P < 0.05). There was a highly significant negative correlation between the major diameter of the tibia (LD) and total calcium content (Ca) (r = 0.71, P < 0.01). The relative diameter of the tibia (RD) showed a highly significant negative correlation with Ca (r = 0.82, P < 0.05). Total calcium content in the tibia showed a highly significant positive correlation with total phosphorus content and ash content (P < 0.01), with correlation coefficients of 0.91 and 0.80, respectively. These experimental results indicate that the thicker the tibia, the lower its hardness and the less calcium it contains, which contradicts previous studies. This discrepancy may be due to the excessive mobilization of calcium from the bones during prolonged egg production. During the mineralization process of the eggshell, the hen's skeleton provides approximately 20 %–40 % of the required calcium (Fu et al., 2024). During shell calcification, the activity of osteoclasts increases by ninefold, and once the medullary bone is depleted, osteoclasts will absorb structural bone, leading to changes in the strength and toughness of the bones and the onset of osteoporosis (Velde et al., 1984). Thicker bones often have a richer calcium reserve, and when the laying period is extended, these hens tend to have more persistent production performance, but they are also more prone to diseases such as osteoporosis and cage layer fatigue syndrome (Petracci and Cavani, 2012; Sinclair-Black et al., 2023).

Fig. 3.

Fig 3

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Correlation diagram of carcass integrity and skeletal traits. DS(Disjointed score), KS(Keel score), MBF(Maximum breaking force), BD(Breaking distance), TL(Tibial length),TW(Tibial weight), LD(major diameter), SD(minor diameter), RD(Relative diameter), Ca(%)(Tibia calcium content), P(%)(Tibia phosphorus content), CA(%)(Crude ash content in tibia).

In this study, there was no significant correlation between dislocation and the various skeletal parameters (P > 0.05), indicating that skeletal health does not directly lead to dislocation as a type of carcass damage but rather that dislocation is more likely to be caused by external factors such as violent handling during capture. During the process of culling chickens, human handling, transportation, and environmental changes during transport can all lead to stress in the chickens (Berry et al., 1990; Knowles and Broom, 1990). Physiological changes caused by stress can affect the quality of chicken meat after slaughter and, in severe cases, can lead to chicken death during transport (Adams and Rinne, 1982). Additionally, stress makes chickens more sensitive to external stimuli, increasing the likelihood of struggling and dislocating during handling. The use of the encircling method or the Swedish chicken-holding technique instead of the leg-grasping method during the capturing process can also reduce harm to the chickens. In the process of manual capture, the flapping and struggling of the wings due to the inability to escape can cause edema or fractures (Knowles and Broom, 1990; Wessel et al., 2022). Studies have found that during machine loading, chicken escape and wing flapping effectively reduce the occurrence of severe skeletal damage (Wolff et al., 2019). Moreover, an appropriate increase in the speed of the conveyor belt can reduce the likelihood of fighting, crushing, and collision among chickens during transport, thereby reducing the probability of injury (Werner et al., 2023). Saraiva et al. found that midnight capture due to factors such as lighting and stress often leads to more struggling by the chickens, making the chest, legs, and wings more prone to bruising. This bruising can cause chickens to flap their wings more vigorously on the slaughter line, leading to further damage (Saraiva et al., 2020). Additionally, the maximum breaking force and breaking distance of the bones reflect the strength and toughness of the bones, with the control group exhibiting better strength and toughness than the experimental group. The tibial weight of the control group was higher than that of the treatment group, suggesting that an increase in bone weight can reduce the risk of fractures and dislocations to some extent. This is because the muscles and ligaments attached to the bones become stronger with increased bone weight, providing an advantage in resisting external force damage (Proietto, 2020). In summary, during the culling process, choosing an appropriate method for capturing chickens and avoiding violent handling that causes carcass damage and stress can improve the quality of slaughter, while also considering the influence of tibial weight on the occurrence of dislocation.

Conclusion

The quality of spent laying hens is crucial for the profitability of the industry, with body weight and carcass integrity being the decisive factors for pricing. This study identifies dislocation as the primary factor affecting carcass integrity, which is predominantly caused by direct or indirect trauma during the culling process rather than being significantly associated with the skeletal health of the hens. Therefore, it is imperative to give due attention to the selection of appropriate culling methods to minimize this issue.

Disclosures

The authors declare that they have no known competing financial interests or personal relationships that could appear to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the China Agriculture Research Systems (CARS-40), the National Key Research and Development Program of China (2021YFD1200803), the National Key Research and Development Program of China (2022YFD1300100) and the Natural Science Foundation of Sichuan Province (Grant No. 2024NSFSC0389).

Footnotes

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.psj.2024.104596.

Appendix. Supplementary materials

mmc1.docx (20.7KB, docx)

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