1. Introduction
Light is a critical extrinsic factor that controls physiological and behavioral processes in birds, especially poultry, where birds are often grown in artificial light environments. The characteristics of light, such as intensity, spectrum (or wavelength), and photoperiod affect living organisms [
1], and are used as management tools to control behavior and improve the well-being and performance of poultry [
2]. Photoperiod, along with light intensity, is an important factor that influences the physical growth and reproductive development of birds. The photoperiodic regimens of broiler chickens can be manipulated to maximize feed intake and efficiency [
3].
Typical photoperiods include continuous light (24L), near-continuous light (23L:1D), intermittent light with repeating light and dark cycles, and step-up light with a gradually increasing photoperiod [
1]. Since rapid growth and high feed efficiency are important parameters for the productivity and economy of the broiler industry, most broiler facilities use the continuous or intermittent light regimens that help maximize the feed intake and growth of broilers [
4,
5,
6]. Although conventional lighting regimens induce rapid growth and improve the performance of broilers, they can also cause several health effects and raise welfare concerns [
7]. Rapid growth increases the incidence of skeletal disorders and metabolic diseases, which may increase the mortality of broilers [
1].
Reducing the growth rate by controlling the photoperiod may reduce the incidence of skeletal and metabolic diseases. Classen and Riddell [
3] reported that a gradual increase in photoperiod from 6 to 23 h significantly reduced leg abnormalities in broilers, compared with those in a control group that received continuous light for 23 h; both groups exhibited similar performance parameters. Several studies have suggested the use of a more natural light pattern instead of the commonly used 23L:1D regimen to limit skeletal abnormalities in broilers [
6,
8,
9].
A moderate photoperiod (i.e., 16L:8D) is beneficial for the welfare of broilers, including reduced physiological stress, enhanced immune response, and increased sleep duration and physical activity [
9,
10,
11].
Darkness is as important as light for the growth and health of birds [
7]. Prolonged dark periods during rearing impede access to feed, which further leads to reduced feed intake and limited growth. However, dark periods provide sufficient resting time for the birds, allowing reduced stress levels that help their health [
4,
12,
13]. Rest and sleep serve vital functions, such as tissue restoration and growth, energy conservation, neurobehavioral performance, etc. [
14]. Animal welfare organizations, such as the Royal Society for the Prevention of Cruelty to Animals (RSPCA) [
15] and Animal Welfare Certification Standards (AWCS) [
16], recommend a minimum dark period of 6 h as the poultry welfare standard.
Stress parameters that can indicate animal welfare status include blood corticosterone, blood glucose, triglycerides, and the heterophils-to-lymphocytes (H/L) ratio. The incidence of footpad dermatitis (FPD) is also used as a welfare parameter in broilers reared in floor-houses [
13]. Animal welfare is a major issue worldwide, and measures to improve the welfare of chickens are implemented in the poultry industry. Although several reports have been published on the influence of lighting regimens on the performance and health conditions of broilers, only few studies involve a comprehensive analysis of parameters such as productivity, various stress indicators, and carcass characteristics. Therefore, the purpose of the present study was to compare the effects of different photoperiods on productivity, blood biochemical profiles, stress and welfare indicators, and carcass characteristics of broilers.
2. Materials and Methods
2.1. Birds, Housing and Experimental Design
Ross 308 male broilers (336, 1-day-old, body weight 48.0 ± 0.17 g) were used in this study. The experiment was performed in a floor-pen broiler house at the Poultry Research Institute (Pyeongchang, Korea) for 35 days. For the first 7 days, the chicks were reared under the same light intensity (30 lx) and photoperiod (22L:2D). From the second week, the chicks were randomly allocated to four treatments (4 replicates per treatment, 21 birds per replicate). The four treatments were 24 h continuous light (24L), 18 h continuous light (18L:6D), 8 h continuous light (8L:16D), and intermittent light (4L:2D). The 18L:6D and 8L:16D treatments were set based on AWCS of Korea [
16], which recommends a minimum dark period of 6 h and a maximum light period of 8 h.
The birds were provided with a corn-soybean meal-based commercial broiler starter diet (CP 22.5%, ME 3020 kcal/kg) for week 1, a grower diet (CP 18.5%, ME 3050 kcal/kg) for weeks 2–3, and a finisher diet (CP 18.0%, ME 3100 kcal/kg) for weeks 4–5. Throughout the experiment, the birds had ad libitum access to feed, and water via a bell-type water dispenser. An LED light bulb was used as the light source, and the light intensity was maintained at 35 lx.
2.2. Growth Performance
For growth performance analysis according to photoperiod, body weight was measured on the initial (7-days-old) and final days (35-days-old) to determine the body weight gain and feed conversion ratio. The feed intake for each treatment was calculated by measuring the remaining feed at the end of the experiment. The feed conversion ratio was defined as the ratio of feed intake to weight gain during the test period.
2.3. Blood Sampling and Measurements
At 35 days, blood samples were collected from the wing veins of 12 birds (3 birds per replication, birds of similar body weight) selected from each treatment. Blood samples were analyzed for leukocytes, erythrocytes, and thrombocytes using a hemocytometer (HematVet 950; Drew Scientific, FL, USA). The biochemical composition of the serum was analyzed using a hematology analyzer (AU480 Chemistry Analyzer, Beckman Coulter Inc., CA, USA).
2.4. Cytokines and Corticosterone
Corticosterone and cytokine stress hormone levels were analyzed in the serum samples. Cytokine analysis was performed using tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) with the Chicken IL-6 ELISA Kit (MBS268769, MyBioSource, San Diego, CA, USA) and Chicken TNF-α ELISA Kit (MBS2509660, MyBioSource, San Diego, CA, USA), respectively. Corticosterone analysis was performed using a commercially available Chicken Corticosterone Kit (ECH0077, Wuhan Fine Biotech Co. Ltd., Wuhan, China), according to the manufacturer’s instructions.
2.5. Litter Moisture Content and Footpad Dermatitis
To determine the moisture content of litter, 40 g of litter was collected on the day before slaughter; dry weight measurements were collected after drying the litter samples for 24 h at 105 °C. Moisture content was expressed as a percentage (%) of the difference between the fresh weight and dry weight of litter samples.
The footpad dermatitis (FPD) score of the broilers was evaluated according to the criteria of Welfare Quality [
17]. The FPD score ranged from 0 to 4 points (
Table 1); 40 birds per treatment were evaluated during the final week. The FPD score was calculated as the average value of the degrees.
2.6. Carcass Yield and Carcass Cut Yields
At 35 days of age, 12 birds per treatment (3 birds per replicate) were selected for the measurement of processing performance. The selected birds were euthanized via CO2 asphyxiation and the carotid artery was subsequently cut and exsanguinated. The feathers, head, feet, and intestines were removed to measure the carcass yield. Carcass cut yields were measured by dividing the breasts, legs, wings, neck, and back. Carcass yield and carcass cut yields were expressed as a percentage of carcass weight and partial meat weight to the living body weight.
2.7. Physicochemical Properties of Breast Meat
Breast meat (
Pectoralis major) samples collected from 12 birds per treatment were used to analyze the physicochemical parameters. The samples were weighed and stored at 4 °C until 24 h postmortem. The chemical composition (moisture, crude protein, crude fat, and crude ash) of the breast meat was determined using the AOAC method [
18]. The pH was measured using a pH meter (pH-K21, NWK-Binar GmbH, Celiusstr, Germany) and color intensity (CIE L*, a*, b*) was measured using a colorimeter (CR301 Chromameter, Minolta Co., Japan), calibrated with a white standard plate (Y = 92.40, x = 0.3136, and y = 0.3196).
The shear force (SF), cooking loss and water holding capacity (WHC) of breast meat were analyzed according to method of Chae et al. [
19]. For the determination of SF, each sample (average weight, 61 g) was heated individually in a polyethylene bag immersed in a water bath at 70 °C for 10 min. The samples were then cooled at room temperature, and cores (diameter, 1.27 cm) were collected in the longitudinal direction of the muscle fibers. SF values were estimated using a Warner–Bratzler shear blade attached to a texture analyzer (TA-XT2, Stable Micro Systems Ltd., Surrey, UK). To measure the cooking loss, each sample was placed in a polyethylene bag that was then heated at 85 °C in a water bath for 45 min. After cooling at room temperature for 20 min, cooking loss was calculated as the percentage of weight loss after heating. WHC was calculated as a percentage of the difference between them by subtracting the free water generated by centrifugation and the total water in the meat. For free water, 0.5 g of a sample from which fat and fascia (tendon) were removed was placed in a tube, heated at 80 °C for 20 min in a water bath, and centrifuged at 448×
g for 10 min. WHC percentage was calculated as the value obtained by dividing the fat coefficient (the value obtained after subtracting the fat content from the sample, %, n = 12) by the weight before and after centrifugation.
2.8. Statistical Analysis
All data were analyzed using the general linear model (GLM) procedure of SAS software (version 9.4, SAS Institute, Cary, NC, USA). Duncan’s multiple range test was used to determine significant differences among treatments. Differences were considered statistically significant at p < 0.05.