All experimental protocols were authorized by the University of Veterinary and Animal Sciences (UVAS), Lahore Ethical Review Committee via letter number DR/985 and followed all laws and regulations. The experiment ran from week 17 until week 50 (34 wks).

The study was conducted at the Department of Poultry Production, UVAS, Ravi Campus, Pattoki. Pattoki is located at 31°1′ 0N and 73°50′ 60E with an altitude of 186 m (610 ft). This city experiences normally hot and humid tropical climate with temperature ranging from 05°C in winter and +45°C in summer.

In total, 150 commercial layer pullets (LSL lite) were procured from a flock that had already been raised at the Department of Poultry Production. Up to 15 weeks, this flock was kept under standard management practices. These pullets were divided into three groups at 16 weeks of age according to live weight using a completely randomized design and five replicates of ten birds each. At the time of photostimulation, the body weight categories used in this investigation were light (under 1,300 g), medium (1,300 to 1,375 g), and heavy (more than 1,375 g). The pullets were housed in independent open-sided laying house with east-west dimensions of 6.10× 6.10 m (37.21 m²) and 3-tier laying cages of 5.18×1.52 m (47.42 m²). To make the process of collecting eggs easier, the cages had a sloping wire floor. Ceiling fans, curtains, and other practical manual methods were used to regulate the ventilation, humidity, and interior temperature. A wet and dry bulb hygrometer (Mason’s type, Zeal, England) positioned in the middle of the home was used to record daily variations in temperature (°F) and relative humidity at 6:00 AM and 6:00 PM (Figure 1). To remove faeces, removable dropping trays were installed beneath the mesh floor. The birds were fed using detachable individual trough feeders that were installed outside the cage. At 6:00 AM, the birds were given a commercial laying diet (Table 1) with an allotment of 90 g/bird/d, and freshwater availability was guaranteed by the automatic nipple drinker system installed inside. The birds were given access to natural day light at the beginning of the experiment, and then the amount of light was increased by 30 minutes every week until a 16 L:8 D photoperiod was reached. The experiment lasted up to the age of 50 weeks.

Throughout the phases of rearing and production, the birds were housed in open-sided house. The birds were kept at their natural day length for the whole growing season (up to 15 weeks) because the flock was in-season, placed in the middle of April. The growth period’s natural daytime length was roughly 12 hours. The photoperiod was extended for photostimulation by 30 minutes per week until a total day length of 16 hours was reached. In the production phase, the birds were exposed to 40 to 50 lux of light intensity after sunset.

Analysis of collected data was performed using one-way analysis of variance (ANOVA) in SAS 9.1. Significant treatment means were separated through least significant difference test.

The statistical model used was:

Where; Y_ij = observation of dependent variable documented on ith treatment; μ = Population mean; t_i = Effect of ith treatment i.e. body weight at photostimulation (i = 1, 2, 3); Σij = Residual outcome of jthobservation in ith treatment NID ~ 0, σ².

The results showed that the heavy birds reached sexual maturity (age at first egg) earlier than the light and medium birds, whereas the light birds produced more eggs overall, gained more body weight, and had better FCR per dozen eggs as well as per kilogram of egg mass. However, compared to the medium and light birds, the heavy demonstrated higher egg mass (Table 2).

At 20 weeks of age, body weight categories had no effect on the characteristics of eggs that determine their quality, except for egg weight, eggshell weight, eggshell thickness, egg yolk index, and HU. However, at 50 weeks of age, significant effects of body weight categories on egg quality were observed (Table 3).

Egg geometry characteristics, except for egg shape index, did not differ significantly among body weight groups at 20 weeks of age, according to an ANOVA, but at 50 weeks of age, those differences became significant (Table 4).

At 20 weeks of age, there were no statistically significant differences between the groups in terms of bird welfare aspects such FPD and FC. However, compared to the medium and light birds at 50 weeks of age, the heavier birds had poorer footpad and better FC (Table 5).

At 20 weeks of age, the physiological responses of the birds were not significantly different for body temperature, heart rate, and respiration rate depending on body weight categories, but at 50 weeks of age, the light birds had higher values for rectal temperature, heart rate, and respiratory rate than the medium and heavy birds (Table 6).

Except for catalase, body weight categories had a strong impact on both productive and reproductive hormones (Table 7). At 20 and 50 weeks of age, heavier birds than medium and light birds displayed higher levels of triiodothyronine (T₃), thyroxine (T₄), GnRH, FSH, and LH. However, light birds had the greatest corticosterone levels, followed by medium and large birds (Table 7).

Cost considerations are important in the business of raising livestock, especially poultry. For instance, between 65% and 70% of the overall cost of growing poultry is attributable to feed costs. Poultry body weight plays a key role in feed intake, feed efficiency and other egg characteristics [9]. Similar to our findings, research has shown that heavier relative to lighter body weight layers consume more feed [10]. This change may be the result of increased body maintenance requirements.

There are many elements that influence the economics of chicken production, but two of them—faster growth and effective feed conversion—play a significant part in the economics of the poultry industry. Lacin et al [9]’s observation that the body weight of the laying hens considerably impacts the FCR corroborated the relationship between the FCR and live body weight in the current research.

When it comes to poultry birds, the age at sexual maturity is crucial. According to Pishnamazi et al [11], the only factor that affected differences in body conformation at sexual maturity was body weight. In comparison to lighter-weight poultry birds, heavier-weight birds reach sexual maturity sooner [12]. The plasma estradiol-17β content may be the cause of this variance. According to Triyuwanta et al [13], variations in egg production and egg weight have been linked to variations in the body weight of layers during sexual maturity. It has been noted that heavier relative to lighter weight layers produce more eggs [14]. The plasma estradiol-17β content may be the cause of this variance.

Romero et al [15], who noted a positive correlation between hen body weight and its egg weight, support the positive association between body weight and egg weight in this research. It has been noted that heavier body weight layers tend to lay heavier eggs than lighter weight layers [10]. This difference may be brought on by growing ova and secreting more albumen. Specific gravity is a sign of the quality and freshness of the eggshell because eggs with a stronger shell have a higher specific gravity than eggs with a thin shell. Eggs that have just been laid have a higher specific gravity than long and old stored eggs. In the current investigation, it was discovered that layers with lighter body weights had eggs with a higher specific gravity. Similar to this, it has been noted that layers with greater body weights have lower egg specific gravities than those with lighter body weights [16]. The relationship between increased body weight and eggs decreased specific gravity may be to blame. Mekky et al [17]’s conclusion that pullets with larger body weight laid eggs with increased eggshell weight and eggshell thickness as compared to layers with lighter body weight supports our findings regarding eggshell weight. This could be explained by the increased calcium deposition in eggshells caused by heavier body weight, which results in higher egg weight.

The calculation of egg geometry is crucial to the poultry business. It has been noted that strains with heavier body weights tend to lay eggs with greater egg length and width [18]. The egg geometry has been shown to be significantly influenced by body weight [9]. It has been noted that layers with lower body weight than medium and overweight have a lowered egg shape index [19]. This change may be brought on by layers’ advancing age and weight. In a similar manner, larger egg weight may have contributed to the overweight group’s highest values for egg volume and surface area.

Footpad dermatitis is a financial and welfare indicator for poultry. Among layers that have been produced commercially, FPD is a frequent and complicated problem. Weight of the bird is one of the causes of FPD [20]. When compared to other body weight groups in the current investigation, FPD was considerably higher in the overweight birds. Shepherd and Fairchild [21], who noted a severe FPD incidence in broiler chicks with high body weight, provide support for these findings. In a similar vein, Hocking and Wu [22] found that FPD was higher in layers with heavier body weights compared to layers with medium and lighter body weights, demonstrating a substantial correlation between FPD and body weight.

For the welfare of hens and as an economic production criterion, feather score is crucial. In the current study, bigger body weight layers exhibited improved FC compared to medium and lighter body weight layers. These findings are further corroborated by Kiani and von Borstel [23], who found that larger laying hens had superior back FC than lighter and medium-weight hens. This might be explained by heavier bodyweight layers producing more eggs and having higher plasma estradiol-17 concentrations [4].

Human literature has long documented the consequences of a high body weight, such as that caused by obesity, on respiratory and physical functioning as well as the vascular system. Large-bodied, productive laying birds were found to have a higher respiratory rate. This accelerated respiration may be a compensatory response to metabolic acidosis during the production of eggshell CaCO3 [24]. In the present trial, a highest body temperature was recorded in light birds. Similar to this, Ghayas et al [25] found that lighter broilers had higher rectal temperatures than heavier broilers. This can be the outcome of elevated T₃, which might have contributed to elevated heart rate in layer.

Triiodothyronine (T3), a plasma thyroid hormone, is important for promoting broiler growth and for accelerating the compensatory growth of birds. Hassaan et al [26]’s revelation of a higher T₃ level in heavier hens as compared to lighter hens supports the current findings. This might be caused by variations in the hormonal composition or metabolic processes between lines. The current study’s heavier birds had greater FSH and LH values at 20 and 50 weeks of age than the medium and light birds. Similar to the current findings, greater FSH and LH levels upon phostimulation have been seen in heavier compared to medium and lesser body weight broiler breeder groups [27]. This variation might be due to body mass, because FSH helps in fat accumulation. The main glucocorticoid secreted in poultry chickens under stress is corticosterone (a stress hormone, in all vertebrate classes); it is the byproduct of the activation of the hypothalamic-pituitary-internal axis and has long-term effects on stress adaption. According to Huff et al [28], varied body weights have different levels of corticosterone, such as heavier and lighter poultry birds. In the current investigation, lighter body weight layers had higher serum corticosterone concentrations than other body categories. Similar to this, it has been noted that birds with lighter body weights have higher corticosterone levels than those with bigger body weights [29]. The metabolic rate may be the cause of this discrepancy.

Hydrogen peroxide (H2O2) is broken into water and oxygen by the enzyme catalase, which is present almost universally in all living things, exposed to oxygen and prevents oxidative cell damage. In the current research, birds with higher bulk had the highest catalase concentrations. Jena et al [30] findings’ that catalase levels are greater in birds with heavier body weights than in birds with lesser body weights are consistent with these findings since catalase concentration rises with age and body weight.