Feed restriction strategies are common management practices in broiler breeders (4), but there are variations in their effects in broiler chicken productions (4, 16, 23). On this account, the current study clarifies the discrepancies of intermittent feeding effects on broiler chickens for commercial production purposes. Specifically, this study explores the impacts of intermittent feedings and fasting strategies on broiler chickens’ growth indices, histological and molecular mechanisms linked with WS phenotype in breast muscle meats.
The feed intake data obtained in the current study are consistent with previous research suggesting that restricted feeding may suppress feed consumption levels in broiler chickens (24, 25). Despite the reduced feed intake observed in our study, the alternative feeding strategies did not impair growth indices. During the feed restriction period, birds could easily adjust to changes in feeding patterns by temporarily storing feeds in the crop and subsequently prolonging digestion in the proximal region of the intestine (26, 27). These results could be attributed to an adaptive mechanism deployed over time by birds fed the alternative feeding strategies, which show improved performance than Velele (23) study that reported stunted growth in restricted fed broilers. Previous findings have also suggested the possibility of compensatory growth rate reprogramming during restricted feeding strategies in broilers production over a long period (28–30). Consistently, our results further reveal that intermittent feedings did not significantly decrease the breast meat yield at the market age, i.e., 42 days of age, even though the AD feeding had the highest weight for breast meat. This observation, in fact, corresponds with the final body weight across the groups and also aligns with Jahanpour et al. (31) that previously suggested that up to 75% of restricted feed regimens had a neutral or non-detrimental effect on broiler’s breast meat yield.
More importantly, consumers are concerned about breast meat quality, accounting for more than 67% of broiler carcass parts. Thus, the current study focuses on the impacts of chronic IF on breast meat quality related to the WS-associated myodegeneration (32, 33). Enlarged breast muscle fiber size, one of the precursors of WS development, has previously been linked with meat hardness and muscular defects such as fibrosis (34, 35). Several researchers agree with this proposition as observed in the AD group (36, 37). On this premise, this study shows that 1.5h-IF reduced the fiber diameter in the breast muscle region, which aligns with the meat quality criterion for improved meat tenderness. The idea has been that enlarged fiber size predisposes the breast muscle region to poor vascularization and lipidosis, consequently leading to metabolic disorders that characterize severe hypertrophic muscle in broiler pectoralis major (14). The reduced muscle fiber diameter in the 1.5h-IF group could promote vascular tone and ensure proper muscular development by increasing interstitial space for vascular tissues without significantly affecting relative breast weight in broiler chickens (38, 39).
Larger fiber sizes attributed to AD feeding predispose muscles to myodegeneration (32, 33). In the current study, the molecular analysis further supports the beneficial effects of intermittent feeding on meat quality compared to AD feeding. The analyses elucidated their roles on myogenic regulatory factors and inflammatory cytokines. The gene expression data revealed no significant differences among all the feeding strategies. These results suggest that chronic intermittent feeding had a similar regulatory effect on myogenic markers such as MYF5, MYOD, and PAX7 gene expressions (40). Both MYF5 and MYOD are involved in muscular cell growth and development (41), whereas PAX7, a skeletal muscle satellite cell, supports continuous cell proliferation for protein turnover in the pectoralis major region (42, 43). As revealed by the immunohistochemistry staining, PAX7+ proliferation density was significantly compromised by AD feeding. With this data, we could extrapolate that decreased PAX7+ density resulted in poor muscular differentiation and repair activities during the later phase of AD birds that caused hypertrophic muscle defects (41, 44). From the histology standpoint, IF might contribute to normal muscle growth essential for optimal muscular cell proliferation, maintenance, and repair during broiler production. Moreover, the myogenic markers examined in this research prove that, despite the lower feeding rate, chronic IF strategies had comparable effects on myogenesis (45).
To understand the effects on fat deposits, lipid metabolic markers were examined in the breast muscle samples. As reported by Papah et al. (46), functional analysis revealed that lipid metabolism-related genes are elevated in the myopathic tissues of broiler chickens. Though the lipid uptake and transport-related genes, including PABP4, C/REP-A, and C/REP-B, were not significantly altered in the current study. However, the relative upregulations of the adiposis-related genes (ZNF423 and PDGFR-A) in AD birds compared to 1h-IF and fasting birds align with a previous whole-genome sequencing that reported a potential higher degree of intermuscular fat in the pectoralis major samples of fast growth broiler chickens (47). ZNF423 and PDGFR-A gene expressions increased pre-adipocyte cells, contributing to muscular adiposity development during muscular disruption as detected in the current study (27, 48, 49). Consistently, the results here also reveal that both chronic intermittent feedings and fasting feeding decreased the TG level – a metabolic indicator of ectopic fat deposit in the breast muscle samples. The excess TG accumulation could exacerbate lipotoxicity in the breast muscle, which is composed mainly of Type IIB non-oxidative muscle (46, 50). From the intramuscular energy supply standpoint, decreased intramuscular TG levels in IF and fasting birds were likely mobilized to provide energy for muscular activities during the restricted feeding period (51). Note, excess TG accumulation causes lipid peroxidation in meat fillets leading to oxidative stress (52) in relation to the etiology of myopathic lesion development (11, 53, 54). These observations, therefore, corroborate lipid-laden perturbation in WS and WB-affected muscle during severe conditions (55). In light of this understanding, we suggest that the IF regimens used in the current study could alleviate lipid spoilage in broiler breast meat.
Besides, collagen accretion in the breast muscle indicates poor cell repair activities that cannot sustain the fast growth rate associated with AD feeding. Interestingly, our results show that IF and fasting feeding significantly decreased TRIM63 and MAFBX protein expressions. These atrophic proteins are responsible for muscular degradation, causing tissue fibrosis and muscle wasting during chronic muscular stress related to hypertrophy in fast-growing broiler chickens (48, 53). Moreover, birds fed restricted feeding regimens showed lower apoptotic activities, demonstrating relieved muscular repair processes. It is likely that the higher muscle protein breakdown in the AD group triggers the increased abundance of CASPASE 3, which might aggravate muscular damage in severe conditions, as reported in a previous study that provided evidence of apoptotic liver damage of birds affected by WB myopathy (56). This means that the enhanced protein degradations in the AD group profoundly contribute to higher collagen aggregate than alternative feeding regimens. Within this context, we could infer that intermittent feeding and fasting improves muscle growth and repair processes in the pectoralis major of broiler chickens (53, 57). This proposition agrees with previous findings that demonstrated timing schedules alleviate collagen modifications and fibrotic myopathy affecting abnormal breast muscles (35, 58, 59).
Although 1h-IF had an insignificant effect on the WS scoring in the current study, both 1.5h-IF and fasting strategies alleviated the WS appearance significantly compared to AD feeding. As generally believed, the fast growth rate is potentially recognized as the fundamental cause of WS (8, 53). In the past, feeding strategies such as lowering energy in diets (6), 85% ad libitum feed restriction (60), and 8 hours daily feed restrictions (61) have alleviated WS phenotype at the end of the studies (62). These previous findings further support why the alternative feeding strategies investigated in this study could produce breast meat fillets with improved muscular conditions. To sum it all, when applied appropriately, chronic IF could potentially solve myopathies associated with fast growth muscle hypertrophy in the future.