1. Introduction
The international farming industry is grappling with a significant challenge stemming from feed scarcity, exacerbated by an escalating demand for meat. The scarcity of feed ingredients has become more noticeable, and there has been a progressive increase in the cost of commonly used feed ingredients. This predicament has prompted exploration into cheap and alternative feed sources for livestock [
1]. Broccoli, a cruciferous vegetable (
Brassica oleracea L. var. italica), is renowned for its abundant nutritional profile, boasting essential nutrients, glucosinolates, sulforaphane, polyphenols, and minerals [
2]. Notably, consuming broccoli products has been linked to human chronic disease prevention [
3]. Over the past decade, broccoli cultivation has surged by 32.1%, reaching a combined production of 37.2 million tons in 2018, largely driven by China and India [
4]. Nevertheless, a substantial portion of the broccoli plant, notably the leaves and stems, is commonly discarded as byproduct waste after processing, resulting in environmental pollution and resource loss due to its high perishability [
5]. Recent studies have explored the application of broccoli residues as sources of bioactive compounds [
6,
7] and demonstrated that the extracts from leaves exhibited the highest antioxidant activities due to phenolic compounds, while stem extracts had the highest antimicrobial activity related to fatty acid derivates [
8]. Another study has also found that a high protein content of 90–300 g/kg dry matter (DM) was present in broccoli residual leaves [
9], but their utilization as animal feeds remains underexplored. Repurposing these broccoli residues as animal feed presents an opportunity for resource conservation and environmentally sustainable practices. Several studies have suggested that incorporating dried broccoli residues (up to 90 g/kg inclusion) into poultry diets can enhance the growth performance and improve nutrient digestibility within the digestive tract [
10,
11]. One study highlighted the positive impact of broccoli stem and leaf meal (up to 90 g/kg inclusion) on poultry meat quality, attributing it to its antioxidant properties [
5]. Broccoli residues have shown promise as a supplementary feed in the diets of small ruminants, with the capacity to substitute up to 30% of traditional feed components without adversely affecting ruminal fermentation. This substitution is feasible, except for a slight deficiency in the supply of rumen-degradable nitrogen [
12]. Moreover, fermenting plant-based materials with beneficial microbes, such as
Bacillus and
Lactobacillus species, can further enhance their nutritional value. In vitro findings indicate that fermentation pretreatment with
Lactiplantibacillus plantarum is a straightforward and cost-effective technique that could lead to a boost in the nutritional content of broccoli stems and a decrease in the drying time required [
13]. Animals consuming fermented broccoli residues (FBR) benefit from the inherent active compounds and the probiotic bacteria that colonize their digestive systems, providing additional health benefits [
14]. Despite the fact that FBR is rich in low-molecular-weight peptides (small peptides), probiotics, and organic acids and can safeguard broilers against harmful bacterial infections when added at 5% or 10% in diets [
15], the influences of FBR on other animals, particularly regarding pig growth, remain insufficiently documented. Our previous study also suggested the beneficial effect of up to 10% FBR supplementation on meat quality in growing pigs [
16]. Thus, 10% might be an appropriate dosage level for dietary supplementation with FBR for its positive effects. On the other hand, it was common to use finishing pigs as experimental animals due to their high tolerance for plant protein sources and being more conducive to meat quality evaluation [
17,
18]. We aimed to conduct a preliminary experiment with 10% FBR inclusion and hypothesized that the dietary supplementation of 10% FBR could also influence the meat characteristics of finishing pigs without compromising their growth efficiency. Consequently, the research assessed the influence of incorporating FBR into the diet on the key metrics of finishing pigs, including growth, nutrient digestibility, and meat physiological characteristics.
4. Discussion
Given our present insights, this is the inaugural investigation into the impact of FBR on finishing pigs. The absence of notable variances in final body weights, ADFI, ADG, and G/F between the CON and FBR groups suggests that incorporation of 10% FBR into the diet exhibited no adverse influence on the fattening performance of fattening pigs. These results may somewhat contrast with a previous study [
15] showing that supplementing 25, 50, and 75 g/kg FBR significantly increased the ADG of broilers. Additionally, our previous research [
16] indicated that supplementing 5% FBR significantly increased the ADFI of growing pigs. These discrepancies can be attributed to the distinct FBR levels and animals involved. Our results align with a previous study [
14] showing that supplementation of 10% FBR did not notably impact the ADG and feed utilization of broilers. The potential reason could be that dietary fiber-rich materials, such as broccoli residues, stimulate the expansion of visceral organs and the production of digestive juices, consequently allocating more energy toward maintenance rather than catabolic metabolism, especially for pigs in the finishing phase [
28]. Thus, our results indicate that FBR may be effectively integrated into the dietary regimen of finishing pigs at concentrations up to 10% without detrimental effects. Given the scarcity of research assessing the impact of fermented broccoli products on livestock, it is not feasible to draw any additional comparisons.
The digestibility of nutrients serves as a crucial indicator linked to the growth performance of animals. In our study, FBR supplementation in finishing pig diets significantly decreased the nutrient utilization of CP, DM, and CF, consistent with a study showing a drop in the digestive efficiency of pigs during the growth-to-finishing phase when fed high-fiber diets [
27]. Similarly, Mustafa and Baurhoo [
10] noted a markedly decreased apparent ileal digestibility of CP and DM in 35-day-old birds when their diet included 10% broccoli residues. Our previous research in growing pigs also indicated a trend (0.05 <
p < 0.1) toward reduced nutrient digestibility when supplementing 5% and 10% FBR [
16]. Moreover, several investigations have shown that incorporating fermented feeds, such as fermented soybean meal and wheat bran, into the diets of growing or finishing pigs leads to better nutrient absorption [
20,
23,
25], as fermentation can degrade complex macro-molecules into small substances, promoting feed digestion [
29]. On the other hand, increasing dietary fiber could decrease nutrient digestibility by reducing nutrient digestion time in the gastrointestinal tract [
30,
31]. Thus, the inconsistencies observed across these studies can likely be ascribed to the elevated levels of dietary fiber present in the experimental diets, which impedes nutrient breakdown and uptake during digestion and counteracts the positive impact of probiotics on intestinal digestive enzymes [
32]. Furthermore, diet composition, the anti-nutritional factors present in FBR, variations in bacterial strains, and experiment duration may all contribute to the variability observed across different studies. Interestingly, although significant inhibition in digestibility of CP, DM, and CF was observed in pigs fed with FBR-supplemented diets, the pig growth was not adversely impacted under the conditions of this study. Previous studies found that
Bacillus spp. and
Lactobacillus sp. could increase the activities of digestive enzymes, promote nutrient transporter expression, and improve intestinal morphology [
33]. Thus, the presence of live micro-organisms in the FBR might contribute, in part, to the compensatory effects on decreased digestibility in finishing pigs. Moreover, acid-soluble proteins (small peptides) are largely present in FBR, which facilitates the uptake of nutrients in the digestive system of animals, thereby offsetting the adverse effects of high crude fiber content in diets [
17]. Further research is needed to explore whether FBR influences the intestine structure and nutrient absorption to better understand this phenomenon.
Slaughter performance metrics serve as vital criteria for evaluating the performance of meat carcasses. We reported that the dietary inclusion of FBR had no adverse impact on the carcass performance of finishing pigs. The finding aligns well with our previous study [
16], which demonstrated no adverse influence on carcass characteristics of pigs in the growing stage fed FBR at up to 10% inclusion levels. The marbling score, a key indicator of fat deposition in pork muscles, is closely correlated with meat eating quality, including flavor and tenderness [
34]. Our finding demonstrated a significant elevation in marbling scores in the FBR-supplemented group, indicating improved meat quality. This observation is consistent with the data presented by Hao et al. [
20] and Liu et al. [
26], who reported increased marbling scores or intramuscular fat content in pigs in the finishing phase fed diets supplemented with fermented mixed feed. The abundance of probiotic strains present in FBR, such as
Bacillus sp. and
Lactobacillus sp., may contribute to these beneficial effects on meat quality [
35]. The optimization of gut microbiota by probiotics after dietary FBR inclusion may lead to enhanced nutrient absorption and improved efficiency of converting feed into body mass, which in turn could elevate the overall metabolism of nutrients, resulting in an increased marbling score [
28]. Additionally, some
Lactobacillus strains were found to produce vitamin E, which contributes to the increase in the marbling score [
36]. The marbling score is an indicator of both the quantity and the distribution pattern of fat within the muscle tissue, correlating positively with the content of intramuscular fat [
18]. Interestingly, the present study reported that the marbling score of the longissimus muscle exhibited a notable increase, without corresponding to any significant alteration in the intramuscular fat content, indicating that FBR mainly affected the fat distribution pattern in muscle. Similar discrepancies between marbling and intramuscular fat were observed in other studies [
17,
18], and their correlation needs further investigation. Our results also showed that gender significantly changed the backfat depth. Barrows had a thicker backfat depth than gilts, indicating a lower leanness in barrows. Similar results were also shown in previous studies [
37,
38], suggesting that barrows use more energy on fat deposits than muscle deposits.
Increased muscle antioxidant ability has been observed in FBR-fed finishing pigs [
39]. Our study also revealed significantly increased T-AOC contents in the muscles of FBR-fed pigs, suggesting a potential protective effect of FBR against muscle oxidative damage. Similar studies have demonstrated that supplementing FBR improved SOD activity in longissimus dorsi muscle in pigs [
20] and SOD and CAT activities and T-AOC in pectoralis major muscle and gastrocnemius muscle in free-range broilers [
14]. A previous study demonstrated that dietary inclusion of broccoli stem and leaf meal at a concentration of 4%, 8%, or 12% significantly improved muscle T-AOC of Ross 308 male broilers [
5]. Due to limited research on the impact of FBR on the antioxidant status of meat, further comparisons are challenging. The abundance of probiotics in FBR may partly explain the improvement in antioxidant capability in pigs [
40]. The genus
Lactobacillus exhibits dose-responsive free radical scavenging capabilities, and its supplementation has been shown to enhance the antioxidant status of finishing pigs [
41]. Nevertheless, small peptides in FBR also contribute to the improvement in muscle antioxidant ability, as a previous study reported that a dipeptide (carnosine) could enhance the oxidative activity of pigs’ muscle [
42]. Additionally, bioactive molecules produced during the solid-state fermentation of broccoli residues, such as flavonoid and phenolic compounds, have been reported to increase sharply during lactic acid bacteria fermentation [
2,
6]. Given the close relationship between meat shelf life and antioxidant capacity, FBR appears to function as a functional feed to elevate the quality of meat by improving the antioxidant status of the longissimus muscle. Furthermore, dietary inclusion of FBR did not yield any significant effect on meat color, which conflicts with our previous findings regarding the influence of FBR on growing pigs [
16]. This result somewhat agrees with a previous study indicating that dietary inclusion of fermented soybean meal does not affect meat color parameters [
25]. Previous studies have shown that the impact of fermented feed on the meat properties of pigs in the growing–finishing stage follows a concentration-dependent pattern [
27,
43]. The FBR supplementation level in our current study may have been too low to significantly affect meat color, as previous studies demonstrated that there was a positive correlation between the effect on meat color and the dietary concentrations of fermented feed [
44]. Further research employing higher levels (>10%) of FBR supplementation is necessary to better understand the influence of FBR supplementation on meat color in finishing pigs.
The amino acid composition of muscle tissue is intricately linked to meat taste and nutritional benefits [
45]. Recent studies have revealed that dietary supplementation with fermented feed effectively increases amino acid concentrations, thereby improving meat quality [
46,
47]. Liu et al. [
26] demonstrated that pigs fed fermented mixed feed had increased levels of methionine, lysine, threonine, alanine, aspartate, glutamate, arginine, and total essential amino acids. Tang et al. [
48] demonstrated that finishing pigs fed fermented feed had higher lysine and glutamate contents than those fed a basal diet. However, in our study, FBR supplementation did not significantly affect longissimus dorsi muscle’s amino acids. The result somewhat aligns with previous research [
49], which reported no significant benefits of fermented feed on the essential amino acid profile of muscle. These discrepancies may be attributed to differences in experimental variables, such as breed and age of the experimental animals, fermented substrates, or fermentation strains used. Given that the precise regulatory mechanism governing amino acid content is not yet fully understood, our results suggest that dietary supplementation of FBR has no adverse influence on the amino acid profiles of finishing pigs.
Muscle fibers, serving as the fundamental units of muscle tissue, are categorized into four distinct categories: Type I, distinguished by their slow-twitch characteristics; Type IIa, exhibiting fast-twitch properties; Type IIx, also known for its fast-twitch nature; and Type IIb, which shares the fast-twitch attribute. This categorization was based on the unique expression patterns of MyHC isoforms present in each fiber type [
50]. The assortment and relative distribution of muscle fiber types are intricately linked to the attributes of muscle color, texture, and flavor, thereby exerting a pivotal effect on the overall quality of meat [
51]. A prior study revealed that the inclusion of fermented feed in the diet substantially elevated the mRNA expression levels of
MyHC I and
MyHC IIa, indicating a potential shift from the fast-twitch to the slow-twitch muscle fiber phenotype [
18]. As is widely recognized, the reduction in the proportion of glycolytic muscle fibers and a concurrent rise in oxidative fibers in muscle tend to result in meat with a more intense red color, as oxidative fibers contain more pigment protein myoglobin than glycolytic fibers [
50]. However, our result showed that FBR supplementation had no significant effects on fiber gene expression level, suggesting no such benefits from FBR. Several signal transduction pathways have been implicated throughout the metamorphosis of muscular fiber classifications, including calcineurin and nuclear factor of activated T-cell c1 pathway [
52], mechanistic target of rapamycin kinase pathway [
53], and AMP-activated protein kinase pathway [
54]. We speculated that this discrepancy may be partly attributed to these influence factors. However, the specific process still warrants further research.