1. Introduction
Rabbit meat is lean meat of high nutritive value, because it is rich in essential amino acids, polyunsaturated fatty acids (PUFA), vitamins, minerals [
1,
2], low in cholesterol contents, and does not contain uric acid compared with other meats [
1,
2,
3]. The profitability of rabbit farms is partly depending on the effectiveness of weaned rabbits to grow healthy and to protect them from high mortality rates during the fattening period. Antibiotics are frequently used in the diets of growing rabbits because digestive disturbances are the main reason for morbidity and mortality in the rabbit industry [
3].
As a consequence of the European ban on antibiotic growth promoters (AGPs) and increased consumer awareness about the consumption of healthy and safer animal products, researchers and feed companies have encouraged to seek new animal feeding approaches to substitute AGPs and synthetic antioxidants [
3]. Phytogenic feed additives and/or their extracts are being progressively used in animal nutrition due to their beneficial phytochemical compounds. These active components have been shown to augment appetite, improve carcass yield, enhance digestive enzyme secretion, and stimulate an immune response, and encourage antibacterial and antioxidant properties [
3,
4,
5].
Moringa oleifera is commonly cultivated in Africa, Asia, and the US [
6]. In Egypt,
M. oleifera is widely cultivated due to its adaptation to various environmental conditions and different types of soil. Accordingly, the leaves of
M. oleifera are plentifully available all over the year. Recently,
M. oleifera leaves (MOL) as animal and poultry feed supplement have gained great interest owing to their high nutritional value and low anti-nutritional factors [
7,
8]. Various literature on the nutrient composition of
M. oleifera showed that their leaves are rich with essential nutrients such as protein (sulfur-containing amino acids), fatty acids (α-linolenic acid), minerals (calcium, iron, and phosphorus), vitamins (A, E, B-complex, folic acid, and ascorbic acid), and several bioactive compounds, including carotenoids, saponins, phenolics, alkaloids, and flavonoids [
7,
8,
9].
The phytochemical components of MOL have been reported to have antimicrobial roles and antioxidant activities and thus, MOL is commonly used in numerous medicinal applications to control various diseases such as digestive disturbance, asthma, inflammatory disease, and cancers [
10,
11,
12]. It has been documented that MOL could possibly avoid oxidation damage [
11,
12], enhance the immune response [
13], exhibit antioxidant activities that can conquer free radicals and reactive oxygen species synthesis [
14,
15], and favorably modulating lipid metabolism [
9,
11] in rats. Moreover, the effective antioxidants in MOL prevent deterioration and increased the shelf life of meat products in goats [
16] and poultry [
17].
Previous studies have been carried out to evaluate the use of MOL as a feed supplement for poultry [
18,
19] and rabbits [
20,
21,
22]. These studies concluded that MOL can be used as a good feed ingredient owing to its high nutritional value and low antinutrients level. Rabbit meat is predominantly more prone to lipid peroxidation during storage, with negative effects on the meat quality traits, owing to its elevated content of unsaturated fatty acids (USFA) [
23]. To our knowledge, little information is available about the effect of dietary MOL supplementation on the meat oxidation stability, cholesterol content, and fatty acids (FA) profile of growing rabbits. Therefore, the objective of the current study was to assess the effects of graded dietary supplementation levels with MOL on performance, carcass characteristics, oxidation stability, blood parameters, meat characteristics, and FA profile of growing rabbits. The hypothesis tested was that the inclusion of MOL in the diets of growing rabbits might improve their growth performance, carcass yield, antioxidant capacity, and enrich meat with essential FA.
4. Discussion
Previous studies reported the beneficial impacts of dietary MOL inclusion on the production performance, carcass traits, immunity, and health status of growing rabbits [
20,
21,
22]. However, to the authors’ knowledge, little information is available on the effect of dietary supplementation with MOL on the oxidative stability, physical and chemical characteristics, cholesterol contents, and FA profile of the rabbit’s meat. The nutrients content of MOL used in the current study was in line with previous studies which indicated that MOL is an excellent source of essential nutrients and represents a valuable feed ingredient for growing rabbits [
21,
22,
39].
Our findings showed that the addition of MOL up to 1.5 g/kg in the diets of growing rabbits improved their production performance. These findings are in agreement with earlier studies [
20,
21,
22]. Cui et al. [
19] reported an improvement in the growth performance of broiler chickens when MOL was included in their diets at a level of 1.56%. The potential benefits of dietary MOL on the production performance of growing rabbits were observed at a dietary inclusion level of less than 20% of the diet [
20,
21,
22]. The greater live weight gain and better FCR in the present study could be attributed to the bioactive compounds present in MOL [
19,
21,
22], which might have a key role in augmenting the nutrient utilization. It was observed that dietary MOL with lower inclusion levels had a beneficial impact on the rabbit performance, but negative effects were detected by feeding rabbits with higher inclusion levels (30%) of MOL [
21,
22]. The main cause for lower live weight gain and FCR with high dietary MOL in these previous studies is not obvious and may contribute to an elevated level of several phytochemical components such as phenols, tannins, and phytate [
19,
21,
22].
In the present study, the dietary supplementation with MOL increased the dressing out percentage, which is related to the higher final live weight. Additionally, the increased digestive tract length of rabbits fed MOL in the current study could augment the small intestine absorptive area and nutrient utilization [
20,
21]. The length and weight of the small intestine were observed to be greater in broiler chickens fed 1.2% MOL [
40]. On the contrary, earlier studies recorded that dietary MOL supplementation did not have a noticeable impact on the carcass yield [
21,
39]. El-Badawi et al. [
20] observed that carcass yield was positively increased by a moderate inclusion level (up to 0.45%) of MOL in the diets of growing rabbits. Moreover, the higher dietary inclusion of MOL at a rate of 30% [
28] can be efficiently utilized by rabbits without inducing any beneficial impacts on the carcass traits. Greater spleen index (within the normal range) of the rabbits fed the MOL
1.5% diet in the current experiment may indicate that MOL could positively impact the immune response of growing rabbits. It has been stated by Sun et al. [
21] that MOL addition up to 30% of the rabbit’s diet increased the spleen index and could boost the immune response against pathogens. Isitua and Ibeh [
41] observed that rabbits fed MOL showed an increase in CD4 (T-helper) cells and help B-cells and they suggested that MOL could act as a positive immune-regulator. The decreased abdominal fat in rabbits fed MOL in the present study might indicate a difference in lipid metabolism induced by dietary MOL supplementation. It was suggested by Cui et al. [
19] that USFA in MOL could augment fatty acid β-oxidation and accordingly reduced abdominal fat deposition. The current findings of spleen and abdominal fat indexes suggested that MOL could beneficially impact the immunity and lipid metabolism of growing rabbits.
Our data observed that dietary inclusion of MOL positively modifies the meat FA composition, resulting in a reduction of SFA and an increase in PUFA, particularly n-3 FA, with an improvement in the ratio of n-6 to n-3 FA. The higher PUFA content in the meat of rabbits fed the MOL-diets might be owing to the greater concentration of USFA in MOL. Moyo et al. [
42] reported that PUFA of MOL represents about 57% of total FA, among which 44.57% of them is α-linolenic acid. Cui et al. [
19] found that MOL contained 26.46% α-linolenic acid. Although, type of cultivation, harvest time, method of preparation could influence the FA composition of MOL. Unfortunately, in the current study, we did not analyze the FA composition of MOL to discuss with previous reports. Rabbits can absorb dietary FA and deposit it in the adipose tissue and intramuscular fat, and thus it is likely to modify the meat FA profile through dietary sources of UFA [
43]. Unfortunately, there are not any published reports on the effects of feeding MOL on the meat FA profile of growing rabbits to discuss with the findings reported herein. In broiler chickens, Cui et al. [
19] observed that dietary inclusion with MOL increased the contents of PUFA, n-3 FA, and n-6 FA in the breast meat. They suggested that a high PUFA content of MOL might be responsible for an elevated PUFA concentration of the breast meat [
19]. However, the mechanism of action underlying the modification of meat FA composition in broilers or rabbits fed MOL remains unclear and needs further investigation. In the review by Woods and Fearon [
44], they documented that PUFA could deposit straightforward in the meat of non-ruminants through dietary supplementation. The high content of PUFA, in particular n-3 PUFA, and decreased SAF content become a necessary characteristic in the meat products for consumers from the nutritional point of view due to the potential benefits of n-3 PUFA on the immunity and cardiovascular system [
45]. The greater concentrations of both PUFA and antioxidants are two significant criteria to consider when a dietary approach is performed.
The meat with a greater PUFA content is always more prone to lipid peroxidation and deterioration, and consequently have a lower shelf-life [
46]. Interestingly, in the present trial, dietary MOL addition increased the meat contents of PUFA and GPx but decreased MDA, suggesting that MOL could be a beneficial dietary approach to improve the meat quality of growing rabbits. In addition, there was a positive correlation between the serum and meat MDA contents, as well as, between the serum and meat GPx levels. The enhanced GPx and the diminished MDA levels in both serum and meat supported the idea that MOL could enhance the antioxidant properties and thus resulting in better meat quality [
7,
19]. Our findings are in line with earlier studies, which reported that dietary inclusion of MOL for broiler chickens [
19] and rabbits [
22] improved meat oxidative stability during storage. Phenols and potent antioxidants in MOL could be deposited in the meat, exerting a potential diminishing effect on the meat lipid oxidation efficiently by augmenting the antioxidant enzymes and scavenging free radicals [
11,
12,
42]. Little information is available in the literature about the effect of MOL on the meat oxidation stability of growing rabbits.
In the current study, greater CP and lower EE contents of LL were observed for rabbits fed the MOL diets. These findings suggested that MOL could impact the meat proximate analysis. Sun et al. [
21] reported no significant difference in the proximate chemical composition of the rabbit meat fed MOL at a level of 10, 20, and 30% of the diet. There is a paucity of literature on the effects of MOL on the meat nutrient composition of growing rabbits. Meat color is a significant measure of the meat characteristics in meat manufacturing [
47]. Dietary inclusion of MOL up to 1.5 g/kg DM improved the meat color after the storage period (72 h), which was supported by an increase in the a ∗ (redness) values and a decrease in the b ∗ (yellowness) values. The augmentation of meat oxidative stability and antioxidant activity due to dietary supplementation with MOL was expected to be responsible for the meat color enhancement. Myoglobin, the sarcoplasmic heme protein, acts as the chief pigment accountable for the red color of the meat, is oxidized during storage, and will cause a change in the meat color [
48]. It was reported that a low meat pH decreases the selective absorbance of myoglobin, causing the meat to look less red and more yellow [
21,
48]. Our results are in agreement with earlier studies in rabbits [
21] and broiler chickens [
19], which reported that dietary MOL could beneficially impact the redness and yellowness of the meat.
The reduced cholesterol content in LL of growing rabbits fed the MOL diets might be related to the phenolic compounds present in MOL. These phenolic compounds would lower circulating cholesterol by serving as a natural hypo-cholesterolemic agent and hence a lesser content of meat cholesterol. This was supported by a positive correlation between serum and meat cholesterol concentrations in the current study. Earlier studies suggested that the effect of MOL extract in lowering lipid levels may result from the phenolic compounds, polyphenolic, and flavonoids [
49,
50]. These bioactive compounds can reduce the uptake of dietary cholesterol from the intestine of rats [
49,
50]. Polyphenolic compounds were reported to induce a variety of biological properties, involving a reduction of blood lipids due to the greater expression of LDL receptors, diminishing lipid synthesis and lipoprotein secretion in the liver, and augmenting cholesterol removal via bile acids [
7,
51]. Furthermore, it was assumed that the higher fiber content of MOL may be responsible for lowering blood cholesterol concentration via augmenting lipid metabolism [
7,
8]. Nevertheless, little information is available about the effect of the dietary inclusion of MOL on the meat cholesterol content in growing rabbits. The latter data can assist to provide additional evidence to the usefulness and safety of MOL as a potential feed ingredient for growing rabbits.
Blood biochemical constituents are considered as an effective method to assess the health status of animals [
52]. The absence of a significant effect of MOL on the serum creatinine, urea, bilirubin, or uric acid concentrations indicated that the inclusion levels of MOL up to 15 g/kg of the diet did not induce any adverse effects on the kidney functions of the experimental rabbits. In this regard,
M. oleifera extract was shown to be effective against gentamicin-induced nephrotoxicity of rabbits due to the detoxification effect of
M. oleifera extract on the kidney [
53]. Greater serum levels of total protein and globulin and lower levels of ALT and AST in the MOL-fed groups in the present study suggested better immunity and liver functions of these rabbits.
M. oleifera extract was reported to have a hepatoprotective activity due to the presence of bioactive flavonoids such as quercetin [
54]. Sun et al. [
21] and Salem et al. (22) reported that dietary inclusion of MOL enhanced the blood parameters of growing rabbits. On the contrary, Makanjuola et al. [
55] observed that dietary supplementation of MOL up to 0.6% of the diet did not affect the serum total protein, albumin, globulin, and AST concentrations of broiler chickens. Greater serum globulin level of the MOL-fed rabbits supported the immune-stimulant effect of
M. oleifera since globulins are mainly responsible for enhancing immunity [
56]. Our findings are consistent with the results reported by Salem et al. [
22].
The addition of MOL in diets of growing rabbits had both linear and quadratic responses regarding the concentrations of serum total triglycerides, LDL, and cholesterol, with lower levels being noticed for rabbits consumed the MOL diets. Consistent with the current findings, Salem et al. [
22] observed that the addition of MOL up to 20% in the diets of growing rabbits lowered serum total lipids, cholesterol, triglycerides, and LDL. Jain et al. [
49] reported that MOL exerted hypo-lipidemic properties in rats. LDL is a major constituent of the total blood cholesterol and is mainly attributed to the occurrence of cardiovascular diseases and hence, should be considered as a key goal of any lipid-lowering agent, such as MOL, as implied in the present trial.
M. oleifera is identified as a potent antioxidant feed ingredient due to its antioxidant nutrients, involving vitamins C and E, carotenoids, flavonoids, and selenium, and beneficial phytochemicals [
7,
8] which might have a crucial role in enhancing the health status of growing rabbits.