**4. Discussion**

#### *4.1. Growth Performance and Feed Utilization*

Vegetable oils, such as olive oil, rice bran oil, corn germ oil, rocket seed oil, and wheat germ oil have been used as a source of energy, essential polyunsaturated fatty acids and fat-soluble vitamins with grea<sup>t</sup> nutritional and health benefits [56]. Rocket, *E. sativa* is considered an important leafy vegetable crop that is high in antioxidant molecules, and its seeds contain oil up to 25–35% [11]. Wheat has been called the staff of life, and people who have many health concerns have used wheat germ oil [57]. Wheat germ oil represents about 10–15% of the whole wheat germ, which is reported to be one of the potential resources for beneficial molecules [21].

In the current study, the final BW, weight gain, and ADG increased significantly in rabbits that were administered orally with RSO and WGO or their mixture compared to the control group. In the same vein, the RSO treatment significantly increased the final BW and daily gain of rabbits [16]. Moreover, the final BW, ADG, and total gain of growing rabbits were higher significantly with the supplementation of RSO, and the mixture of RSO and onion oil [17] or with the supplementation of watercress oil and the mixture of watercress oil and coconut oil [18]. Furthermore, dietary supplementation of WGO increased the BW of male broilers [29]. In conflict with our results, the addition of RSO (1–3 mL/kg BW) in rabbit's diet for 2 weeks tended to decrease BW as the dependent dose [19].

In the current study, all the groups were fed the same diet (isocaloric), but the groups receiving the supplemental oils (0.3 mL/kg BW) had more caloric value by ≈3 kcal/kg BW as an energy intake, representing a 0.7% increase in the daily gross energy intake. This increase is not significant, and it could not affect the obtained results of growth, feed utilization, physiological performance, and digestibility. Previous studies revealed that ADG, slaughter live BW, and carcass weight were not affected either by the fat source or fat level [58]. In this study, the difference in gross energy was 3.44%, which was obtained using different fat sources (linseed and black soldier fly). In addition, the increase in fat levels by 3% did not affect the growth rate of rabbits due to the decreased feed intake, but it increased energy digestibility and feed efficiency [59,60]. Moreover, rabbits fed three diets with different energy levels (2707, 2436, and 2276 kcal DE/kg) did not affect live BW, weight gain, FCR [61]. However, the increase in gross energy by 8.79% could affect the rabbit BW [62].

Accordingly, the improvement in BW and BWG with vegetable oil supplementation in the present study could be attributed to the content of active molecules in rocket seeds, such as carotenoids, vitamin C, glucoerucin, and flavonoids, which are health-promoting agents [63]. In addition, the volatile oil of rocket seeds contains isothiocyanates, which have antimicrobial, antioxidant, and anticarcinogenic activities [64]. Moreover, WGO is a natural source of α-tocopherol, which increases (*p* ≤ 0.05) the BW and BWG of growing rabbits [65]. Mustacich et al. [66] supported these findings by stating that the natural α-tocopherol has higher natural activity than synthetic α-tocopherol. Furthermore, linoleic acid found in WGO operates as the precursor of cell membrane phospholipids [67], which could also participate in the growth improvement in the current study.

The oral administration of vegetable oils had no effect on daily feed intake and total feed intake during the whole experimental period in the present study. The route of administration in the present study was gavage, which could not affect the diet palatability and therefore maintained the normal feed intake. Meanwhile, FCR improved significantly with RSO and WGO or their mixture treatments compared to the control group. In accordance, the feed intake did not affect the 12-weeks aged rabbits that were given orally different levels of watercress oil plus coconut oil compared to the control group. However, the FCR of rabbits was improved with watercress oil or watercress oil plus coconut oil [18]. The improvement in the FCR of growing rabbits may be attributed to the properties of these oils that act as antibacterial, antiprotozoal, antifungal, and antioxidants [18,68,69]. Moreover, rocket seed cakes increased the FCR of male rabbits, which may be due to their beneficial effect on stimulating and activating the digestive system [16].

#### *4.2. Digestion Coefficients of Nutrient, Nutritive Values, and Nitrogen Balance*

The different experimental oil treatments had a significant (*p* ≤ 0.05) improvement in nutrient digestibility compared to the control group. This improvement in nutrient digestion coefficients resulted in a significant increase in body weight gain and an improvement in the FCR (Table 2). Whereas, rocket seed cakes have a beneficial effect on stimulating and activating the digestive system [16]. In parallel to our results, using onion or moringa oils or a mixture of them significantly (*p* ≤ 0.05) improved digestibility coefficients of CP and EE compared to the control group. Meanwhile, the digestibility of DM, OM, CF, and NFE was not affected significantly by the addition of different oils [70,71]. In this line, the addition of 1 g RSO/kg diet or 1 g onion seed oil/kg diet and their combination in the growing rabbit's diet increased (*p* ≤ 0.05) the digestion coefficients of CP and EE compared to the control group [17]. In this regard, the addition of fennel seeds, oregano leaves only or the mixture to the rabbit's diet had a significant (*p* ≤ 0.05) improvement in DM, OM, and CF digestibility [72].

Concerning the nutritive values and digestible energy of the experimental diets, the results showed that TDN, DCP, and DE increased significantly in RSO, WGO, and their mixture groups. In accordance with the current results, the fat-containing rabbit diets with or without a herbal mixture formulation containing fennel seeds and oregano leaves improved both TDN and DCP values [72]. Moreover, there was a significant (*p* ≤ 0.05) improvement in nitrogen intake, absorbed nitrogen, and NB in rabbits that received Mos, followed by rabbits that received RSO and WGO compared to the control group. The improvement in N absorbed and NB of the experimental diets of rabbits that received vegetable oils resulted in a significant (*p* = 0.050) decrease in serum blood urea nitrogen in the current study (Table 5).

#### *4.3. Lipid Content and Fatty Acid Profile of Rabbit Meat*

Currently, consumers are increasingly praising foods that contain not only macronutrients, but also beneficial compounds for health and welfare [73]. Rabbit meat is a good source of protein with a low fat and cholesterol content, and it has a lower energetic value than red meat [74]. Moreover, the fatty acid composition of rabbit meat consists of a high polyunsaturated fatty acid content [74]. Several studies have suggested that n-3 fatty acids play an important role in human nutrition since they help reduce the incidence of lifestyle diseases such as coronary artery diseases, atherosclerotic diseases, hypertension and diabetes, as well as certain inflammatory diseases, such as arthritis and dermatitis [23]. The increase in the n-3 PUFA level in rabbit meat is feasible and could be achieved by feeding them n-3 PUFA rich diets. Furthermore, the supplementation of natural antioxidants in feed declines lipid peroxidation and enhances the stability of unsaturated fatty acids [29].

The current findings show that rabbits that received RSO, WGO, and their mixture had a significant (*p* = 0.001) decrease in the meat content of lipids and had a positive effect on meat fatty acid composition, including a significant increase in TUFA and linolenic c18:3 n-3 and had a significant reduction in the meat content of SFA, palmitic c16:0, and oleic c18:1 compared to the control rabbits.

These results are in agreemen<sup>t</sup> with other studies that have shown that dietary manipulations in monogastric animals, including rabbits, can change the quantity and chemical composition of the fatty acid. Whereas, vegetable oils can lower the PUFA/SFA ratio and increase the n-3 value in rabbit meat [6,75,76]. In addition, supplementing the rabbit feed with 8% linseed increased the C18:3 n-3 concentration and decreased the n-6/n-3 ratio in rabbit meat compared to the control group [77]. Daily linseed oil supplementation for 30 days increased the amount of ω-3 fatty acid in the muscular tissue lipids of bucks from 4.49 to 7.72%, i.e., by 1.72 times [78]. Moreover, the feeding of linseed oil rich in n-3 PUFA can be an effective method for increasing the tissue levels of these fatty acids in broiler chickens [79–81]. Blending different vegetable oils can modify the fatty acid composition

and raise levels of natural antioxidants and bioactive lipids in the mixes, thus, leading to an improvement in the nutritional value and oil stability [82].

#### *4.4. Serum Biochemical Parameters*

Oral administration of RSO, WGO, and their mixture improved the serum total protein as well as albumin and globulin levels compared to the control group. In agreemen<sup>t</sup> with the current results, the serum total protein as well as albumin and globulin levels improved significantly (*p* ≤ 0.05) when the rabbits were given the mixture of moringa oil and RSO, followed by individual moringa oil and RSO supplementation or growing rabbits that received moringa oil alone or with onion oil [70,71]. The significant increase in serum total protein and albumin observed in rabbits that were administered orally with RSO, WGO, and their mixture indicated the ability of these oils to stimulate the regeneration of hepatic tissue, which increased protein synthesis in the liver and improved the functional status of the liver cells [12,83] or these oils had a positive effect on thyroid hormone secretion, which could affect the metabolism of nutrients (Table 5). In addition, the rocket seeds contain vitamin C and carotenoids [63], which play an important role in the protection against oxidative damage [84] as indicated in improving the serum antioxidant status of rabbits in the present findings (Figure 3). Moreover, WGO has a high content of other nutritional and health-benefit factors, such as vitamin E and phytosterol [85], which may be the reason for its improving effect on the blood protein profile. The increase in serum total protein and globulin is a general indication of the immune status of the animal, since the liver can synthesize enough globulin for immunologic action, as mentioned by Sunmonu and Oloyede [86]. These improvements signify better disease resistance and increase immune response and animal resistance against any physiological or physical stressors [87].

Oral administration of RSO and WGO or a mixture of them caused (*p* = 0.050) a decrease in serum blood urea nitrogen. This reduction in urea nitrogen may be due to the improvement of NB as indicated in the present results (Table 3). In terms of lipid profile, it has been observed that oral administration of RSO, WGO, and MOs had a positive effect on serum total lipids, HDL-c, and LDL-c compared to the other groups.

In agreemen<sup>t</sup> with the current results, the supplementation of 1–3 mL/kg BW of RSO to the rabbit's diet decreased (*p* ≤ 0.05) the level of serum LDL-c and increased the serum HDL-c level compared to the control group [19]. In addition, Abozid et al. [88] discovered that RSO had a clear effect on improving the lipid profile due to its high concentration of plant sterols. Whereas, plant sterols reduce the incorporation of dietary and biliary cholesterol into micelles, leading to low cholesterol synthesis and cholesterol absorption as well as increasing LDL-c receptor activity, which eventually results in lower serum LDL-c concentration [89]. Moreover, total cholesterol, triglycerides, and LDL-c concentrations (*p* > 0.05) decreased as well as the HDL-c (*p* ≤ 0.05) increased with the WGO treatment [90]. The lowering effect of WGO on triglyceride, cholesterol, and LDL-c may be attributed to the high content of vitamin E and phytosterol in WGO [85]. Furthermore, linoleic acid found in WGO leads to the abstraction of cholesterol and acts as the precursor of cell membrane phospholipids [67,91]. WGO contains policosanol, which can reduce the high concentration of blood total cholesterol [92].
