Rabbits have a short gestation period, rapid growth, and excellent feed efficiency. As a result, rabbits have a higher metabolic rate, which results in the generation of cellular free radicals. These free radicals may affect cellular functioning by increasing oxidative stress, inflammation, and immunosuppression ^[1][26]. Researchers recently investigated the ability of various botanical substances and their bioactive components to scavenge free radicals and restore cellular activities required for rabbit production and health.

Plant/botanical products are natural and vital substances widely used as dietary additives in livestock farms. They contain phytobiotics, phenols, flavonoids, tannins, and essential oils, which play a multitude of roles in a rabbit’s body. The nutritional and medicinal properties of these substances mainly influence animal performance by improving digestion and enhancing immunity/health ^[2][3][4][19,20,27]. Additionally, several botanical products, including herbs, are considered phytogenic feed additives for livestock to improve growth performance and health status ^[5][28].

Thyme (Thymus vulgaris L.) is a popular medicinal plant widely used in livestock farms to improve animal productivity and health due to its antioxidant, antibacterial, and therapeutic properties ^[6][7][8][29,30,31]. Dry thyme essential oil (active extract) contains a mixture of monoterpenes, primarily thymol (2-isopropyl-5-methylphenol) and the phenol isomer carvacrol (2-methyl-5-(propan-2-yl) phenol) ^[9][32]. Adding plant essential oils to the diet can reduce animal gastrointestinal lesions and enhance the ratio of villus height to crypt depth ^[10][33]. Catalase activity was boosted by essential oil phenolic components, which detoxified hydrogen peroxide and converted lipid hydroperoxides to harmless molecules ^[11][34]. By breaking up free-radical chains, thyme extracts have the potential to increase the oxidative stability of animal meat. This would provide a product with better oxidative stability ^[12][35]. It works by stimulating intramuscular fat and deposition of flavor amino acids. Phytogenic products containing thymol and carvacrol improved animal growth, gut health, and antioxidant enzyme activity ^[13][36]. Moreover, thyme extract contains flavonoids (>50%), which are considered antimicrobial and antioxidative agents ^[14][37]. Flavonoids increased vitamin C activity, which acts as an antioxidant, as well as improves immunological functions ^[13][15][16][36,38,39]. Thyme extracts can also reduce bad cholesterols, such as low-density lipoprotein, and this reduction can be related to HMG-CoA reductase activity inhibition which is among the enzymes responsible for controlling cholesterol metabolism in the body ^[17][40].

Ezzat Ahmed et al. ^[18][41] found that thyme leaf supplementation (4–16 g/kg of diet) can improve the rabbit’s appetite, growth performance, and market body weight compared to the control. Thyme’s antibacterial activity may be associated with increased rabbit body weight. The feed conversion ratio and weight gain were proportionate to the amount of thyme leaves consumed. Feeding with thyme leaves significantly reduced fecal ammonia ^[18][41]. Biomarkers showed that thyme leaves significantly improved liver and kidney function. Furthermore, testosterone levels and sperm quality were significantly higher in the thyme leaf-treated groups when compared to the control ^[18][41]. Thus, at an optimum dose of 16 g/kg of diet, thyme leaves could be a beneficial feed addition for rabbits living in hot climes, according to Ezzat Ahmed et al. ^[18][41]. In agreement, thyme oil supplementation (up to 150 mg/kg) can improve feed intake and growth performance compared to the control according to Abdel-Wareth et al. ^[19][42]. Thyme oil supplementation improved carcass quality by lowering perirenal and scapular fat without damaging internal organs. Thus, thyme oil can be used as an effective feed additive to improve rabbit productivity under hot environmental conditions ^[19][42]. Additionally, Benlemlih et al. ^[20][43] reported that supplementing rabbit diets with 5% of thyme can reduce mortality rates by 39%. According to Abdel-Wareth and Metwally ^[21][44], thyme essential oil levels supplementation (60–180 mg/kg of the diet) significantly improved the feed conversion ratio and body weight gain of treated rabbits compared to the control. This supplementation also significantly increased serum testosterone concentration and decreased aspartate transaminase, alanine transaminase, urea, and creatinine compared to the control group. Furthermore, it enhanced the semen characteristics of male rabbits compared to the control group. Likewise, Abdelnour et al. ^[22][5] revealed that thyme essential oil at a level of 100 mg/kg of the diet increased total proteins, albumin, globulin, blood hemoglobin, total antioxidant capacity, hematocrit, and glutathione of treated rabbits while decreasing malondialdehyde level. Immunological variables (IgG and IgM), milk production, and ovulation rate were also improved by this supplementation. Hence, dietary thyme essential oil can alleviate the negative impacts of heat stress on female rabbits by improving their antioxidant and immunological statuses ^[22][5]. Generally, plant essential oil supplementation positively influenced animal gut microbiota and boosted crypt depth in the ileum. Moreover, it decreased claudin-1 mRNA expression, as well as increased interleukin-1β and toll-like receptor (TLR) 2 mRNA expression in the ileum ^[10][33]. Furthermore, thyme oil significantly improved blood antioxidant capacity and glutathione peroxidase activity in the liver ^[23][45]. It also decreased malondialdehyde levels in the duodenal tissue and increased transepithelial electrical resistance values ^[23][45].

On the other hand, thyme can be added to the rabbit diet combined with other natural botanical substances to increase the diet’s nutritive value or to decrease diet costs. Spirulina (Arthrospira platensis) is a unicellular cyanobacterium alga with numerous health benefits, therapeutic properties, and other biological properties ^[24][46]. Supplementing growing rabbit diets with 3% thyme (in substitution of alfalfa meal) and 5% spirulina (in substitution of soybean meal) for 3 or 6 weeks did not significantly impact rabbit growth performance and health status, but it reduced diet costs and improved rabbit meat quality ^[25][26][47,48]. Kovács et al. ^[27][49] noticed that dietary spirulina supplementation had no discernible effect on rabbits’ antioxidant status of blood, but it enhanced immunity by increasing IgG production in the rabbit’s body. El-Ratel et al. ^[28][50] reported that the combination of spirulina (Spirulina platensis) (5 g/kg of the diet) and selenium nanoparticles (0.5 mg/kg of the diet) for 5-week pre-mating significantly increased live litter size and viability rate at birth, hemoglobin and red blood cells, and plasma T₃, T₄, insulin, and total protein compared to the control. Plasma estradiol 17-β (pre-mating), progesterone (mid-pregnancy), and prolactin (day-7 postpartum) were significantly increased by selenium nanoparticles supplementation (0.3, 0.4, and 0.5 mg/kg of the diet). The supplements also reduced white blood cells, cortisol, and lipid profile, and improved liver and kidney functions. Furthermore, superoxide dismutase was increased by selenium nanoparticles supplementation at the levels of 0.4 and 0.5 mg/kg of the diet, whereas malondialdehyde was reduced by 0.3, 0.4, and 0.5 selenium nanoparticles mg/kg of the diet. Sexual receptivity, ovulation rate, and pregnancy rate were significantly increased by increasing selenium nanoparticles above 0.1 mg, whereas embryo yield was increased by >0.2 mg selenium nanoparticles/kg of the diet. Therefore, a combination of spirulina and selenium nanoparticles in combination with thyme could potentially be used as feed additives to rabbit diets to improve heat regulation and rabbit reproduction, particularly in subtropical countries. Working on dietary supplementation of thyme oil (1000 mg thyme oil/kg of the diet) with betaine (1500 mg) and their combination, Abd El-Azeem et al. ^[29][51] indicated that feed dry matter digestibility and digestible energy were significantly improved in treated NZW rabbits compared to the control. Hematological parameters, feed intake, and carcass quality were not significantly affected by these supplementations, but daily weight gain was significantly increased in treated rabbits. Feed conversion ratio and economic efficiency recorded the best results with the treated rabbits.

Garden cress seed (Lepidium sativum) is a valuable medicinal plant product that has recently gained global attention due to its nutritional and pharmaceutical properties, resulting in the development of novel feed additive sources ^[30][22]. These seeds have a high protein content (~25%) and are nearly equal in healthy fats. They are also high in vitamins and minerals, which are necessary for animal physiological and biological functions ^{[29][30][31][32]}[22,51,52,53]. Furthermore, they contain phytochemical components that contribute to their high antioxidant properties ^[31][32][33][52,53,54]. Polyphenols, flavonoids, antioxidants, and polyunsaturated fatty acids (PUFA) are abundant in garden cress seeds ^[30][22]. These substances influence a number of biological processes in an animal’s body, including the stimulation of growth genes, the advancement of metabolic processes, and the improvement of product quality by raising PUFA and lowering bad cholesterol levels ^{[10][34][35][36]}[23,33,55,56]. They also contain aromatic essential oils that have a variety of therapeutic and nutritional benefits for farm animals. Several types of radicals are lowered by natural vital substances that exist in garden cress oil such as tocopherol (an antioxidant), carotenoid, oleic acid, and α-linolenic acid ^[37][38][57,58]. In addition, it contains α-linolenic acid, which can be converted in the body to eicosapentaenoic acid and docosahexaenoic acid ^[39][59]. Therefore, garden cress can be added as a potential feed additive for better biological and productive performance, health status, and economic efficiency ^[40][41][60,61]. El-Gindy et al. ^[42][62] reported that including 3% garden cress seeds in the female rabbit diet increased the litter weight of heat-stressed rabbits on the 7th, 14th, and 21st days of age. On the 28th day of weaning, it increased milk yield. Furthermore, blood lipid profile parameters (such as cholesterol and triglyceride levels) and serum urea levels were reduced in the treated rabbits, whereas antioxidant status was improved, particularly with 6% garden cress supplementation. Morshedy et al. ^[43][63] reached the same conclusion, recommending 3–4.5% garden cress seeds as a dietary supplement for growing rabbits to improve growth, feed utilization, digestibility, and immune response.

Azolla (Azolla pinnata) is a tiny aquatic fern that can withstand high temperatures and lives afloat on the water’s surface. It is high in proteins (~30%), vitamins (20–30%), and minerals (10–15%), and has a low fiber content (11–13%) ^{[44][45][46][47]}[64,65,66,67]. Moreover, it contains a high level of carotenoids, which have beneficial biological properties that support therapeutic benefits, such as immunological, anti-inflammatory, and antibacterial implications, and improve animal growth and productivity ^[48][68]. Nonetheless, Azolla can be successfully used as an unconventional feedstuff in livestock diets because of its high nutrient content ^[49][69]. Azolla also is an inexpensive alternative protein source for animals, and because of its high nutritive value and low lignin content, it is easily digestible and could increase animal feed efficiency, average daily gain, and milk production by 15–20% ^[44][50][64,70]. The Azolla plant’s high antioxidant enzyme content can reduce free radical activity in the animal’s body ^[44][64]. In general, there are a few articles that discussed using Azolla in rabbit farming. Unfortunately, they offered a different chemical analysis of Azolla. It might relate to employing various Azolla species or where this plant is grown. The main issue with using Azolla on a large scale in livestock farms was the high concentration of water (~88%), but now, there are advanced procedures and inexpensive techniques to address it.

According to Anitha et al. ^[51][71], adding fresh Azolla to rabbit diets up to 60% of the basal diet had no impact on the vital organs, carcass traits, and meat chemical composition. Therefore, the unconventional feed Azolla can be recommended for growing rabbit diets and reducing diet item costs. Sireesha et al. ^[52][72] concluded that the replacement of conventional protein sources with sundried Azolla in rabbit diets by up to 10% improved the body weight gain and feed conversion efficiency in New Zealand rabbits under tropical conditions. Abdelatty et al. ^[53][73] found that incorporating 10% dried Azolla leaf meal into growing rabbit diets improved production performance and meat quality by increasing body weight and modulating the meat amino acid profile. The effect was achieved in part through rapamycin (mTOR) activation (involved in the initiation of protein synthesis). Nevertheless, higher doses of Azolla leaf meal did not appear to have positive effects on growing rabbits. On the other hand, El-Deeb et al. ^[54][74] found that the inclusion of dried Azolla substitution of up to 40% of soybean protein in the diet of growing rabbits during 5–14 weeks of age positively affected growing rabbits’ productive performance and feeding economic efficiency with no adverse impact on blood biochemical and carcass traits, particularly under Egyptian environmental conditions.

Turmeric (Curcuma longa L.) is a rhizomatous herbaceous plant belonging to the ginger family. It has a significant amount of curcumin, which is frequently used as a spice and food color ^[55][75]. Turmeric has been classified as a natural polyphenol nutraceutical and is well known for lowering oxidative stress and repairing the harm that oxidative stress has produced. Therefore, it can be used in animal diets to mitigate heat stress, especially in tropical and subtropical zones where high temperatures can delay growth and impact reproduction ^[56][57][76,77]. According to Sharma et al. ^[58][78], curcumin (the active extract of turmeric) has been linked to a number of biological processes, including antioxidant and anti-cancer actions. In addition, numerous scientific investigations have demonstrated the anti-inflammatory and lipid- and cholesterol-lowering benefits of curcumin ^[59][60][79,80]. Alagawany et al. ^[61][81] found that turmeric addition (2–6 g/kg) had no effects on rabbit growth, but improved the immunity responses and hepatic antioxidant activity in treated rabbits as well as lowered the lipid profile in the blood and lipid peroxidation in the liver. According to Sirotkin et al. ^[62][82], dietary turmeric powder at 5 and 20 g per 100 kg of the diet increases rabbit viability and fecundity (the number of liveborn and weaned pups), as well as the production and growth of ovarian follicles by altering ovarian hormone release and ovarian follicle response to gonadotropin. Moreover, Kaegon et al. ^[63][83] revealed that supplementing the growing rabbit diet with turmeric at the levels of 0.5–0.9 g per kg of the diet resulted in beneficial effects on serum metabolites, particularly creatinine, glucose, urea, cholesterol, conjugated bilirubin, total bilirubin, albumin, and total protein. El-Rawi et al. ^[64][84] stated that turmeric powder (4–8 g/kg of diet) could improve rabbit growth, digestibility, and some carcass traits. Furthermore, it enhanced immunity and served as an exogenous antioxidant. It can reduce free radicals by activating glutathione (GSH), catalase, and superoxide dismutase (SOD), and limiting ROS-producing enzymes ^[65][66][85,86]. Likewise, turmeric extract (1–2 mg/kg of the diet) improved rabbit productive performance (such as body weight gain and feed conversion ratio), has remarkable antioxidant properties, and can be used in conjunction with a miticide to treat rabbit sarcoptic mange ^[67][87]. In agreement, Okanlawon et al. ^[68][88] concluded that 1% turmeric supplementation was found to be the most cost-effective level of inclusion for weight gain, efficient feed utilization, optimum profit, and economic benefit.

Olive (Olea europaea L.) is among the most widely cultivated crops in Mediterranean countries, accounting for more than 95% of global olive production. It provides several by-products, such as leaves, cake, and oil, which are regarded as some of the most significant nutraceutical substances due to their nutritional and healthful properties. Olive leaves, cake meal (olive oil extraction by-product), and oil are employed in animal feeds as a source of energy and antioxidant substances ^[69][89]. Olive leaves display excellent preventative effects against oxidation due to their high antioxidant activity, which is derived from phenolics ^[70][71][90,91]. Olive cake meal has a high nutritional value (~10% crude protein and ~15% crude fat) ^[72][73][92,93]. It has a high concentration of non-starch polysaccharides and lignin ^[69][74][89,94]. Moreover, it contains a good amount of vitamin E, calcium, phosphorus, potassium, magnesium, sodium, and iron ^[75][95]. The use of olive cake meals as a plant source for animal diets is widespread and available in many nations for an affordable price ^[76][96]. The high oleic acid (common monounsaturated fatty acid) concentration of olive oil distinguishes it from other vegetable oils ^[77][97]. Polyunsaturated fatty acids, polyphenols, and other vital substances found in olive products increased the animal’s productivity and immune response by significantly affecting the metabolism and white blood cells and cytokines production ^{[10][78][79][80][81][82]}[33,98,99,100,101,102]. The large concentrations of monounsaturated fatty acids and the presence of underrepresented substances such as alpha-tocopherol, phenolics, chlorophyll, and carotenoids provide olive oil its usefulness ^[83][103]. The therapeutic effects of olive by-products may be owing to their high quantity of important and biological substances such as monounsaturated fatty acids, polyunsaturated fatty acids, and polyphenols, as well as their strong antioxidant capacity ^[84][104]. Additionally, olive oil promotes the absorption of vitamin A (another fat-soluble vitamin) which may raise the level of high-density lipoprotein cholesterol in the blood ^[85][105]. Consequently, olive oil is a good vitamin D solvent and can raise the levels of calcium in an animal’s body ^[86][106]. In this manner, Salama et al. ^[87][107] reported that the incorporation of olive cake with the nucleus at the level of 30% of the rabbit diets (by substituting clover hay and provided with 1% bentonite) significantly increased globulin concentration while decreasing the value of plasma cholesterol and total lipids in the treated rabbits, as well as resulting in the best total volatile fatty acids concentration in the cecal. Likewise, Dal Bosco et al. ^[88][108] mentioned that olive cakes have a high phenolic concentration and peroxide value and are rich in critical nutrients. They also emphasized the benefit of the stoning procedure, which minimizes the oxidative destruction of phenols in seed, which has the highest peroxide activity. Furthermore, the drying procedure used on the fresh pomaces may have helped to keep them from oxidizing ^[88][108]. Consequently, the addition of olive cake to animal diets can impact innate immunity as well as oxidative condition. Therefore, rabbits given a high quality of this by-product had better oxidative damage protection. Younan et al. ^[89][109] reported that under Egyptian environmental conditions, dietary supplementation with olive leaf extract at levels of 1 and 1.5 mL/kg of diet could be successfully saved and useful for growing rabbits in terms of improved production performance, alleviation of post-weaning stress, and high profitability. Furthermore, Mattioli et al. ^[90][110] investigated the use of selenium-fortified olive leaves (10% olive leaves enriched in selenium (2.17 mg of selenium per kg of dried olive leaves)) as a dietary supplementation for growing rabbits to improve the nutritional characteristics of rabbit meat. They discovered that the meat of rabbits treated with this supplementation had a better oxidative status and nutritive value. The use of selenium-fortified olive leaves improved the lipid oxidative stability of rabbit meat. In addition, Mattioli et al. ^[91][111] observed that the combination of selenium and olive leaves in the growing rabbit diet increased plasma selenomethionine and the ferric-reducing ability of plasma and reduced leukocyte DNA damage. Working on dietary polyphenols obtained from the olive oil industry to improve mammalian reproduction, Maranesi et al. ^[92][112] found that polyphenolic concentrate (282.4 mg/kg of the diet), obtained from olive mill wastewaters, inhibited inflammatory and apoptotic activities in rabbit ovary by modulating gene and protein expressions of cyclooxygenase-2 and BCL2-associated X protein, which has a significant positive impact on rabbit female reproduction.