A review of the possibilities of utilising medicinal plants in improving the reproductive performance of male ruminants

ABSTRACT

Our review condensed evidence on the potential of medicinal plants to improve the reproductive performance of livestock. The success of any livestock farming operation is highly dependent on the reproductive performance of animals. However, infertility has limited the proficiency of livestock and resulted in economic losses. For centuries, farmers utilised medicinal plants extensively in managing reproductive disorders. These plants have few to no side effects, are cheap, easily accessible and readily available. Among others, the inclusion of Moringa olifera leaf extracts for 14 days at levels of 100–300 mg/kg body weight improved sperm characteristics. Zingiber officinale root extracts at levels of 500–1000 mg/kg body weight for 3 weeks increased sperm count, viability and mobility and testosterone. Furthermore, the increase in the volume of ejaculate and sperm concentration has been observed in sheep when Leucaena spp were added to their diets at 100–300 g/sheep/day for 60 days. However, there is little literature regarding the use of medicinal plants on ruminants, as the majority of studies have been laboratory-based and have used experimental animals, including rats and mice. Thus, future research is required through in vivo and in vitro studies to ascertain the efficacy of these medicinal plants in male ruminants.

KEYWORDS:

• Plant extracts
• reproductive capacity
• nutrition
• farm animals

Introduction

Livestock is considered by most communal farmers in African countries to be an essential part of their farming system. Becker (2015) and Nkonki-Mandleleni (2019) reported livestock farming as playing a crucial role in the lives of communal farmers through the provision of meat, milk and manure, and through improved livelihoods and socio-economic relief. In addition to the benefits mentioned above, the value of livestock has been increased by 35% of total agricultural production in sub-Saharan Africa when the contribution of ruminant animals, such as cattle, sheep and goats, towards the production of draught power and manure is taken into account. These contributions partially substitute for fossil fuel-driven tractors and for the fertilisers used in developed countries (International livestock centre for Africa (ILCA) 2012).

Despite all these benefits, the decline in the production performance of livestock in many developing countries has accelerated due to unstable climatic conditions that have resulted in feed shortages and diseases, including those that affect the reproductive systems of farm animals (Hedge 2019). While a great deal of research has focused on tackling issues relating to feeding shortages and diseases, little attention has been paid to the reproductive systems of these farm animals (Tangka and Mohammad 2005). This may be because there are several factors that affect the reproduction capability of livestock, including the environment and nutrition (Smith and Akinbamijo 2000). According to Ibtisham et al. (2018), the association between nutrition and reproduction has been a focal point of late, and it has been noted that the influence of poor nutrition on reproduction can, in turn, have detrimental effects on the reproductive performance of animals. In addition, good nutrition in livestock correlates with better sperm quality and reduced risks of abnormalities in parameters such as sperm count, sperm concentration and motility (Ibtisham et al. 2018).

Over the years, farmers have used medicinal plants to correct deficiencies in the diets of livestock which has improved not only productivity but also reproduction. Generally, medicinal plants amount to 95% of the total traditional medicinal preparations in Africa (Gude 2013; Ozioma and Okaka 2019) and other parts of the world. This is because these plants have long been trusted by resource-challenged farmers as the only source of health care that has no side effects and that is to be found freely within their communities (Saleh et al. 2015). These medicinal plants contain an enormous variety of nutrients, such as minerals, vitamins and phytochemicals, to name a few. Apart from the mineral components of these plants, such as zinc and selenium, the role of omega-3 fatty acids, antioxidants and vitamins cannot be overemphasised, since their action is primarily based on the minimisation of oxidative stress which, in turn, has an influence on reproduction (Shakoor et al. 2021).

Studies about the effects of medicinal plants on fertility have provided enough evidence for them to be declared as a solution to livestock infertility (AI-Snafi; Jalal et al. 2000; Biressaw 2017) Thus, the purpose of this review was to (1) summarise the potential of medicinal plants in improving the reproductive performance of male ruminants; (2) outline the medicinal plants available in Southern African that can augment the reproductive performance of male ruminants and (3) and describe the compounds available in these medicinal plants that are of great significance in reproduction.

Methodology

The focus of this review was on the possible use of medicinal plants to improve the reproductive performance of male ruminants. This is because the level of infertility in farm animals keeps on rising and is threatening the gene pool of indigenous livestock species, mostly in developing countries (Kumaresan et al. 2020). Veterinary services and access to pharmaceutical drugs are beyond the reach of most small-scale farmers, resulting in poor-performing animals being culled and causing financial distress to farmers. Thus, over a number of years, researchers have actively tried to come up with possible ways of reducing costs resulting from the poor reproductive performance of farm animals (Kahan 2008; Koketsu et al. 2017; Deka et al. 2021). Taking this into account, it was of paramount importance to review the literature in this field concerning the possible use of medicinal plants to improve reproductive performance. The literature was drawn from peer-reviewed journals, books and conference proceedings published in various databases. Keywords and catchphrases such as ‘medicinal plants and reproductive performance; reproductive disorders of farm animals; the role of medicinal plants in livestock; efficacy of medicinal plants on reproductive performance’ were included in the search of each database. A critical mixture of words and phrases was used to obtain the required information, leading to author searches on databases such as Scopus, Web of Science, AGORA, Science Direct and Google Scholar. In the first step of elimination, the search process was conducted, using the Google Scholar search engine, where 307,000 studies were listed that included ‘medicinal plants’ and ‘reproduction’ as keywords. The majority of these references had a different focus from the current study and were declared irrelevant to the scope of this study. Accordingly, the keyword ‘ruminant’ was added to the search to eliminate results falling outside the desired focus. As a result, 44,300 references were obtained. These were scrutinised solely on the basis of title and keywords. Following this process, some studies were excluded from our search for a variety of reasons and the 280 remaining full-text studies were assessed for eligibility. The remaining additional literature was included based on academic resources (Master's and Ph.D. dissertations), PLoS ONE and the Directory of Open Access Journals.

The origins of herbal medicine

In the past, people relied on the nature of medications to treat their health problems. When humans first started using medicinal plants they judged the efficacy of these plants according to their instincts; there was no point in referral (Pan et al. 2014; Jamshidi-Kia et al. 2018). During those times, the experience was a useful measure of the aetiology of diseases and of the efficacy of various plants in eradicating such diseases, as little to no knowledge existed on ethnoveterinary medicine. Pan et al. (2014) reported ethnoveterinary medicine as dating back to 3000 years BCE, and this knowledge existed worldwide. As time passed, herbalists discovered that selecting some plants over others would be effective in curing certain diseases. Thus, the use of medicinal plants progressively abandoned the empiric framework and became founded on explicatory facts. Before the arrival of pharmaceutical drugs and iatrochemistry in the sixteenth century, people relied on plants for the treatment and prevention of diseases (Kelly 2009; Dzoyem et al. 2020). Populations, such as the Indians, Egyptians, Chinese, the Greek and Roman civilisations proved the healing power of medicinal plants when they used them to treat various diseases and to energise their body systems (Pan et al. 2014). The use of medicinal plants by Indian nations is based on theories and beliefs, as they are selected for their healing potential; and such plants also find a place as processed products of the pharmaceutical industry (Pan et al. 2014). In addition, the use of medicinal plants has increased over the years, especially in light of the discovery of the downsides of synthetic drugs; among others, the decrease in their efficacy, resistance, toxicity and increasing concerns over the presence of drug residues in animal products which increase the health risks posed to consumers (Suleiman et al. 2014; Iwu et al. 2020).

Reproductive disorders of farm animals

Diseases, poor nutrition, the low genetic potential of indigenous breeds and deficient management systems, negatively impact the productivity of farm animals in African countries where livestock farming is at the core of local livelihoods (Perry and Grace 2009; FAO 2011). Fertility is mostly affected by various diseases or syndromes that quite often lead to sterility in some animals (De Kruif 2003); and structural defects can be seen on animal reproductive systems during physical examination. However, at times, this problem requires analysis of the semen. Some conditions change the morphology of sperm cells, leading to poor semen quality, while other conditions make it difficult for the male animal to mount (Tohura et al. 2018). Hedge 2019, reported that farmers experience tremendous economic losses due to the unplanned culling of poorly performing animals with poor mating performances, physical challenges and unhealthy sperm counts. Mating challenges in male animals affect the entire production system since it results in a need for additional male animals which extends the breeding season and lowers the number of offspring born per animal (Gowane et al. 2019). Yahya et al. (2016) reported bacterial infections to be the major contributing factor to infertility. Most farmers relied on antibiotics as the most important type of antibacterial agent for fighting bacterial infections. Antibiotics may either kill or inhibit the growth of bacteria. However, the downsides of antibiotics, including resistance, make it difficult to control fertility problems associated with bacterial infections (Yahya et al. 2016; Fakhri et al. 2019), resulting in increased rates of infertility worldwide, along with varying etiological factors, including physical and psychological conditions (Malviya et al. 2016). Infertility can lead to the loss of income for farmers because of decreased milk production in dairy cattle, a decrease in the calves produced and increased culling rates (Perumal et al. 2013). The culling of cattle due to infertility and sterility per annum is estimated at 18–40% in India (Kaikini 2002), 25% in France (Colleau and Moureaux 1999), 21% in Iran (Ghaderi-Zefrehei et al. 2017) and 18.5% in South Africa (Muller 2017). Kumar et al. (2012) reported a negative relationship between reproductive indices that are used to assess reproductive management and increased knowledge on the subject matter over the past decade. When artificial insemination is practised, failure to time oestrus contributes to low fertility. Literature and practical experiments provide evidence that nutrition has a direct impact on reproduction and has the ability to lower the effects of other factors. Thus, infertility, the effects of harsh environmental conditions and poor management techniques can be managed through dietary manipulation. Moreover, nutritional factors, more than any others, positively interact with reproductive indices to ensure positive results (Smith and Akinbamijo 2000). These challenges result in lower fertility in farm animals, leading to poor economic benefits from the sector (Lobago et al. 2006). Despite the advancements in modern technology in diagnosing and curing infertility, the levels of infertility keep on accelerating in farm animals (Kumaresan et al. 2020). Flock profitability is positively associated with reproductive performance (Pardos et al. 2008) in which the ability of the male to successfully mate and produce viable spermatozoa to impregnate the female is of paramount importance. It is noteworthy that medicinal plants not only play a role in nutrition but also potentially assist in overcoming reproductive disorders, due to their ample phytochemicals, such as steroids and folic acid. Although there is increased accessibility of pharmaceutical drugs for the management of livestock fertility-related disorders, herbal remedies increased the inventory, of available treatment options, especially for resource-challenged farmers (Dada and Ajilore 2009). Plant remedies to enhance the reproductive performance of livestock are under investigation worldwide and are being incorporated into medical practice (Kumar et al. 2012). In addition, herbal remedies are rich in compounds showing that antioxidant and antimicrobial activities have the potential of fighting impotence and improving fertility (Zhang and Liu 2005). Furthermore, when garlic (Malviya et al. 2011) and Garcinia kola (Ralebona et al. 2012) are added to feeds for experimental animals, their erectile dysfunction and sexual performance are successfully improved. Thus, for a farming system to succeed, the reproductive performance of each animal in the herd should be optimised (Abbas 2017).

The role of medicinal plants in livestock reproduction

Sarswat and Purohit (2020) reported medicinal plants to be effective in treating reproductive disorders, thereby, increasing the reproductive potential of livestock. Extracts from plant preparations are easily accessible and safer to use, compared to veterinary drugs (Mahomoodally 2013; Saleh et al. 2015). For centuries, Cochlospermum planchonii has been appreciated by the majority of farmers in Southern Africa for its therapeutic properties, the plant having been used in the treatment of intestinal worms, typhoid and urinary tract infection (Blench 2007; Togotla et al. 2008), of intestinal worms, typhoid and urinary tract infection (Blench 2007; Togotla et al. 2008), Furthermore, when Cochlospermum planchonii was used as a supplementary diet in West African Dwarf goats their sperm quality was improved, thus, an increase in total sperm count, concentration and motility was noted (Mhomga et al. 2019). These findings are per Abu et al. (2012). Abu et al. (2012) further explained that antioxidant constituents such as flavonoids, terpenoids, saponin, tannins, steroids and cardiac glycoside constituting Cochlospermum planchonii have a positive influence on the reproductive indices of livestock. Consequently, these positive results could have been possibly influenced by the availability of rich antioxidants that are known to have the ability to prevent lipid peroxidation by boosting the testicular non-enzymatic and enzymatic antioxidants to effectively scavenge the free radicals (Abu et al. 2012.; Mhomga et al. 2019). These effects are witnessed in the overall sperm characteristics and decreased abnormal sperm morphology. Moreover, other important metabolites contained in Cochlospermum planchonii include flavonoids which are known to enhance spermatogenesis also, the multivitamins and phytochemicals, which could be linked to the increase in the volume and density of germinal epithelium of animals (Aitken and Roman 2008). Studies have reported the effects of different medicinal herbs, such as Asparagus adscendens Roxb, on the male reproductive organs of rats (Bansode et al. 2015). Asparagus adscendens stem has been reported as treating seminal weakness and can act as an aphrodisiac (Bansode et al. 2015; Sharma et al. 2013). In addition, Asparagus adscendens Roxb is rich in compounds such as saponins and flavonoids which have the ability to synthesise steroid hormones, these hormones provoke a surge in steroidal biosynthesis, increase testosterone and gonadotrophin activity, to improve the reproductive capacity and mating desire and abilities of animals (Haren et al. 2002; Chauhan and Dixit 2008 and Thakur et al. 2009). Chlorophytum borivilianum has been reported to improve scrotal homeostasis and prevent testicular cell death, thereby increasing the male reproductive capacity (Ray et al. 2014). The efficacy of aqueous root extract of Lecaniodiscus cupanioides Planch. Ex Bth, a plant indigenous to African and Asian countries to improve reproductive performance was tested in male rats, the plants exhibited the potential to improve reproductive indices by restoring the action of acid and alkaline phosphatases, gamma-glutamyl transferase and lactate dehydrogenase, as well as testosterone levels in the testes (Nurudeen and Ajiboye 2012). Alkaloids, anthraquinones, phenolics, saponins and tannins are highly associated with positive reproductive indices (Ajiboye et al. 2014). Other studies reported Securidaca longepedunculata to have therapeutic and reproductive benefits; and, in addition, its root extracts have been reported to have the potential for treating various diseases or conditions, including toothache, tuberculosis, rheumatism and pneumonia, and is also a blood purifier. It is, furthermore, used as an aphrodisiac for men (Mongalo et al. 2015). Several studies have attested to the efficacy of S. longepedunculata in the treatment of fertility and general health-related challenges in human beings (Maroyi 2013; Mustapha 2013; Nordeng et al. 2013; Mongalo et al. 2015). However, there is scant information on the use of this plant to improve fertility in ruminants. The root and leaf extracts of S. longepedunculata have been praised as having biologically active compounds of importance to both humans and livestock. The extracts of this plant have been successfully utilised in the treatment of bacterial, parasitic, diabetes and inflammatory conditions in humans (Mongalo and Mafoko 2013). In addition, other studies have reported the plant to have antimalarial, insecticidal, pesticidal and anticonvulsant properties (Chibougwu et al. 2017). However, both the in vivo and in vitro studies have suggested the rootbark of S. longepeduculanta to be lethal at relatively high levels of above 100 mg/kg body weight (Mongalo 2013; Mongalo and Mafoko 2013; Zongo et al. 2013). Despite that when Chibougwu et al. (2017) introduced S. longepedunculata rootbark extract at a level of 50 mg/kg body weight in rabbit diets, positive results were observed where there was an increase in testicular morphometrics. Thus, it can be attested that supplementation of S. longepedunculata root bark extract at correct levels can promote reproductive indices of male animals. In addition, Zongo et al. (2013) reported that the inclusion of S. longepedunculata in low concentrations also improves spermatogenesis. The increase in weight and length of paired testes, as reported above, is due to the presence of steroids and saponins (Chibougwu et al.2017).

Medicinal plants are components of nature and most of them have no undesirable secondary effects on human and animal bodies (Mahomoodally 2013). Perumal et al. (2013) reported that the use of Phoenix acaulis (water chestnut) in treating reproductive disorders in cattle has increased sperm cell production and has been effective as a common aphrodisiac. The efficacy of medicinal plants has been appreciated as they promote the reproductive performance of livestock and are also used for treating livestock disorders (Sarswat and Purohit 2020). Garcinia kola has been claimed to be used in the treatment of seminal weakness (Panda et al. 2011), in addition, an improvement in fertility; increase in testicular size, scrotal circumference, sperm concentration and viability have been noted (Tahvilzadeh et al. 2015).

Garcinia kola, commonly known as bitter kola, is claimed to possess aphrodisiac effects in African traditional medicine (Yakubu and Quadri 2012). In a study, where 70% ethanolic extract of G. kola was administered at 100, 200 and 400 mg/kg daily for 56 days, the testes of the rats that were the subjects of the trial were later examined and the findings showed that the plant had potentially improved the sexual capacity of the rats and improved testicular weight (Sewani-Rusike et al. 2016). These findings are in line with reports by Ebenebe et al. (2017), who reported Garcinia kola to have the ability to increase peripheral testosterone and sperm motility. This increase in testosterone and tissue enhancement has been attributed to antioxidant compounds present in the bitter kola (Oluyemi et al. 2007). They further observed that bitter kola can increase sperm cell count in line with earlier reports to the effect that it increases spermatogenic activity through its tissue enhancement ability; thus, increasing the production of spermatids (Guyton and Hall 2000) and, invariably, increasing the sperm count. The increase in sperm count is associated with the effect of antioxidants, such as carotenoid, present in Garcinia kola that protects spermatogenesis (Attehsahlin et al. 2006).

The search for natural compounds in hormonal therapy has increased recently, and the efficacy of herbal extracts has been demonstrated in incremental increases in testosterone levels and enhanced fertility (Tahvilzadeh et al. 2015). Administration of an alkyl amide-rich ethanol solution extract of Anacyclus pyrethrum DC roots effectively increased reproductive hormones, such as serum testosterone, luteinising hormone (LH) and follicle-stimulating hormone (FSH) concentrations in male rats (Sharma et al. 2013). JianFeng et al. (2012) reported a notable rise in the serum testosterone levels of male rats to which A. lappa L. root extract had been administered. Despite all the reported benefits of medicinal plants on reproduction, further research is required as inconsistencies have been observed in the effects of the same medicinal plants on spermatogenesis. Chibougwu et al. (2017) reported an increase in the testicular parameters of male rats with the administration of S. longepedunculata at 50 mg/kg body weight. However, when the dose was increased to 100 mg/kg body weight, lower testicular parameters were measured, this is in agreement with Mongalo and Mafoko (2013) and Zongo et al. (2013) who reported the increased dose of the plant to have some toxicity effects. In addition, Mekoya et al. (2009) reported the increase in testicular degeneration to be higher in sheep than goats when fed Leucaena pallida for six months at 200 g/kg body weight, suggesting that sheep may be considerably more susceptible to L. pallida than goats. This may be due to differences in research models, the extracts, the administration route, doses and the experimental period; and differences in the part of the plant used and variations in plant harvesting (Abbas 2017). Jinadu et al. (2018), in their study on semen characteristics and scrotal size of pubertal West African dwarf rams fed diets containing Garcinia kola seed meal, reported an increase in scrotal diameter with increased crude protein. The scrotal circumference is positively correlated to testis weight, sperm production and semen quality, age at puberty and body weight (Puglisi et al. 2016). The role of medicinal plants in reproduction was given more attention in studies on experimental animals; however, there is scant information on their roles in other livestock species, including poultry and pigs. Moreover, this warrants further studies on medicinal plants shown to have a significant effect on reproductive parameters.

The efficacy of medicinal plants on the reproductive parameters of farm animals

Owing to their effectiveness, safety, few to no side effects and the low costs of medicinal plants, they are trusted by most livestock farmers worldwide in treating livestock disorders (Mahomoodally 2013). Phytochemical analysis of these plants has revealed safer and cheaper sources of chemical compounds such as alkaloids, flavonoids, pigments, phenols, terpenoids, steroids and essential oils that possess a diverse range of bioactivity (Iwalewa et al. 2007). Supplementation of medicinal herbs in the diets of animals has been found to positively aid in physiological processes in the body, for instance, it works against stress, promotes growth stimulates appetite, hosts antimicrobial activities and strengthens the immune system (Citarus 2010). These medicinal plants are used in varying forms – powdered, fresh, dried, chopped or added to solvents such as water, ethanol or petroleum ether; solvents play a major constituent of traditional medicine (Mukherjee 2002).

The therapeutic uses of several medicinal plants and their influence on the reproduction of farm animals, as described in the literature, are summarised in Table 1. However, the section below will provide an elaborate explanation of the role and efficacy of these medicinal plants.

Table 1. Therapeutic plants for reproductive disorders in male animals.

Plant name	Extract	Active compound	Dose and duration	Species tested	Observed effect	References
Chlorophytum borivilianum L.	Roots	Saponins, flavonoids, steroids	125 mg/kg body weight, 14 days	Rats	Aphrodisiac activity and enhanced libido, sexual vigour and sexual arousal	Malviya et al. (2016)
Zingiber Officinale	Roots	Volatile fatty acid gingerols	500–100 mg/kg body weight	Rats Buffalo bulls	Increase in sperm count, viability and mobility testosterone	Morakinyo et al. (2008), Kankanam et al. (2020)
Mixture of (Leucaena leucocepala and Garcinia kola)	Leaves	Steroids and folic acid	–	Boars	The boars showed an increase in sperm production and libido	Mekoya et al. (2009), Kuo et al. (2004)
Dacus Carota	seed	Volatile oils, Phenols, Flavonoids, carotene	200 and 400 g/kg of the diet.	Rats	Increase in testosterone levels	Nouri et al. (2009)
Lannea acid	Stem bark	Alkonoids and vitamins	50, 100, 200 and 400 mg/kg of the diet	Rats	Protect morphology of the testis and decrease spermatozoa abnormality	Ahmed et al. (2013)
Dana racemosa	Whole plant	Flavonoids and vitamins	200 and 400 mg/kg of the diet	Rats	Increased sperm mobility and viability
Hylocereus co-staricensis	Flesh	Ascorbic acid and polyphenols	250, 500, 1000 mg/kg body weight	Mouse	Increased sperm count, viability and production rate	Aziz and Noor, (2010)
Securidaca longempeduculata	Roots	Steroids, saponins, volatile oils	50–100 mg/kg; 60 days	Rabbits	Increased testicular weight	Zongo et al. (2013), Chibougwu et al. (2017)
Cochlospermum planchonii rhizome	Leaves	Saponins, steroids, terpenoids	0, 5, 10, 20 and 40% C. planchonii rhizome in the diet	Goats	Increased sperm concentration, Increase in the length and diameter of the testis.	Mhomga et al. (2019)
Smilax ornate	Leaves	–	–		Increase in testosterone and progesterone levels	Perumal et al. (2013)
Moringa oleifera	Leaves	Saponins, steroids	100–300 mg/kg body weight	Rats, Albino mice	Increased sperm motility, increased sperm density, motility and reduced head defect and DNA damage	Jaiswal et al. (2009), Sadek (2014), Nayak et al. (2017), Abarikwu et al. (2017)
		Flavonoids, vitamins, proteins, amino acids	70 g/kg diet	Chickens	Increased semen pH, volume and progressive motility.	Tubulang et al. (2022)
Moringa oleifera	Leaves	Zinc, saponin, alkaloid, flavonoid, ferulic acid, and chlorogenic acid	15% of the weight of the concentrate	Bulls	Increase in plasma testosterone concentrations, libido and sperm motility.	Syarifuddin et al. (2017), Prabsattroo et al. (2015), Chauhan and Dixit (2008)
Leucaena pallida	Leaves		100, 200 and 300 g per herd per day	Sheep	Sheep displayed an increase in the volume of ejaculate and sperm concentration	Dana et al. (2000)
Panax ginseng	Root	Ginsenosides	5% of the extract	Rats and Mice	Increased blood testosterone, serum levels of LH and FSH which increased spermatogenesis. Increased copulatory behaviour and dose-dependent increase in mounting, intromission and penis licking in mice.	Oremosu et al. (2013), Leung and Wong (2013)
Boesenbergia rotunda	Rhizome	Flavonoids, Polyphenols essential oils (Nerol, camphor, cineole)	–	Bulls	Improvement in semen ejaculate

Moringa oleifera

Moringa oleifera is a tropical plant with extraordinary nutritional and medicinal significance (Alegbeleye 2018). Its anti-inflammatory, antioxidative and anti-diabetes modulations have made the plant one of the more popular medicinal plants (Kohei et al. 2011). Mercury has been noted among the industrial hazards that adversely contaminate the environment where human beings and livestock live. The substance adversely affects the reproductive systems of humans and animals (Boujhiha et al. 2009). However, there is a minute of literature on how mercury causes male reproductive dysfunction. Additionally, Abarikwu et al. (2017) reported that Moringa olifera to prevent testicular damage in rats, plants have the ability to prevent the effects of mercury (HgCl₂)-induced testicular catalase (CAT) and superoxide dismutase (SOD) activities, and further, improve sperm quality and testosterone levels. Moringa also blocks the adverse effects of CO + HgCl₂ (2 ml kg⁻¹ body weight + 5 mg kg⁻¹ body weight) on the investigated endpoints in mercury-induced testicular toxicity in rats. This was proven when the administration of 2 ml/kg⁻¹ body weight of Moringa oil prevented mercury from inducing any damage to the testes of the rats concerned. Furthermore, positive results on testicular mass were shown when the medication was amended to leaf extract and the dose increased to 500 mg/kg⁻¹ orally for 60 days’ (Sadek 2014). Perhaps Moringa had such an influence on these reproductive parameters, as indicated above, due to the presence of chemical compositions in the leaves as an excellent source of nutrients. Moreover, Moringa oleifera is popularly known for being rich in vitamins A, C and E, beta-carotene and strong antioxidative phytochemicals (kaempferol, quercetin, rutin and caffeoylquinic acids) (Jaiswal et al. 2009; Vongsak et al. 2013; Sebola et al. 2015), these compounds have a positive influence on reproductive indices hence the aforementioned effects (Prabsattroo et al. 2015). This conclusion thus corresponds with Roy et al. (2013) and Syarifuddin et al. (2017), who reported the zinc minerals in Moringa oleifera leaves provoke the Leydig cells to produce testosterone for the normal function of the hypothalamuspituitary–testes axis. They, furthermore, showed that the lack of zinc can result in detrimental damage to the LH receptor mechanisms in controlling, storing and releasing testosterone. Furthermore, Tubulang et al. (2022) noted that the inclusion of Moringa olifera leaf meal at 70 g/kg of the diet of locally bred chickens had increased semen pH, volume and progressive motility. What goes unanswered in the literature is the dose of this extract that would be suitable to enhance the reproductive parameters.

Rhodiola spp

Rhodiola sacra is a herb, commonly known as golden root, which is found in the cold mountains of Europe and Asia (Tao et al. 2019). The roots of this herb contain more than 140 active ingredients, and it is, today, widely used as a dietary supplement for humans and livestock due to its many health benefits. Several researchers have explored its abilities as a reproductive enhancer in animals, with Zhao et al. (2009) showing that R. sacra aqueous extract enhanced biochemical and sperm characteristics in cryopreserved boar sperm. Their findings are similar to those of Malo et al. (2011) who reported an improvement in sperm characteristics and fertility after cryopreservation with supplementation of Rosemary (Rosmarinus officinalis) in a freezing medium. Furthermore, the antioxidant properties of Rosemary (Rosmarinus officinalis) are brought about by bioactive substances, such as diterpenes, triterpenes, flavonoids and polyphenols and sesquiterpenes, known to have positive effects on reproductive performance (Rašković et al. 2014; Pintus and Ros-Santaella 2021). In addition, Hualin Cao et al. (2013) reported the positive effects of Rhodiola sachalinensis saccharide (RSS) on bull sperm during cryopreservation at levels (0.02–0.10 mg/mL); thus, the extenders with added RSS resulted in a higher percentage of cryopreserved sperm motility, mitochondrial activity, membrane and acrosome integrity than those in the control group. Owing to the increased antioxidase and antiapoptosis activity, RSS has the ability to protect bull sperm from the damage of freeze-thawing (Tian and Wang 2006) and abilities to eliminate O² and restrain lipid peroxidation (LPO; Zhang and Liu 2005). Related studies show that RSS also has functions of antistatic and dynamic fatigue (Chen et al. 2000). These characteristics may help to reduce sperm damage during the process of freeze-thawing.

Curculigo orchioides (Black musli)

Curculigo orchioides, commonly known as the golden eye or black musli, is an endangered flowering plant species in the genus curculigo (Chauhan and Dixit 2008). The plant is native to India, China and Japan. When the effects of Curculigo orchioides were studied in rats, it was found that the plant has the ability to improve overall mating capacity. The researcher concluded that this effect may be due to the presence of chemical compounds, such as alkanoids, steroids and saponins (Thakur et al. 2011).

Zingiber officinale (ginger)

This plant is of the family Zingiberaceae and is popularly known as ginger, it grows as a brownish coloured rhizome with shoots. Traditionally, the plant has been used in the treatment of stomach irritations, nausea and arthritis in humans. In addition, ginger is used worldwide in the treatment of common cold and flu-like symptoms, headaches and even with painful menstrual periods. Morakinyo et al. (2008) investigated the effect of ginger on the reproductive functions of male rats. Their results showed that ginger has a beneficial effect. These effects were proved by a recorded increase in sperm count, viability and mobility and testosterone. Meanwhile, decreased malondialdehyde levels were observed when male albino rats were dosed with 500–1000 mg/kg body weight of Zingiber officinale. Thus, the authors concluded that the presence of volatile acids, gingerols and shogaols resulted in improved sperm quality. The increase in testicular and epididymal weight may have resulted from the increased effects of androgens on the testis, as proven by a substantial increase in serum testosterone levels in the experimental rats. The influence of androgens on gonadal development and functioning is attested in the literature and it has further been noted that testosterone levels are positively correlated with scrotal contents (Perumal 2014).

Aloe spp

Aloe vera is a perennial herbaceous plant of the family Liliaceaee. The ideal medicinal properties of this plant make it an appraisable pharmaceutical plant. Modaresi and Khodadadi (2014) investigated the effect of aloe on spermatogenesis and reproductive hormones in mice. The findings showed that the administration of Aloe vera extract potentially brought about a significant increase in the number of stem cells and primary spermatocytes in all treated mice groups. In addition, similar results were observed by Mehrdad and Alireza (2014), when Aloe barbadensis was given at a dose of 50–200 mg/kg body weight for 20 days. These researchers, therefore, concluded that the apoptosis factor in Aloe vera which increases spermatozoa increased in the parameters mentioned above. Furthermore, aloe is rich in flavonoids and vitamins A, C and E that are capable of reducing free radicals and supporting sperm production and the health of sperm against oxidative stress (Mehranjani and Taefi 2012).

Leucaena spp

Leucaena is a small, fast-growing memosoid tree native to southern Mexico and northern Central America and is now also found in all tropic regions. Countries, such as Ethiopia, have suspended the use of Leucaena as feed for animals (Middleton et al. 2002). Non-protein amino acid, mimosine (b-(N-(3-hydroxy-4-oxypyridyl))-aminopropionic acid) and the rumen degradation product 3-hydroxy-4 (1H)-pyridone (3, 4-DHP) are the ones which resulted in this prohibition (Liener 2003). Mimosine, found in various parts of the plant, has been associated with poor reproductive performance in animals, revealed by abortions and infertility (Crawford et al. 2015). However, Dana et al. (2000) opposed the aforementioned toxic effects of this plant on reproduction, their study showed no traces of toxicity on animals and the plant significantly increased feed intake, sperm production and semen quality. Anti-oestrogenic compounds and the presence of nitrogen in ruminants, increase feed intake through improved rumen fermentation (Niderkorn and Baumont 2009), resulting in improved digestibility and faster rates of the passage of digesta through the gastro-intestinal tract (Niderkorn and Baumont 2009).

Panax ginseng

The Panax species of the ginseng plant have been studied well. The plant has been reported to demonstrate aphrodisiac effects in both animals and humans (Leung and Wong 2013). In addition, De Jong et al. (2005) and Kim et al. (2005) reported the plant to possess antioxidant properties and enhance immune function and synthesis of nitric oxide. From ancient times the Chinese community has relied on the plant for treating sexual dysfunction and enhancing the sexual behaviour of their animals. When 5% of ginseng extracts were administered orally in rats, an increase in serum levels of LH, FSH and spermatogenesis was observed (Oremosu et al. 2013). Ginsenosides, which is the major pharmacological active compound of ginseng, is believed to have been attributed to the actions mentioned above. The ginsenosides has the potential to cause erection through the use of nitric acid which causes relaxation of the smooth muscle and allows for blood flow to enter the erectile bodies known as corpus cavernosum.

Boesenbergia rotunda

B. rotunda grows naturally in tropical regions, mainly in South and Southeast Asia and China. Its common name, fingerroot, is derived from the appearance of its rhizome which is like a finger. An investigation of the extracts of B. rotunda, using the extracts of its leaf, stem and rhizomes revealed the plant to be pharmacologically active, inducing aphrodisiac activity. When B. rotunda leaf, stem and rhizomes extract were given to rats at levels 0.06, 0.12 and 0.24 g/kg of ethanolic extract for 60 days, the plant exhibited some aphrodisiac activity, thus there was an increase in the diameter of seminiferous tubules and increase in the weights of the testicular and seminal vesicle. However, there were no effects on sperm parameters (Sudwan et al. 2007). The researchers concluded that active compounds such as flavonoids, alkanoids and saponins could be the reason for these results. In pharmacological studies, flavonoids have been shown to have oestrogenic (Resende et al. 2013) or androgenic activities (Nishizaki et al. 2009). These compounds exhibit phosphodiesterase inhibitors like Viagra (Ko et al. 2004). Additionally, the alkaloids have the ability to mimic the effects of the cyclic adenosine, monophosphate, by binding competitively to the phosphodiesterase regions and acting as a nonselective phosphodiesterase inhibitor (Rahimi et al. 2010). They also have the capability to manipulate the receptors in the hypothalamus to induce erection (Hagel and Facchini 2013). While Saponins, on the other hand, are the non-nitrogenous secondary metabolites that are the components of the higher plants. Some saponins have the enzymatic functions to break a phosphodiester bond and some are anti-stress agents (Güçlü–Ustündag and Mazza 2007; Wang et al. 2014)

Conclusion

This study highlighted the possible use of medicinal plants in improving the reproductive performance of male ruminants. Several studies have attested to the efficacy of various medicinal plants for reproductive performance in experimental animals. It can be concluded that literature on the use of medicinal plants to enhance the reproductive performance of animals is scant. However, medicinal plants are capable of alleviating sexual dysfunction and can be a solution to poor reproductive performance in livestock due to their antioxidant and antimicrobial activities.

Limitations of the study

The current review attempted to summarise the medicinal plants with the potential for enhancing the reproductive performance of male ruminants. However, despite the advances that have been made in traditional medicines, there is still scant literature on using these plant extracts to improve reproduction, particularly livestock. Trials on the efficacy of medicinal plants in improving reproductive performance have increased recently; however, the focus has been on humans and experimental animals, with few to no studies on most domestic ruminants. Moreover, most research attained only experimental results and did not explain the detailed mechanisms.

Future recommendations

Infertility is a key constraint to the accomplishments of high competence levels and commercial production in a livestock operation. Taking into account the limitations of the current study, it is recommended that further research be conducted to identify the bioactive substances and their chemical structures in plant extracts. Further in vivo and in vitro studies must be conducted to find the individual effects and effective dosages for different animals. This will allow for detailed explanations of the results and exact mode of action of plant-derived antioxidants in ruminants’ reproduction to be drawn up. It is further recommended that future research focus on clarifying the conflicting results observed from various items of literature when similar medicinal plants yield varying effects on the reproductive parameters of the same species. The latter may be a result of unstandardised extraction methods, routes of administration and dosage. Identifying active compounds, and standardising extraction methods and doses may reduce the cases of toxicity observed in varying studies when doses of similar plants are increased, or when the experimental periods are prolonged. This will result in improved reproductive performance in most livestock species.

Acknowledgements

All these authors were involved in the conceptualisation of the review and worked together in the drafting of the manuscript. In addition, all parties took part in revising the work critically for important intellectual content, they agreed on and approved the final version to be sent for publication to the authors further, ensured the accuracy and integrity of the work and all agreed to take accountability for all aspects of the work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing is not applicable to this article as no data sets were generated or analysed during the current study.

Additional information

Funding

We are grateful to the Gauteng Department of Agriculture and Rural Development (GDARD) for their support.