1. Introduction
The basis of the high productivity of animals is balanced normalized feeding. It satisfies the need of animals for nutrition elements (proteins, fats, carbohydrates, amino acids, vitamins, macro-, and microelements) against the background of their safe, sanitary, and hygienic maintenance. Simultaneously, the growth and productivity of animals are closely related to the functional activity of the nervous, immune, and endocrine systems. With industrialized livestock, stress and immunodeficiency precede many diseases and cause pathological conditions of varying severity, reducing the quantity and quality of livestock products [13, P. 1]. Additionally, contamination of the microflora of the stomach with pathogenic microorganisms and feed with synthetic and natural toxicants comprehensively affect the health of the animal. It affects its immunity and severely limits the actualization of genetic potential in practical animal husbandry.
To combat bacterial infections in animal husbandry, since the early 1950s, antibiotics, producers of fungi (e.g., penicillin), or bacteria (e.g., tetracycline) have been added to the feed. They can be synthetic or semi-synthetic substances
[42][44, P. 14]
The negative effects of antibiotics are greater deposition of subcutaneous fat instead of dietary protein (deterioration of meat quality); with constant use, it is necessary to increase doses, but which, on the contrary, reduces productivity. Another issue is that antibiotics in the body of animals tend to accumulate in individual organs and tissues. During the culinary processing of such products, some antibiotics are not destroyed. The third issue is that antibiotics can weaken the animal’s body, concerning adverse environmental factors. Therefore, their use is not recommended for breeding animals. Antibiotics are potent substances that are released according to list B. It is forbidden to take them for other purposes and mix two or more types
[42]
Since January 1, 2006, all types of feed antibiotics have been completely banned in the EU countries, except for the ones prescribed by veterinarians
[11][7][12]
Simultaneously, in the 1960s and 1980s, tranquilizers (psychotropic drugs with hypnotic and sedative effects) began to be used to protect against stress
[42][19][30]
Currently, all synthetic means of stimulating the growth and productivity of animals (hormonal anabolic, tranquilizers, antibiotics) are banned in most countries or are under prohibition. Even their trace amounts can have a very harmful effect on human health [27]. It is also believed that many feed antioxidants of synthetic origin, used to level oxidative stress, can inhibit the immune system, which in turn leads to suppression of the mechanism of protection against pathogenic microflora, and therefore are prohibited in many countries.
Therefore, to significantly reduce the share of infectious diseases in animal husbandry, one should apply preventive measures more widely, switching to phytobiotics (alternative herbal antimicrobials)
[11][20]
2. Materials and methods
The research goal is the systematization of scientific literature data on natural biologically active substances in the composition of feed additives and their sources according to the criteria of anabolic and anti-stress effects and safety issues when used in animal science and veterinary medicine. The following tasks are set to achieve the goal:
1.
2.
3.
4.
The search for literary sources (as of July 2021) was performed in the following scientific electronic libraries with built-in search engines: Wiley Online Library
[45][29][33][10][8][24]
3. Results and discussion
3.1. Phytobiotics: assortment of plants, active substances, and restrictions
Phytobiotics or phytogenic feed additives are products of plant origin used in animal feeding to improve health and stimulate growth and productivity. The primary mechanism of action promoting growth is the result of stabilization of feed hygiene and beneficial effects on the ecosystem of gastrointestinal microflora by controlling potential pathogens. Other mechanisms of action include increased appetite, digestibility, and absorption of nutrients, improved immune response, induction, or inhibition of metabolic enzymes. The antioxidant effect of some phytogenic compounds is crucial due to the protection of the quality of feed and products obtained from animals fed with these biologically active substances. The highest correlation of the decrease in the antioxidant defense system is observed in connection with the stress factors accompanying care and cultivation.
The plants used in practice usually belong to the following botanical families: Labiatae
[20][21][26]
The most active secondary metabolites of phytobiotics belong to the class of terpenoids, flavonoids, glucosinolates, steroids, and saponins. Their effects are antimicrobial, anti-inflammatory, antioxidant, antiparasitic, and antiviral; they also include increased feed intake and digestibility of nutrients. Polycondensed phenols (tannins) bind proteins, polysaccharides, and other biopolymers and protect them from decomposition by the microflora of the stomach, have a tart astringent taste, and inhibit the growth of pathogenic microorganisms. Some polyphenolic compounds are metabolized in the gastrointestinal tract through bacterial enzymes, which play an essential role in the bioavailability of phenolic glycosides.
The antioxidant potential of medicinal, spicy, and essential oil plants is primarily associated with the concentration of flavonoids (quercetin, rutin, etc.), hydrolyzable tannins, proanthocyanidins, phenolic acids (benzoic, cinnamon, coumarin derivatives), phenolic terpenes (volatile essential oils), vitamins (A, C, and E), and carotenoids
[41][12][14]
According to the international analytical data
[12][46][12]
Other limitations are the variability and inconstancy of the composition of phytobiotics. The composition of phytobiotics varies widely depending on the botanical origin, substance composition, and technological processing. Therefore, they are difficult to quantify. However, there is still no standardization of phytobiotics for active substances and attempts to perform this reveal cytotoxicity in very small dosages
[25]
3.2. New and promising bioactive components in phytobiotics
Recently, due to the weak effectiveness of phytobiotics in stressful situations, plants containing potent substances have begun to be involved in research. Among them is Macleya which synthesizes isoquinoline alkaloids. The research results are carried out on sows (farrowing stress) and piglets (weaning stress) and demonstrate that in low doses, they can regulate the stress response [1].
However, isoquinoline alkaloids and plants containing them (any parts) are classified by regulatory authorities as toxic substances of natural origin by the European Food Safety Agency, as they can pose a danger to human and animal health [28].
The following plants with saponins and alkaloids have been used to modulate immune reactions as part of phytobiotics, and when one overdoses, there may also be health problems. These plants include alfalfa )(, Spanish licorice and [12].
Saponins are substances of glycosidic nature. They dissolve well in water and cause hemolytic poisoning when they penetrate the blood. The five-volume U.S. Review of Toxic Plants indicates that echinacea and alfalfa can lead to kidney failure and renal acidosis. Echinacea can also cause allergic reactions, rash, or exacerbate asthma
[3][4][6][5]
Unfortunately, the set of cultures as phytobiotics is extremely limited in Russia. However, there is a rich gene pool of wild flora. Specific types of plants used in Russia as phytogenics are coniferous flour (fir, spruce, pine), Jerusalem artichoke, beetroot, carrot, pumpkin, alfalfa, and sea buckthorn
[2][42]
Therefore, searching for new and unconventional plant species with unique properties is urgent. The goal is to find sources of new, highly effective natural compounds of preventive, therapeutic, anabolic, anti-stress, and adaptogenic orientation, which are simultaneously economically profitable. In other words, feed production from the viewpoint of animal science and veterinary medicine needs a significant expansion of the range of species. Simultaneously, attracted species should be combined with traditional cultures.
One of the promising directions for improving the feed production system is the inclusion of ecdysteroid synthesizing plants in it. In Russia, there are unique plant sources containing phytoecdysteroids (PES) as biologically active components (Fig. 1). Distinctive positive properties of phytoecdysteroids are not available in currently widely used phytobiotics. Feed additives with them relieve severe stress, which conventional phytobiotics cannot do, have a direct anabolic effect of influence due to interaction with estrogen receptors, and have pleiotropic (multiple) effects of action due to the influence on important genes. Their use in animal husbandry does not cause concern, as they are safe substances [9], [18], [34], [47].The most important among the representatives of the ecdysteroid class, based on the practical significance, availability, and biological activity, is the bioactive substance ecdysterone (20-hydroxyecdysone, ecdysterone, 20E) [9].
General structural composition of phytoecdysteroids
3.3. Industrial sources of ecdysterone for phytobiotics
The sources of PES for phytobiotics are higher plants. Primary chemical synthesis of ecdysteroids is impossible in practice; biotechnological methods are ineffective. After several reproduction cycles, strains lose their ability to synthesize
[9][37][22]
Despite the theoretical possibility of obtaining ecdysterone-containing substances from a variety of plants of the world flora, in fact, there are severe restrictions on attracting them to feed production for phytobiotics. It follows from an analytical review of the achievements and issues in the cultivation of ecdysteroid-synthesizing plants. As wild-growing raw materials for the production of ecdysteroids in various countries, rhizomes of ferns from the forests of Europe and South America (
[39, P. 4]
Plants that, are considered in European countries as good sources of PES and deserve attention for the large-scale production of substances with ecdysterone in sufficient quantities and at a reasonable price are
1) species from the genera
Amaranthaceae
2)
Commelinaceae
3)
Amaranthaceae
4)
Asteraceae
5)
Asteraceae [9]
According to experts and specialists
[23]
However, the value of plants is assessed not only by the ability to increase the synthesis of target substances but also by a low predisposition to the concentration of various toxic compounds of natural or anthropogenic origin. In this regard, representatives of the first three genera with a high content of ecdysteroids, including related species (
[15]
Studies with partially purified ecdysterone substances from
[23]
The most acceptable in the conditions of Russia is the use of two plants in the composition of phytobiotics – carthamoides rhapontic (maral root)
[31]
Industrial source of ecdysterone from Rhaponticum carthamoides agropopulations
The Serratula coronata agropopulation is a source of phytogenic substances with ecdysterone
[16][36]
These two tall-herbed perennial species have undergone a long stage of introduction. They synthesize large amounts of ecdysterone in leaf organs in
[38][39][9][23]
Ecdysterone, as part of the PES complex in their aboveground phytomass, stimulates protein synthesis in muscles and other organs in animals while preventing fat deposition. Unlike chemically synthesized hormonal agents, herbal stimulators of protein synthesis based on ecdysterone do not cause life-threatening side effects
[9][27][32][35][18][37][43][47]
As a candidate for the PES plants described above, a new species of carthamoid rhapontic
[40]
4. Conclusion
The review summarizes the achieved results and trends in the use of biologically active substances synthesized by plants in the diet of farm animals for health improvement and growth stimulation. The specific composition, active substances, effectiveness, and limitations in industrial use have been studied and personalized. Promising new and unconventional plant species and their promising components for producing phytobiotics with improved qualities based on isoquinoline alkaloids, saponins, and ecdysteroids are analyzed.
It has been shown that ecdysteroid-containing phytobiotics, where the primary bioactive component is ecdysterone (20-hydroxyecdysone), are alternative substances in comparison with prohibited synthetic androgenic and estrogenic hormonal stimulants. Simultaneously, they have a direct anabolic and anti-stress effect, are economically beneficial for the manufacturer of products, are free from the disadvantages of chemically synthesized hormonal agents and tranquilizers, and have no issues with safety and toxicity. It is possible to combine such substances with other antimicrobial agents in order to improve the bioavailability and prolong the action of the active substance ecdysterone.
Groups of plants considered good industrial sources of PES and deserve attention concerning the large-scale production of substances with ecdysterone are species from the genera
In the perspective of future research, plant species characterized by an increased content of ecdysterone and simultaneously a high yield of aboveground mass, do not synthesize toxic impurities, and are capable of long-term growth in agropopulations are of interest for the production of phytobiotics. Other important upcoming studies concern the examination of the variability and stability of the production of PES of selected plants during the life cycle in agropopulations. They also concern establishing the causes of the presence or absence of low- and low-active ecdysteroids in phytomass, the possibility of influencing the content of ecdysterone through the optimization of cultivation technology, and the experimental animal tests to establish a possible synergistic or antagonistic effect of ecdysterone containing substances with other components of phytobiotics, etc.