Tropical Forage Legumes in India: Status and Scope for Sustaining Livestock Production

Livestock contributes enormously in food and nutritional security apart from livelihood security to rural population all over the world. India has the largest number of livestock, representing over 17% of world population. Availability of forage legumes is essential for better animal health, production and increasing the nutritive value of forage-based rations, besides providing a source of biological nitrogen fixation for enriching soil, reducing land degradation and mitigating climate change. However, supply of quality green fodder in India is extremely precarious, and the gap is huge against demand. The major fodder legume crops cultivated in India are Medicago sativa, Trifolium alexandrinum, Vigna unguiculata, Vigna umbellate and range legumes are Stylosanthes spp., Desmanthus virgatus, and Clitoria ternatea. Indian subcontinent represents wide spectrum of eco-climates and reported diversity of 21 forage legumes genera viz., Desmodium, Lablab, Stylosanthes, Vigna, Macroptelium, Centrosema and browse plants Leucaena, Sesbania, Albizia, Bauhinia, Cassia, Grewia, etc. Diversity of forage legumes were collected (>3200 accessions), evaluated and sources for different biotic and abiotic stress tolerance were identified, apart from >50 cultivars developed. Considering these aspects, tropical legumes for livestock production, soil health and ecosystem services, diversity, evaluation and breeding for improved varieties are discussed in this chapter.


Introduction
Cultivated forage legumes and range legumes are contributing in sustainable agriculture production apart from nutritional security to the livestock population of India. Cultivated forage legumes and range legumes are also crucial for the nutritional security for mankind as they are integral component for increased availability of animal protein and product which has higher biological value than the plant proteins. The major fodder legumes crops cultivated in India are Medicago sativa, Trifolium alexandrinum, Vigna unguiculata, Mucuna pruriens, Vigna umbellate and range legumes are Stylosanthes spp., Desmanthus virgatus, Clitoria ternatea and others. Among these, Medicago sativa, Trifolium alexandrinum and Vigna unguiculata are more popular among cultivated legumes and Stylosanthes in range legumes because of easy availability of seeds of improved varieties and well developed technology to increase the forage yield and quality. To understand the current status and scope of tropical forage legumes of India for sustaining income through livestock sector, their importance in livestock production, soil health and ecosystem services and diversity among germplasms, evaluation and breeding for improved varieties are discussed in this chapter.

Forage legumes in livestock production
India has the largest livestock population in the world with more than 512 million heads. It supports 56.7% of the world's buffaloes, 12.5% of the world's cattle and 20.4% of the world's small ruminants (sheep and goats) [1]. Besides, the country hosts 17% of the world human population [2]. India is also the leading milk producing country in the world but milk productivity per animal basis is very low. Deficiency in quality of fodder is one of the major reasons for the low animal productivity. Although India is very rich in varied flora and fauna but there is deficiency of quality green fodder to the tune of around 35%. The animals need proper feeding to meet their nutrient requirement to express their full genetic production potential.
In fact, the sustenance of Indian rural agricultural economy depends on crop and animal farming, the two key components of a mixed farming system. Although the contribution of agricultural sector in the Indian economy is steadily declining (from 36.4% in 1982-1983 to 14.1% in 2012-2013), it still contributes employment to over 50% of the work force [3]. The contribution of livestock sector to agriculture GDP has increased to more than 28% and is likely to increase further. In the recent past, the lifestyle of people has been changed with a marked shift in food habits towards milk, milk products and meat leading to increase in demand of livestock products. Economic scenario in animal husbandry is also changing with emergence of peri-urban livestock farming and fodder markets. This indicates the huge pressure on available land, most of which, is used for arable farming and food production.
Forages form the main stay of our animal farming to reduce the competition between human beings and animals due to increasing demand for land and other inputs. Sole feeding of green forages to dairy animals is much cheaper than feeding concentrates with crop residues and has the potential of higher level of milk production. Nearly 65% of the total expenditure of milk production in cows is attributed to the feeding of animals when both concentrates and green fodders are fed as mixed ration. When the milk production is primarily depend upon concentrate based feeding, the cost of feeding towards milk production reaches to 80%, however, in case of forage (legumes) based feeding, it is reduced to only 40% of the total expenditure [4]. Hence, any attempt towards enhancing availability of quality green fodder, and economizing the feed cost would result in better remuneration to livestock farmers/producers.
From an animal perspective, one of the largest benefits provided by legume forages is that they provide a better level of nutrition than cereal forages/grasses at a similar stage of growth, leading to greater forage intake by livestock and increased animal performance. The symbiosis between legumes and Rhizobia provides the plant with an ample supply of N and it is one of the reasons why crude protein (CP) concentrations of legumes are higher than cereals/grasses. In addition to higher concentrations of CP, forage legumes also provide a higher quality protein which may be of equal or greater importance in case of non-ruminant livestock species like equines. Legumes also contain more concentrations of digestible energy than grass/cereal forages due to the structure and development of the legume cell wall. Indeed, the cell wall of legume plants contains fewer hemicelluloses and more pectin compared to that of cereals, thus increasing their digestibility by livestock. However as the cell matures, a secondary cell wall consisting of cellulose and lignin is deposited on the interior of the primary cell wall and reduces the overall availability of the structural carbohydrates in the digestive system. In cereal forages, this phenomenon occurs in all tissues types (i.e. leaves, stems, etc.) while being primarily restricted to the vascular tissues of legume stems. The lignin of non-legumes is also more esterified to hemicelluloses and is more recalcitrant in composition (e.g. higher proportion of syringyl subunits) indicating a more suppressed degradability than in legume species.

Forage legumes in soil health and ecosystem services
Forage legumes is essential for providing a source of biological nitrogen fixation (BNF) for enriching soil fertility (15-40 kg fixed N/ha), reduction in land degradation, disease breaks and for mitigating climate change. Estimating biological N 2 fixation of the forage and fodder legumes precisely is challenging because statistics on the areas and productivity of these legumes are highly difficult to obtain. Therefore, N 2 fixation values of forage and fodder legumes will be less reliable and also estimates of %Ndfa (nitrogen derived from atmosphere) of fodder legumes in those lands. There are very few reports available on forage legumes-BNF in India. But, all works mainly focused on application of Rhizobium inoculants to fodder legumes and testing their potential for enhancing fodder production (fresh and dry weight, crude protein content, forage quality aspects, nodulation properties, etc.). Appreciable amount of atmospheric N (~60-100%) is fixed by forage legumes annually, fixing up to 380 kg N ha −1 [5]. Quantity of forage residues available for soil incorporation range from 80 to 143 kg N ha −1 and rice cultivated following forage legumes yields the same as rice with 24-50 kg fertilizer N ha −1 [6]. About 100-120 Mha of land is under fodder and forage legumes and green manure crops, with assumed average N 2 fixation rates of 200 kg N/ha/year for alfalfa, 150 kg N/ha/year for clovers (Trifolium spp.), 100 kg N/ha/year for other forages and 50 kg N/ha/year for legume-grass pastures [7]. From this assumption, total nitrogen fixation by forage and fodder legumes was calculated at 12 Tg annually (average of about 110 kg N/ha/year). But fixation by legume-grass mixtures is much more variable, ranging from a just a few kilograms to more than 250 kg N ha −1 .
In addition to BNF, many forage legumes have soil-covering growth habit similar to most grasses and deep root system which can contribute to the mitigation of many soil problems, viz., soil conservation by legume cover crops such as Stylosanthes, Crotalaria, Sesbania, Arachis and Desmodium to prevent erosion; contour-hedges with leguminous trees such as Leucaena; rehabilitation of degraded soils by legumes such as Stylosanthes spp., which are deep-rooted and adapted to infertile soils, cycle minerals from deeper soil layers resulting in soil improvement and enhanced concentration of soil organic matter through litter production [12]; the potential of legumes like Stylosanthes hamata can be exploited to ameliorate compacted soil [13]. When used as cover crop forage legumes can also control weed growth, which can be exploited as an attractive alternative to the use of herbicides. They supplement part of N fertilizer application, thus reduce nitrate leaching and eutrophication of water bodies as a consequence of surface runoff as a result of N fertilization in tropical pasture production process. Tropical forage legumes have considerable potential to increase productivity of forage-based livestock systems, while providing benefits to the environment [14]. The environmental benefits, referred as 'ecosystem services', comprise positive effects on: soil conservation and soil chemical, physical and biological properties; mitigation of global warming and of groundwater contamination; saving of fossil energy; and rehabilitation of degraded lands [14]. These features make tropical forage legumes particularly valuable at all levels of the system because of their interaction with plants, soil, animals and the atmosphere.

Genetic resources of tropical forage legumes
Plant genetic resources (PGR) are the basic platform for screening, improving and developing fine cultivars, and the important materials for biodiversity studies including classification, evolution and origin. Therefore, maintenance of enormous genetic diversity is mandatory for broadening the genetic base of the present and future forage improvement programmes to achieve the national goals. Extensive collection, proper evaluation, in depth study of genetic attributes and cataloging of germplasm is prerequisite for its efficient utilization. According to an estimate there are about 650 genera, 18,000 species of legumes (Leguminosae) in the world. Out of these, only about 30 legumes are used to an appreciable extent for forage production [15]. Information regarding the centre of origin of different forage crops is furnished in Table 1.  [20]. Besides, India possesses enormous diversity of minor and under-utilized fodder species such as Agrostis alba, Desmodium parvifolium, Leptochloa fusca, Potentilla fruticosa, Rhynchosia minima and Salvadora persica [21]. The forage genetic wealth of India distributed in 15 agro-climatic zones has been summarized in Table 2.

World
The National Bureau of Plant Genetic Resources (NBPGR) is the nodal agency for characterization, evaluation, maintenance, conservation, documentation and distribution of germplasm resources in India. Currently a total of 4594 accessions of different forage crops including cereal forages (1167), grasses (11,160, range legumes (1443), forage millets (781) and others [85] are being maintained at long term storage (LTS) module of National Gene Bank at NBPGR, New Delhi [22]. Indian Grassland and Fodder Research Institute (IGFRI) is a unique R&D organization in South Asia for sustainable agriculture through quality forage production for improved animal productivity. IGFRI being the National Active Germplasm Sites (NAGS) on forages works with its three regional stations and All India Coordinated Research Project (AICRP) on forage crops with 18 coordinated centres. At present IGFRI maintains more than 8000 accessions of 19 major forage crops including cereal forages, forage legumes, grasses and fodder tree at midterm storage [23].

Problems associated with breeding of tropical forage legumes
Tropical forage legumes breeding programmes are associated with certain unique problems.
Most of the tropical pasture legumes still possess traits of wild plants that include seed shattering, small seed size, seed dormancy, relatively slow germination rates, etc. In most of the cases we have very little knowledge about the basic biology of the species. Some of the problems include overlapping of vegetative and reproductive growth phases, uneven pod setting, nonsynchronous maturity and seed shattering in forage legumes [24]. Inherent heterozygosity as most forage species are cross pollinated. Self-incompatibility limits the extent to which they may be inbred; small floral parts make artificial hybridization tedious; poor seed producers; or produce seed with low viability as well as inherently low seedling vigor and competitive ability. Many forage species produce weak seedlings and stands are not easily established. Strains may perform differently with different systems of grazing management. Persistence of perennial tropical forage legumes is not as a single trait, but rather as a complex of traits dependent on various factors, such as diseases, insects, abiotic stresses, or management stress. Fertility barriers of one sort or another are very common in tropical forage legume breeding viz., berseem [25], owing to the wild nature of the species and inadequate knowledge of interor intra-specific variation.

Egyptian clover (Trifolium alexandrinum L.)
The genus Trifolium from the tribe Trifolieae of the family Leguminosae (Fabaceae) is important for its agricultural value. A few of the 237 species of this large genus have actually been cultivated [26], out of which 25 species are important as cultivated and pasture crops [27]. Egyptian clover or berseem (T. alexandrinum 2n = 16) is commonly cultivated as winter annual in the tropical and subtropical regions. Berseem is popular due to its multicut [4][5][6][7][8] nature, providing fodder for a long duration (November to May), very high quantum of green fodder (85 t/ha) and better quality of fodder (20% crude protein), high digestibility (up to 65%) and palatability. Berseem was introduced in India from Egypt in 1904, and has been established as one of the best Rabi (winter season) fodder crop in entire North West Zone, Hill Zone and part of Central and Eastern Zone of the country, occupying more than two million hectare [28].
Berseem being an introduced crop in India, the most important drawback in genetic improvement has been the lack of genetic variability [29,30]. Variability in the existing gene pool has been induced through mutation, polyploidization and inter-specific hybridization. High biomass production potential along with extended growth period and resistance to biotic stresses specially root rot and stem rot have been the main target traits that were to be improved genetically. Different genetic improvement programmes carried out in various research institutes/universities by utilizing breeding approaches like selection, polyploidy and mutation resulted in the development of >15 varieties for different berseem growing regions of India. Inter-specific hybridization have been used to improve resistance to biotic and abiotic stresses and extended length of the vegetative period because genes for wide scale adaptability are widely distributed in several wild species of Trifolium (Table 3). Interspecific hybrids of berseem with Trifolium apertum [31], T. constantinopolitanum [32], T. resupinatum [33] and T. vesiculosum [34] were successfully developed and progenies of interspecific hybrids showed introgression of various desirable traits, including late flowering and resistance to root rot and stem rot diseases.
A major breakthrough in berseem breeding in India was achieved through induction of polyploidy. The work on polyploidization of berseem genome was started with the aim to induce greater leaf and stem size [35,36]. Autotetraploid induced by using colchicine treatment, and selection at tetraploid level resulted in the development of first polyploid variety 'Pusa Giant' with more fodder production and good regeneration capacity, uniform and higher yield throughout the season than diploid varieties released for general cultivation in India [37]. Another big achievement in polyploidy breeding was achieved at IGFRI, Jhansi by developing an autotetraploid variety namely 'Bundel Berseem-3' through colchiploidy followed by recurrent single plant selection followed with mass selection [28].

Major success in Berseem breeding was achieved by induction of longer duration mutant in Mescavi variety through gamma ray treatment which resulted in 'BL-22' a variety released
Species Chromosome number (2n)

Stylosanthes
The genus Stylosanthes comprises approximately 40 species, distributed in the tropical [42], subtropical and temperate regions areas of America, Africa, and Southeast Asia. It can be grouped into two subgeneric sections, Stylosanthes and Stylosanthes. Most species are diploid (2n = 20) but polyploid species (2n = 40 and 2n = 60) also exist. Six species, namely Stylosanthes scabra, S. seabrana, S. hamata, S. guianensis, S. humilis and S. viscosa, are predominantly used as fodder legume in humid to semi-arid tropics of India (Table 4). These are very popular and have been widely adapted due to their ability to restore soil fertility, improve soil physical properties, and provide permanent vegetation cover as well as to provide nutritious fodder. The most specific problems associated with Stylosanthes are the limited variations of available germplasm and the susceptibility to anthracnose disease caused by the fungus Colletotrichum gloeosporioides. In the past, mainly five species of Stylosanthes  (S. hamata, S. scabra, S. humilis, S. viscosa and S. guianensis) have been introduced primarily from Australia and evaluated at different sites in India [43][44][45]. This was in addition to the native perennial S. fruticosa Alston, which is widely distributed throughout the southern peninsular regions [46].
Testing and evaluation of wide germplasms carried out at IGFRI on acid and saline soil which contribute major part of the soils of India, indicated better adaptation of S. hamata and S. seabrana lines over other species in salinity. The potential of S. seabrana for tropical and subtropical regions of the country with clay and heavy soils, cool winters and distinct wet-dry seasonal conditions directed the use of this species in developing new breeding approach. The one could be based on the finding that it is the second progenitor of S. scabra which in turn elucidated the evolution of one of the most important Stylosanthes species, S. scabra may lead to important impacts on the efforts of improving S. scabra [47]. It may be possible to artificially synthesize S. scabra using pre-selected S. viscosa and S. seabrana accessions [48]. These artificial S. scabra genotypes could be used directly or more likely, be used in breeding programs. By doing so the genetic variation existing in the two diploid progenitor species would become available in improving the allotetraploid S. scabra. So far developed map and linked markers with anthracnose resistance also provide the opportunity to use them after converting them in sequence tagged sites (STS) or sequence characterized amplified region (SCAR) and then using them in direct breeding programs.

Alfalfa (Medicago sativa L.)
Genus Medicago is one of the oldest forage legume comprising 60 perennial and 35 annual species, distributed mainly around the Mediterranean basin, cultivated throughout the world in diverse environments ranging both temperate and tropical environments [49]. It is generally agreed that the basic chromosome number for the genus Medicago are x = 7 and x = 8. Its ploidy varies from diploid (2n = 16) to polyploid (2n = 32, 48, 64). Perennial species are mainly tetraploids (2n = 4x = 32) and allogamous, however diploid (2n = 2x = 16) and hexaploid (2n = 6x = 48) cytotypes have also been reported [50]. Medicago sativa (alfalfa or lucerne) is widely cultivated as the most important forage legume in the temperate areas of the world. Lucerne is native to South West Asia as indicated by occurrence of wild types in the Cancasus and in mountainous region of Afghanistan, Iran. M. sativa complex, comprises of several members at the same ploidy level e.g., M. falcata, M. media and M. glutinosa, which freely intercross, without any hybrid sterility in the F 1 or later generations [51]. In India, it is grown in Maharashtra, Gujarat, Andhra Pradesh, Karnataka, Tamil Nadu, Haryana, Madhya Pradesh, Rajasthan, Punjab. The major breeding objectives in the crop include vigorous tall growing plants, better branching, quick regeneration, and balance between seed and forage yield and persistence.
Genetic resources for alfalfa improvement are limited and restricted to the M. sativa complex but tolerant sources for biotic and abiotic constraints are lacking in the complex [52].
The annual and perennial species of the genus Medicago are the reservoir of several useful agronomic traits, including disease and insect resistance and potential salt and drought tolerance having direct implication in cultivated alfalfa improvement ( Table 5). Most of the lucerne cultivars grown in the country and worldwide are susceptible to many diseases and insect pests and the most serious constraint is the alfalfa weevil (Hypera postica Gyll.) [53]. Resistance to weevil has been reported in several annual species such as M. scutellata, M. prostrata, M. turbinata and M. intertexta [54][55][56][57]. Genes conferring resistance to aphid have been identified in M. rugosa, M. scutellata and M. littoralis [58]. Similarly, three woody species viz. M. arborea, M. strasseri and M. citrine of the section Dendrotelis have been reported as excellent sources for incorporating drought and salt tolerance in M. sativa [59][60][61]. However, due to post fertilization barrier, interspecific hybridization is difficult, so we may need to use biotechnological tools like ovule-embryo culture and electroporation.
Inter specific hybrids of M. sativa with some of the perennial species viz. M. cancellata, M. glomerata, M. papillosa, M. prostrata, M. rhodopea and M. saxatilis have been recovered by conventional crosses [51]. However, pollen and embryological studies demonstrated that there exist strong post fertilization barriers for recovering hybrids between M. sativa and annual species [62]. Utilizing embryo culture and fertilized pod culture techniques interspecific hybrids were obtained between M. sativa and many other annual species however, no hybrids were produced between M. sativa and weevil resistant M. scutellata [63,64]. Bauchan and Elgin [65] reported chromosomal incompatibility and presence of two SAT chromosomes in M. scutellata as the major barriers for getting interspecific hybrids between M. sativa and M. scutellata. Utilizing protoplast fusion technique S 1 plants were obtained between M. sativa and M. rugosa and it was confirmed by genomic in situ hybridization (GISH) that small portions of M. rugosa chromosomes were present in the hybrid however, it is not clear that in which chromosome the resistance genes are present [50].
A lot of molecular information has been generated across species. However, information from M. truncatula on marker-trait association is unlikely to be exploitable in lucerne, considering the large differences between annual and perennial [66]; in addition to the differences due to the ploidy level which may further contribute to the inconsistent genetic control of some morpho-physiological traits between the two species [67]. Some breeding goals such as region-specific adaptation; drought-tolerance; improvement for forage quality should be considered [68]. Attempts have been made to produce transgenic alfalfa containing fungal chitinase gene for resistance against fungal pathogens [69], tolerance to abiotic stresses such as salt and cold [70,71], improved forage quality [72], and sulfur-containing amino acids [73], value addition by making it an edible forage vaccine [74]. In recent years the breeding strategies for Lucerne are more towards utilizing potential of polycross methods followed with phenotypic selection. It has resulted in development of a few cultivars in recent years. The future strategies should include development of cold and drought hardy lucerne with degree of persistence for pasture and meadows, increasing genetic base, high seed production, stress tolerance, diseases and pest resistance etc.

Cowpea (Vigna unguiculata (L.) Walpers)
Cowpea (2n = 2x = 22, genome size = 620 Mb) also known as 'black eye pea' or 'hungryseason crop' is an annual food and forage crop mostly grown throughout the semi-arid tropics in parts of Asia, Africa, Southern Europe, Southern United States, and Central and South America (Singh 2005). It can be grown throughout the year due to its short duration and fast growing nature. It is suitable for inter, mixed and relay cropping system. Cultivated cowpea, which is in subspecies unguiculata, is divided into five cultivar groups namely Unguiculata, Sesquipedalis (yard-long-bean), Textilis, Biflora and Melanophthalmus [75]. The commonly cultivated cowpea belongs to cultivar group Unguiculata the most widespread and economically important group of the species. They are pulse and vegetable and forage types. Other cultivar group Biflora also known as 'catjang cowpea' mainly cultivated in South Asia (India, Sri Lanka) as a pulse or as forage for hay and silage, and as a green manure crop. In Australia and Asia cowpea is primarily a fodder crop, but is also used for green manure or as a cover crop [76]. In India, the crop is cultivated around 6.5 lakh ha with 3 lakh as fodder crop in Rajasthan, Gujarat, Maharashtra, Karnataka and Tamil Nadu [24].
Cowpea was first introduced to India 1000-1500 years ago and now Indian-subcontinent appears to be a secondary centre of diversity. In India a large numbers of varieties for vegetable, pulse and fodder purpose have been developed. The breeding objectives have focused around developing lines with terminal drought tolerance, early maturity, erect growth to fit in cropping systems and enabling improved radiation use efficiency, high harvest index and resistance to diseases. The desirable traits in forage cowpea varieties are leafiness with indeterminate growth to get green fodder for a longer period. International Institute of Tropical Agriculture (IITA) has developed several dual purpose cultivars of cowpea with high grain and biomass yields and erects habit for intercropping/mixed farming purposes. In future development of cowpea lines against various forms of root-knot nematode, cowpea aphids and Fusarium wilt, is required. Further, development of transgenic cowpea lines with resistance to major insect pests can also be a breakthrough in cowpea breeding.

Conclusion
Tropical forage legumes were promoted in the past with the major focus on livestock production in India. This has led to a substantial decrease in research on tropical forage legumes.
In view of current climate change problems and environmental concerns, research on forage legumes should be resumed with adequate funding support at national and international levels. Newer biotic and abiotic stress tolerant varieties should be developed for the changing environmental conditions. Forage legumes have potential to contribute significantly to environment-friendly agricultural land use and sustainable livestock production in the tropics.