Rural economy of many developing countries in the world is agrarian based. The livelihood of resources poor farmers is supported by agriculture and animal husbandry. Country like India occupy about 2 per cent of the total world’s geographical area and sustains livestock population of 536.76 million (
Anonymous, 2022). In recent years the growth of human and livestock population has posed a tremendous pressure on land resources. Indian agriculture is oriented towards mixed farming in which role of livestock form an integral part of rural India. Livestock productivity in India is far below the desired level due to considerable gap between the requirement and availability of quality fodder.
At present, the country is facing a deficit of 11.24 per cent in green fodder, 23.4 per centin dry fodder and 28.9 per cent in concentrates
(Roy et al., 2019).The regional deficits are more important than the national deficit, especially for fodder which is not economical to transport over long distances. The pattern of green and dry fodder deficit/surplus varies in different parts of the country. In India, Union territories face a deficit of 76.2 per cent, North East zone of 23.1 per cent, East zone of 41.2 per cent, Hill zone of 24.9 per cent, the North zone experiences an overall surplus of 133.05 per cent, West zone faces a modest deficit of 6.3 per cent, the Central zone faces a deficit of 4.8 per cent and the South zone faces a significant deficit of 43.9 per cent in green fodder. Among the dry fodder availability, Union territories face a deficit of 59.1 per cent, North East zone experiences an overall surplus of 14.0 per cent, East zone faces an overall deficit of 43.9 per cent, West zone of 43.5 per cent, Central zone of 16.4 per cent, in Hill zone there exists an overall surplus of 55.9 per cent, in North Zone faces an overall surplus of 31.7 per cent and an overall deficit of 27.0 per cent in South zone (
Roy et al., 2019).
Sufficiency of nutrients to forage crops is vital for ensuring higher crop productivity and better animal nutrition. Forage plants, especially the high biomass producing perennial ones, are heavy feeders of plant nutrients and remove large amount of nutrients from soil. Perennial grasses have been found to remove 9.4, 1.45, 14.2, 4.6, 2.65 and 1.95 kg N, P, K, Ca, Mg and S, respectively for each tonne of dry matter produced (
Hazra, 1994). Development of a manageable association between different forage crops (Table 1) and beneficial organisms would greatly increase forage production efficiency.
Biofertilizers
Biofertilizers, more commonly known as microbial inoculants, are artificially multiplied cultures of certain soil organisms that can improve soil fertility and crop productivity. Although the beneficial effects of legumes in improving soil fertility was known since ages and their role in biological nitrogen fixation was discovered more than a century ago. Biofertilizers has a potential to substitute the quantum of organic and inorganic source of nutrients in agricultural production systems. Bio-fertilizer contains microorganisms which promote the adequate supply of nutrients to the host plants and ensure proper growth by regulating plant physiology. Living microorganisms are used in the preparation of bio-fertilizers. Microorganisms which have specific functions to enhance plant growth and development and make the nutrients available to the plants are used as bioagents for the preparation of biofertilisers. Presence of various N fixers in the rhizosphere of different grasses has been documented by
Hazra in 1994 (Table 2). Biofertilizers play a significant role in improving the soil fertility and plant growth by fixing atmospheric nitrogen through symbiotic and non-symbiotic association, solubilising insoluble soil phosphates and also producing plant growth stimulating substances.
Microorganisms as biofertilizers
Organisms that are commonly used as biofertilizers are nitrogen fixers (N-fixer), phosphorus solubilizer and potassium solubilizer as sole culture as well in combination with few moulds or fungi (Table 3). Most of the bacteria used in biofertilizer have close relationship with plants and these bacteria possess ability to fix atmospheric nitrogen (Table 4).
Rhizobia
Rhizobia are a group of symbiotic nitrogen-fixing (SNF) bacteria that have the capability to form nodules on the roots and in certain instances, on the stems of their host plants in particular of legumes. The symbiotic relationship between rhizobia and host plants is mutually beneficial, in which both bacteria and host plants are benefitted (
Sprent, 2008). Legumes host rhizobia and supply them with carbon and energy sources, while in return, these plants receive ammonia from the rhizobia (
Lindstrom and Mousavi, 2020). Through symbiosis, atmospheric nitrogen is converted into a form that can be easily assimilated by the host plants
(Gothwal et al., 2007). The specificity of this symbiosis varies among different
Rhizobium strains and legume species. To better understand the compatibility between
rhizobium and legume hosts, scientist have categorised them into cross-inoculation groups. These groups consist of legume species that can share compatible
rhizobium strains, meaning they can develop nodules when inoculated with bacteria from any member of the same group. The concept of cross inoculation groups helps streamline the selection of appropriate
rhizobium strains for legume crops, optimizing nitrogen fixation and overall plant productivity. Cross-inoculation groups of
rhizobium are pivotal in agricultural practices, aiding in the selection of optimal strains for inoculating specific legume crops. These groups categorize
rhizobium strains based on their compatibility with particular legume hosts. For example,
R. japonicum adept at nodulating and fixing nitrogen in soybeans, whereas,
R. leguminosarum exhibits greater efficacy with peas or clover (Table5).
Nitrogen-fixing bacteria (NFB) transform inert atmospheric N
2 to ammonia through their biological processes, and this reaction is catalysed by the oxygen-sensitive enzyme nitrogenase, present in the bacteria
(Bakulin et al., 2007; Maitra et al., 2023). Rhizobium inoculation is a well-known agronomic practice to ensure adequate N supply for legumes with an objective to minimize inorganic nitrogen requirement. In plant root nodules, the O
2 level is regulated by special haemoglobin called leg haemoglobin. This globin protein is encoded by plant genes but the haem co-factor is produced by the symbiotic bacteria upon infection of the plant with
Rhizobium. The process of nitrogen fixation does indeed require specialized cells with organelles containing cytoplasmic compartments known as symbiosomes. Within these symbiosomes, rhizobia ultimately differentiate into a specialized cell type called bacteroids, which then fix atmospheric nitrogen for the plant in exchange for sugars
(Kumar et al., 2020).
Recent studies have revealed that rhizobia beyond their symbiotic association with leguminous plants also function as endophytes and enhanced the growth and productivity of various plants through multiple processes such as solubilization of inorganic compounds and phytohormone production
(Gopalkrishnan et al., 2015). Long term studies under ICAR-All India Coordinated Research Project (Forage Crops) on inoculation of
Rhizobiumin diverse cultivated fodder legumes, pasture legumes and shrubs, indicated increase in green fodder yield of cultivated forage legumes to the tune of 10 to 28 per cent, whereas, the magnitude of increase in pasture legumes and shrubs was 11to 31 per cent over non-inoculated control (Table 6).
Rhizobium inoculation demonstrated a favourable response in yield of multi-cut species like alfalfa, Egyptian clover and Persian clover compared to single cut species. Among perennial species butterfly pea, Stylosanthes and Centro showed better response to
Rhizobium inoculation (
Hazra, 1994). However,
Rhizobium inoculation had positive effect on the growth and yield of cowpea in clay loam soil of Ankara (Turkey) with appreciable economization of inorganic nitrogen application
(Albayrak et al., 2006).
Under prevailing ecological challenges of climate change and environmental degradation due to excessive use of chemical fertilizers the
rhizobium inoculation of alfalfa seed along with application of poultry manure, rock phosphate and phosphobacteriaenhanced the herbage yield appreciably
vis-à-vis mitigated the adverse effects of inorganic fertilizers on the soil health (
Bama, 2016).
Bio-inoculants had favourable effects on the growth, development, yield attributes and yield of fodder crops by enhancing the availability of nutrients to the crops. This has been very well demonstrated by
Dahiya et al., (2019) and
Ijaz et al., (2019)when appreciable increase in fodder yield of Egyptian clover was observed with the application of
Rhizobium + PSB. Various studies have established the positive effects of bio-inoculants on soil health. In zero tilled soils
Rhizobium inoculation of cowpea improved soil health, fodder yield and minimise the dose of inorganic nitrogen up to 25 per cent
(Mallikarjun et al., 2022). In sandy loam soils
rhizobium inoculation of cowpea had notable response of plants to inoculation owing to better nodule development, increased nitrogen fixation and improved yield. Although,
Rhizobium inoculation had favourable effect on crop yield but studies have also established the positive role of inoculation on protein, total phenolic, chlorophyll and beta carotene contents in cowpea (
Kandil and Unlu, 2023).
Phosphate-solubilizing bacteria (PSB)
Phosphorous is one of the most prevalent elements in the crust of the earth and can be found in both inorganic and organic forms in soils
(Gyaneshwar et al., 2002). Plants absorb it in the inorganic form i.e. orthophosphate (H
2PO
4- and HPO
42-)(
Hinsinger, 2001). Phosphorous nutrition plays a crucial role in photosynthesis, energy transfer, signal transduction, nitrogen fixation in legumes, crop quality, and resistance to plant
diseases(Khan et al., 2014). Acidic soils of tropical and subtropical regions are severely deficient in phosphorus, with significant capacities for phosphorus sorption. Phosphorus is only present in micromolar or lower proportions in soil solution, although the majority of mineral nutrients are often present in millimolar amounts (
Ozanne, 1980). The strong reactivity of soluble phosphate with aluminium ions in acidic soil and calcium ions in alkaline soils is the cause of the low phosphorus availability
. Many heterotrophic microbes released organic acids that solubilize P, chelate cationic companions of P ions and release the P directly into solution (
Ingle and Padole, 2017). PSB constitute 1 to 50% of the total microbial population in soil, whereas
Penicillium,
Aspergillus,
Rhizoctonia solani, and
Trichoderma, make up only 0.1 to 0.5% of the P solubilization potential
(Chen et al., 2006). PSB inoculation of maize seed increased the green fodder yield, dry fodder matter yield, crude protein and crude fibre content by 3.67%, 5.4%, 7.1 % and 2.1%, respectively, over uninoculated check
(Ayub et al., 2014) (Table 7).
Microbes affect soil fertility by mineralization, decomposition and conversion of P from inorganic form to its accessible forms.PSB +
Trichoderma with 50% recommended dose of fertilisers and 10tFYM/ha increased fodder yield of oat, economise inorganic fertilisers doses due to rapid decomposition of organic matter, higher availability of nutrients, increased availability of phosphorus due to mineralisation and prevention of crop from seed borne diseases
(Singh et al., 2015).In sandy loam soils of Ranchi (Chhattisgarh), application of 75% of recommended fertilizer (RDF) along with
Azotobacter and PSB was as efficient in promoting growth and yield of oat crop as of 100 per cent recommended dose of fertilizer (Kumar and Karmarkar, 2015).PSB inoculation in lablab bean (
Lablab purpureus Linn.) sown in sole stand or in combination with cenchrus grass (
Cenchrus setigerus, Vahl.) enhanced fodder yield and crude protein content owing to mineralisation of fixed phosphorus by P solubilising micro-organisms and make it available to crop plants
(Sharma et al., 2015).
Vesicular Arbuscular Mycorrhiza (VAM)
Among various bio-agents, vesicular arbuscular mycorrhiza is recognised for its ability to accelerate physiological activities that enhance the growth and health of plants
(Johansson et al., 2004). Vesicular arbuscular mycorrhiza exhibits a symbiotic relationship between fungi and the roots of higher plants. These fungi colonize near plant roots forming a network of hyphae that extends into the soil, increasing the root length, thereby enhancing water uptake and nutrient absorption by the plants. It is believed that Arbuscular Mycorrhizal Fungi (AMF) have the potential to reduce the use of chemical fertilizers up to 50 per cent and achieving optimal crop production
(Begum et al., 2019).
VAM enhances nutrient uptake, improves tolerance to drought and salinity and also regulates plants defence mechanism
(Hause et al., 2007). VAM also stimulates plant growth through physiological effects or by mitigating the severity of diseases caused by soil pathogens (Gupta, 2004). Mycorrhizal fungi play crucial role in enhancing forage production and restoring soil fertility. Early studies indicated that mycorrhizal association increased the herbage yield of guinea grass by 24 per cent under hot and dry weather conditions of Bundelkhand region of India (
Hazra, 1994).VAM inoculation also hold very good potential in tropical environment of coastal region of India which was evident from vigorous growth and higher forage yield of guinea grass
(George et al., 1998).
In nutrient-deficient soils, mycorrhizal association play a vital role in plant nutrition due to their ability to absorb nutrients and in particular of phosphorus from the soil, consequently decreasing the reliance on expensive phosphatic fertilizers. Several fodder crops underwent testing with mycorrhizal association, demonstrated an increase in fodder yield of 5 to 18 per cent in annual fodder legumes, 3 to 5 per cent in annual cultivated cereal fodders, 13-33 per cent in perennial range legumes and 8-12 per cent in perennial grasses (Table 8). VAM has also been reported to have synergistic effect with other bioagents. In sandy loam soils dual inoculation of
Rhizobium and AM fungi significantly enhanced the nodulation, nodule biomass and fodder production in
Stylosanthes species. Furthermore, the treated plants had elevated levels of crude protein and total digestible nutrients, alongside reduced neutral detergent fibre and acid detergent fibre contents
(Mishra et al., 2009).
Azotobacter
Azotobacter is a group of Gram negative, free-living, nitrogen fixing aerobic bacteria inhabiting in the soil capable of fixing an average of 20 kg N/ha per year
(Gandora et al., 1998). In 1901, the Dutch botanist and microbiologist Beijerinck made the discovery of the
Azotobacter genus. These bacteria utilize atmospheric nitrogen to fuel the synthesis of their cellular proteins, which gets mineralized in the soil and increase the availability of nitrogen to crop plants. They have positive effects on crop growth and yield through the development of phytopathogenic inhibitors, rhizospheric microbial induction, modification of nutrient uptake, and ultimately enhancement of biological nitrogen fixation. Besides, nitrogen fixation,
Azotobacter also produces various growth hormones like, riboflavin, indol acetic acid and gibberalin
(Jnawali et al., 2015).
In sandy loam soils ecosystem, integrated use of organic and inorganic sources of nutrients with biofertilizer resulted in significantly higher green fodder yield of maize over organic and inorganic treatments owing to the higher availability of nutrients indicating beneficial effect of
Azotobacter owing to its ability to fix atmospheric nitrogen also helps to enhance nutrients availability in the root zone during the early growth stages of the crop. Use of
Azotobacter culture in integration with inorganics and organics had more pronounced effects on the growth and fodder yields of the crops
(Kumar et al., 2016). Seed inoculation with
Azotobacter also indicated complimentary effects with micro-nutrients application on the productivity of crops. Foliar application of ZnSO
4 @ 1% at 20 and 40 DAS + inoculation of seeds with
Azotobacter significantly improved the productivity of maize over no inoculation.
(Tejaswi et al., 2021). Studies have also established the beneficial effect of
Azotobacter on the quality constituents like crude protein, acid detergent fibre and neutral detergent fibre contents of fodder crops. Crude protein content is an important constituent in forages and acts as an indicator of quality of fodder produced by different crops (
Aseefa and Ledin, 2001).
Azotobacter also established its superiority in fodder maize crop grown with recommended primary and micro-nutrient application. Crude protein an important constituent in forages acts as an indicator of quality of fodder. Higher crude protein indicates better quality of the fodder. Acid Detergent Fibre (ADF) and Neutral Detergent Fibre (NDF) represent the fibre constituents in the forage and govern the digestibility and intake potential, respectively (Zhang
et al., 2022). Studies have reported beneficial effect of
Azotobacter biofertilizers on crude protein, acid detergent fibre and neutral detergent fibre contents of fodder crops. In sandy soils
Azotobacter also hold its potential for the improvement of herbage quality constituents. Better dry matter and crude protein yields of oat were obtained in sandy soils with
Azotobacter inoculation by
Sharma (2009). Nitrogen fixation and phosphorus mobilization by
Azotobacter and PSB, respectively improved input use efficiency of inorganic nitrogen and phosphorus sources in fodder crops like oats and sorghum. Studies advocated the substitution of about 25 % recommended doses of nitrogen in forages by
Azotobacter which resulted in better economic returns and economic efficiency of production systems
(Patel et al., 2018).
Azospirillum
Azospirillum is the foremost common plant growth-promoting rhizobacteria, generally associated with grasses and other crops like rice, wheat and sugarcane
etc.
(Suhameena et al., 2020). This bacterium is rod-shaped, gram-negative and aerophilic.
Azospirillum fix the atmospheric nitrogen in the rhizosphere but also known to promote plant growth by enhancing plant root growth, uptake of water and nutrients by plants, due to the production of phytohormones, polyamine and trehalose. The efficiency of
Azospirillum effect on plant growth and development under varied conditions depends on soil and climatic conditions. Beneficial effects of
Azospirillum have been obtained consistently in variety of crops. After the discovery the diazotrophic
behaviour of Azospirillum, several studies have established its capacity to fix N2 and to replace N-fertilizers requirement when associated with grasses and cereals like paddy, wheat and sugarcane. Some
Azospirillum strains can solubilize inorganic phosphorus, making it more readily available to the plants and resulting in higher yields and another important feature of
Azospirillum is related to biological control of plant pathogens.
The potential effect of
Azospirillum on crop productivity depends on the strain specificity used in a particular crop. Variable response of
Azospirillum strains on the green fodder yield (Table 9) of
Pennisetum trispecific hybrid (
P. typoides ×
P. purpureum ×
P. squmulatum) in sandy loam soil was observed by
Ramamurthy (2002).
Under integrated nutrient management system,
Azospirillum inoculation maintained its significant superiority over no inoculation in terms of crop yield (
Patil, 2014). Complimentary effects of
Azospirillum with
Azotobacter as well with PSB on yield attributes
viz. plant height, number of leaves, leaf area index and leaf stem ratio as well as green and dry yields of fodder crops. This bioinoculants has also shown equal response as of
Azotobacter in different studies
(Verma et al., 2014; Yadav
et al., 2010;
Gawai and Pawar, 2007).
Azospirillum has shown its ability to substitute about 25% recommended dose of nitrogen in forage crops
(Patil et al., 2008). Although,
Azospirillum had positive effect on crop productivity but studies have also reported an improvement in quality attributes i.e. enhancement of protein content and reduction in fibre constituents (
Yadav et al., 2007).
Constraints in adoption of bio-fertilizers
• Availability of improper, less efficient strains for production.
• Unawareness amongst the consumers.
• Lack of knowledge about the application of technology.
• Practical difficulties in implementation and adoption of technology.
• Non-availability of inoculants at the time of requirement.
• Short shelf-life of inoculants.
• Problems in the adoption of the technology by the farmers due to different methods of inoculation.
• No immediate visual difference in the crop growth like that of inorganic fertilizers
• Unreliable results may be due to lack of proper quality, improper method of application and adverse edaphic conditions.