The main constraints for productivity of fodder crops are lack of availability of soil nutrients, quality seeds and various pests and diseases. These constraints not only influence on the productivity, but also quality of fodder.
Resource poor farmers often cultivate forage on low productive soils to make use of the idle land and do not apply the required quantity of nutrients. Such farmers have several options to boost crop yields by applying low cost inputs such as soil amendments, organic manure and bio-fertilisers.
Unfortunately, most of them do not bother to apply biofertilisers and soil amendments due to their ignorance and difficulty in procuring them. Even some farmers using pesticides in cultivated fodder crops to control pests and diseases. Extensive use of chemicals is non-economical and may cause the problems of environmental pollution and health hazards in livestock.
In this regard, creating awareness on natural resources and their beneficial association in fodder production is very important aspect for the quality production of fodder. Therefore this article we emphasizing the direct or indirect role of beneficial association of pollinators, soil arthropods and arbuscular mycorrhiza fungi in major forage crops
1. Role of Pollinators in forage crops
In fodder insects play important role in the pollination of many plants including important leguminous fodder crops like Lucerne, berseem, clovers and so on. They mainly belong to the orders Coleoptera, Diptera, Lepidoptera and Hymenoptera.
Among these, hymenoptera key order for pollination and having a most specialized and economically valuable pollinators like honeybees, solitary bees and bumblebees. Pollination does not result in production of the commodity itself, the process contributes to crop propagation such as seed production and quality produce.
Alfalfa seed, a bee pollinating crop with an annual value of $ 109 million, is used to produce hay for livestock forage that is valued at $46 billion per year. Fodder legumes having specialized pollination mechanisms, which results in cross pollination. Hence for successful pollination they must need aid of insect’s pollinators.
Alfalfa flowers have 5 petals, with the lower two petals modified into a "keel" that encloses the reproductive organs. Pollinators must have the strength and ability to force their way between the keel petals to access nectar and pollen.
The stamens are held under tension and when a flower visitor “trips” the flower, the pistil and stamens strike the head of the insect. This is where the pollen is deposited, and where it is likely to be dislodged by future trippings in other flowers. A flower has 10-12 ovules, but only a small proportion usually develops into seed.
A flower must be tripped before fertilization can take place, so it must be visited at least once by an insect for seed to set. Various species of bumble bees with varying tongue or proboscis length are pollinators. These include Bombus spp., Melitta leprorine and Anthidium punctatum.
The flower structures is such way that it must be tripped and cross pollinate to set high yields of vigorous seeds. In addition the stigmatic surface of flower is covered with membrane which avoids the germination of pollen grains hence results loss of seed setting.
The pollinators’ activity during flowering time results tripping and rupturing of membrane leads to successful pollination. Important pollinators of Lucerne include Apis spp., Pithitis smaragdula, Megachile spp., Certina binghami, Andrena levilabris, Xylocopa fenestrate, Chalcidoma rubripes and Nomia spp.
The supplementary pollination especially with Apis mellifera doubled the seed yield and improved the seed quality.
Berseem having piston type of pollination. As pollen and stigma hide within the keel when honey bees forage nectar from the base of the gynoecium, the keel is pressed sufficiently to give access for the bees tongue and the central column of the style and stamen, which was kept under tension is released explosively. Hence, this mechanism must need assistance of pollinators for good pollination. Important pollinators of include Apis spp., Pithitis smaragdula, Megachile flaviceps, Certina binghami, Xylocopa sp, Lassioglossum cattulum, Halictus sp., Noimoides variegate and Bombus sp.
2. Soil Arthropods
The soil systems are in dynamic equilibrium like most natural systems. One of the important processes on soil ecosystem is the decomposition and mineralization of organic matter of soil providing the required nutrients and energy to the plant.
The decomposition is largely a biological process with three major determinants viz., soil organism, the physical environment and the substrate quality. In this contest arthropods that live in the soil perform a number of critical functions in arid and semiarid regions.
Arthropods comprise a large proportion of the meso and macrofauna of the soil. Within the soil system, five groups are chiefly represented such as Isopoda, Myriapoda, Insecta, Acari, and Collembola, the latter two being by far the most abundant and diverse.
In natural grasslands and forages large diversity of species terms of species richness, arthropods may represent as much as 85% of the soil fauna. The mites alone constitute 79-94% of the total population. Soil is made mostly of the feces of arthropods.
Micro grazing by arthropods mineralizes to complete the nutrient recycling process. Arthropods function on two of the three broad levels of organization of the soil food web: they are plant litter transformers or ecosystem engineers.
Litter transformers fragment, or comminute, and humidify ingested plant debris, which is deposited in feces for further decomposition by micro-organisms, and foster the growth and dispersal of microbial populations. Large quantities of annual litter input may be processed (e.g., up to 60% by termites).
The comminuted plant matter in feces presents an increased surface area to attack by micro-organisms, which, through the process of mineralization, convert its organic nutrients into simpler, inorganic compounds available to plants. Ecosystem engineers alter soil structure, mineral and organic matter composition, and hydrology.
The burrowing by arthropods, particularly the subterranean network of tunnels and galleries that comprise termite and ant nests, improves soil porosity to provide adequate aeration and water-holding capacity below ground, facilitate root penetration, and prevent surface crusting and erosion of topsoil. Also, the movement of particles from lower horizons to the surface by ants and termites aids in mixing the organic and mineral fractions of the soil.
The feces of arthropods are the basis for the formation of soil aggregates and humus, which physically stabilize the soil and increase its capacity to store nutrients. The population of collembolans and mites higher in winter months (November to February) whereas, their activity greatly reduced during summer months (March to June). The incorporation of FYM increases the population of these micro arthropods.
The interesting fact that population of micro arthropods is not affected by seed dressing fungicides and among the forage crops, Lucerne increases the arthropods population.
3. Arbuscular Mycorrhizal fungi
Root system is a unique micro site for the association of symbiotic and non-symbiotic microorganisms, AM fungi is one of them. AM fungi constitute a monophyletic group, of the order Glomales (Phycomycetes), consisting of 6 genera belonging to 3 families. AM fungi have unique features that create, ecologically, a functional group of organisms.
On global basis mycorrhiza occurs in 83% of dicots and 79% monocots where as all gymnosperms are mycorrhizal. Dense infection more common in species of leguminaceae and graminae. Compare to other agriculturally important crops, AM root colonization more in the fodder crops. Different species of AM viz., Glomus sp. Gigaspora sp. Acaulospora, and Sclerocystis sp. are reported from India.
Mycorrhizal fungi are known to affect growth of most plant species through various ways. AM fungi are beneficial to their hosts by improving the uptake of water and minerals. They can also improve drought stress tolerance, one of the proposed mechanisms for which is improved phosphorus and micronutrients uptake in dry and poor soil.
Growth promotion is often observed due to improved acquisition of mineral nutrients through the AM fungal hyphal network. Changes in the root exudate patterns repel nematodes and induce changes in the soil microbial community, possibly attracting antagonists of pathogens. AM fungi show protective effect of colonization by mychorrhizal fungi against infections by microbial pathogens in different plant systems especially on soil borne pathogens such as Fusarium, Rhizoctonia, Pythium and Phytophthora.
In most cases the protective effect is not only related to damage compensation or tolerance but frequently reduce the damage also correlates with the decrease in pathogen content in the plant tissues. Similarly a clear reduction of the detrimental effects by endoparasitic nematodes. The effect of AM fungi on various forage crop’s growth and nematode is presented in the Table 1.
Nematodes like Meloidogyne sp, Heterodera cajani and Tylenchorhynchus vulgaris are major constraints for higher fodder production in cultivated fodders such as Sorghum, Berseem, Lucerne, Cowpea, Oats and Stylo in central and northern India.
While AM acts as obligate symbionts enhancing plant growth, the nematode as obligate parasite take out vital nutrients from the roots making reduced plant growth. Though largely based on greenhouse experiments recent research works indicated that AM fungi play a potential role in the biological control agent when both groups of organisms occur simultaneously in the rhizosphere of the same plant.
Thus the plants colonized with mycorrhizal fungi are able to grow well inspite of the presence of damaging levels of plant parasitic nematodes. Even AM provides protection against root feeding insect, Otirhychus or weevils. The larvae of insects are rhizophagous, whereas the adults feed on the damage on the foliage of the same plant.
Most interesting point that colonization of AM fungi increases the visitation of pollinators, which results increase in the seed setting.
Table 1: Effect of VAM fungi on plant growth and nematodes in forage crops
|Host and Nematode species||Effect of mycorrhizal plants|
|Pratylenchus zeae||Increased plant growth and low nematode population|
|Tylenchorhynchus vulgaris||Increased plant growth and low nematode population|
|Tylenchorhynchus vulgaris||Improved plant growth, nematode population unaffected|
|Meloidogyne hapla||Improved plant growth, reduced nematode numbers and adult|
|Meloidogyne incognita||Increased plant growth, fewer nematodes and root knots|
|Heterodera cajani||Improved plant growth, reduced larval penetration and reproduction|
|Meloidogyne incognita||Increased plant growth, fewer nematodes|
|Meloidogyne hapla||Increased plant growth, fewer nematodes|
|Meloidogyne incognita||Improved plant growth and fewer nematodes|
(Source: IGFRI, Plant Protection bulletin, 2002)
This article has attempted and signifies the importance of beneficial association of organism in forage crops. The benefits provided by these organisms are long term and have greater potential to combat constraints of fodder production in ecofriendly manner. Therefore creating awareness in farmers in the fodder production about this aspect will improve the soil health, fodder production and as well as fodder quality in an economical manner.
Manjunatha, N., R. P. Sah and A. K. Singh
ICAR- Indian Grassland and Fodder Research Institute