Unconventional Feed Resources and Anti-Nutritional Factors
With increasing ruminant population, there is a need to identify and introduce new and lesser-known food and feed crops capable of growing in poor soils, which in addition to controlling soil erosion, bring economic benefits to farmers, enhance biodiversity, create jobs and bridge the wide gap between supply and demand for animal feeds; and avoid competition with human for food resources. The successful use of these resources is possible only after their nutritional evaluation using appropriate techniques coupled with quality control approaches, and after understanding nutritional principles underlying their utilization.
There is a shortage of feeds and fodders of the conventional type such as grains and
their milling byproducts, viz. oilcakes, brans and polishes, forages, and cultivated green and dry fodders. The scope for increasing areas under fodder cultivation is limited. There are many non-traditional or non-conventional materials which are available in abundance and which can form potential source of feedstuffs for livestock. The chapter deals with important aspects related to use of non-conventional feeds in livestock diets.
The feed ingredients, which are not popularly used in the rations even though they are fairly good nutritious, are known as unconventional feeds. For example tapioca, mango seed kernel, sal seed, tamarind seed, guar meal, sunflower and safflower cakes, corn gluten meal from wet corn milling, dried yeast from brewing industry, urea from fertilizer Industry, dried poultry manure, etc. are some examples of non-conventional feeds.
Why unconventional ingredients are needed
1. To minimize the competition of livestock with the human race food.
2. To economise on the cost of feeding.
3. Because of the limited availability of conventional foodstuffs. At present in India there is a shortage of 11 % dry fodder, 38% of green fodder and 44% of concentrates. To bridge the shortage the remedy is only using unconventional feed ingredients in livestock feeds.
Most unconventional feeds are either crop residues or agro-industrial byproducts. They include cereal and legume straws and stalks. Agro-industrial byproducts result from processing of crops such as cereals (e.g. milling, alcohol fermentation), oilseeds, sugar beet, and citrus fruits. Developing feeding programs for ruminants based on unconventional feeds should take into accounts several problems that might be associated with these feeds. Several unconventional feeds are made of heterogeneous materials, which make it difficult to obtain a uniform product. A second challenge is the fact that processing (e.g. grinding, pelleting, ammoniation) might be necessary for many unconventional feeds to improve animal performance. Proper utilization of unconventional feeds by ruminants will not only benefit the animal industry but will also increase the economic return of many cash crops.
Crop residues are a major source of roughage to Indian cattle. These feed resources are used as livestock feeds since time immemorial. In arid and semi arid tropics where natural pastures are only seasonally available because of the shortage of moisture, crop residues assume great importance in decreasing the level of feed deficit. Fibrous crop residues like rice and other cereal straws are the major feed resources of the country accounting for 59-60 % of the total dry matter (total feedstuffs inclusive of green fodder, pasture grasses and concentrate feeds) produced. Even though the livestock feedstuffs are in short supply, still most of these crop residues are not effectively utilized. Usually in many parts of the country these materials, especially rice straw and other cereal straws, are wasted as they are fed in long form without chaffing. Further, the methods developed for improved utilization of these low-quality crop residues are not commonly practiced by the farmers as the advantage of these methods have not been demonstrated at the farm level. Hence, the nutrients from these crop residues are also wasted to a great extent. To utilize effectively the available crop residues, the methods for improved utilization of low-quality roughages like chaffing, ammoniation with urea, ensiling, supplementing with leguminous fodder, and enrichment with deficient nutrients would be demonstrated in each village so that the farmers can be convinced of the advantages.
Crop residues usually consist of the aboveground part of cereal plants after grain removal. They are potentially rich sources of energy because up to 80 % of their dry matter (DM) consists of polysaccharides. Due to the prevalence and intensity of agriculture in most regions of India, crop residues represent a high proportion of total feed for herbivores. However, they are not all well utilized as energy sources at present, since their digestibility is often low. They partly resist rumen microbial action so their digestion is far from complete. Due to their rigid structure and poor palatability, intake of crop residues is low. These constraints are mostly related to their specific cell wall structure and chemical composition, but there are also deficiencies of nutrients essential to ruminal micro-organisms, such as nitrogen, sulphur, phosphorus and cobalt.
Chemical composition of cell walls
As parts of plants, crop residues contain five different tissue types: (a) vascular bundles containing phloem and xylem cells; (b) parenchyma bundle sheaths surrounding the vascular tissue; (c) sclerenchyma patches connecting the vascular bundles to the epidermis; (d) mesophyll cells between the vascular bundles and epidermal layers; and (e) a single layer of epidermal cells covered by a protective cuticle on the outside. These tissues are digested to different degrees in the rumen. In general, the extent of tissue digestion by ruminal bacteria is as follows: mesophyll and phloem > epidermis and parenchyma sheath > sclerenchyma > lignified vascular tissue. These differences in tissue digestion explain the wide range in nutritive value of crop residues compared to conventional feeds.
The cells have two major components: cell contents and walls. The cell content fraction contains most of the organic acids, soluble carbohydrates, CP, fats and soluble ash. The cell wall fraction includes hemicellulose, cellulose, lignin, cutin and silica. In most crop residues, the cell wall fraction accounts for 60-80 % of dry matter (DM). Cell walls of crop residues consist mainly of polysaccharides, protein and lignin. These substances, with small amounts of other components, like acetyl groups and phenols, are organized in a complex three-dimensional structure. Other wall components include suberin, cutin, tannins, waxes and minerals.
Polysaccharides: Major polysaccharides in primary cell walls of most of the higher plants include cellulose, xyloglucan and pectic polysaccharides, while secondary cell walls contain mainly cellulose and xylans. Cellulose is a highly ordered linear homopolymer of glucose linked by b-1,4-bonds. In all higher plants, cellulose in primary and secondary walls exists in the form of microfibrils. The crystallinity of cellulose microfibrils is highly variable depending on the source and age of the tissue.
Hemicelluloses are a wide group of polysaccharides that basically share only the property of being soluble in dilute alkali and being able to bind to cellulose by multiple hydrogen bonds and to bind to lignin by covalent bonds. In grasses, the main fraction of hemicellulose is xylans, with a backbone of 4-linked xylose residues and short side chains of arabinose, glucuronic acid and 4-O-methyl-glucuronic acid residues. Most of xylose residues in higher plants are acetylated, mainly on the C-2 hydroxyl groups, but also on C-3. Hemicellulose polysaccharide concentrations in grasses can range anywhere from 150 to 400 g/kg DM, whereas in legumes, the concentration is much lower, generally between 80-150 g/kg DM. For both grasses and legumes, xylose usually comprises half or more of total sugars of hemicellulosic fraction. Furthermore, rhamnose only exists in the hemicellulosic polysaccharides of legumes.Pectic polysaccharides are present in the primary cell walls of all seed bearing plants and are located particularly in the middle lamella. They are the major components of the primary cell walls of dicotyledons (e.g. legumes) but account for relatively less of the primary walls in monocotyledons (grasses). Three pectic polysaccharides have been structurally characterized from the primary walls of both monocotyledons and dicotyledons: rhamnogalacturonan I, rhamnogalacturonan II, and homogalacturonan. Pectic polysaccharide concentration is quite low in grasses (monocotyledons), generally <10 to 40 g/kg DM, while fairly high in legumes (dicotyledons) ranging from 50 to 100 g/kg. The distribution of the different pectic polysaccharides within the cell walls is largely unknown.
Proteins: Proteins make up 2 to 10 % of the primary cell wall of many dicotyledons and some monocotyledons, and may become cross-linked by the formation of isodityrosine or dityrosine. Cell wall proteins may also be involved in covalent bonding with polysaccharides. Glycoproteins seem to be invariably found in primary cell walls. Apparent covalent protein-lignin linkages have also been observed in wheat internodes. Of the several types of structural proteins known, the best-characterized are the family of hydroxyproline-rich proteins, or extensin. These glycoproteins with rod-like conformations are components of the wall matrix in dicotyledons and in grass walls (e.g. maize pericarp). Other wall proteins, e.g. glycine-rich proteins, have been found in walls of herbaceous dicotyledons.
Lignin: Lignin represents between 5-20 % of crop residue DM. Lignin is described as three-dimensional networks of phenylpropane units. It is generally recognized that the precursors of these building stones are coniferyl, sinapyl, and p-coumaryl alcohols, which are transformed into lignin by a complex dehydrogenative polymerization process. These three aromatic monomers in lignin are referred to as p-hydroxyphenyl, guaiacyl and syringyl residues, respectively. Depending upon the number and type of functional groups on the aromatic rings and propane side chains, lignin has variable solubilities. Wheat straw lignin has higher alkali solubility than wood lignin. When wheat straw lignin is methylated with diazomethane, the number of free phenolic hydroxyl groups in the guaiacyl monomeric units resembled that in pine lignin. Grass lignin is esterified by cinnamic acids, chiefly p-coumaric acid through hydroxyls on its monomers. In addition, ether-linked ferulic acids have been observed in lignin from maize stalks, wheat straw, rice straw and bagasse.
Lignin in plant cell walls is physically and chemically associated with wall polysaccharides and proteins. The association between lignin and polysaccharides includes glycosidic linkages, ether cross-linkages, ester cross-linkages and cinnamic acid bridges. On the association between lignin and proteins in straws, limited information is available. Only a covalent protein-lignin linkage was reported in wheat internodes. The strong linkage between lignin and polysaccharides or proteins would definitely prevent cell wall components from enzymatic hydrolysis by ruminal micro- organisms, and thus limit the digestion of cell walls.
Others: These components - including cutin, suberin, tannins, waxes and minerals - are also found in the cell walls. Cutin and waxes are attached to the epidermal walls on plants surface. Cutin is a three-dimensional polyester composed of w-hydroxy and mid-chain hydroxy fatty acids. It is often esterified with phenolic acids, and maintains a close association with pectin in the epidermal cell walls. Cutin appears to be embedded in wax and pectin; these components serve as diffusional barriers that impede ruminal digestion of the intact tissue. Suberin is a functional component of cell walls. The polyesters that appear in suberized tissue can be esterified with phenolic monomers, oligomers or lignin. Silicon is an important inorganic element in plant cell walls and mainly present in the form of silica in the walls of epidermal cells and leaf hairs. The presence of silica in the cell wall of rice straw can limit the rumen digestibility of polysaccharides. Tannins are phenolic compounds synthesized by some plants as a defence. They may inhibit the activity of specific enzymes, such as cellulases. Since tannins are often insoluble, they can contaminate the crude lignin, resulting in higher analytical value. As a result of complexes with protein, tannins would depress its utilization, but may not affect cell wall carbohydrates.