Nutritive value of crop residues

Nutritive value is generally determined by feed composition, intake and utilization  efficiency of digested matter. Thus, the value of a feed depends on chemical composition, digestibility, intake and efficiency. Table 7b.1 contains the nutrient content of some cereal crop residues. Such high fibre contents are believed to be negatively correlated with voluntary intake, rate of organic matter fermentation, microbial cell yield per unit organic matter fermented, and propionate: acetate ratio in fermentation end products. Crop residues also have a low mineral content, especially P, and are deficient in vitamins. Therefore, supplementation of crop residues before feeding is necessary, in addition to various treatments. In rice straw, another unwanted constituent is oxalic acid, which can cause rumen disorders. However, oxalic acid can be eliminated by alkali treatment, washing or ensiling. Various crop residues have their own nutritional values and are used for different animal species. Sweet potato vines and peanut hay are relatively rich in protein, available energy and vitamins, and are mainly fed to pigs in most rural areas. Wheat straw and rice straw have high contents of cell walls, and are basically used for feeding ruminants.Sugarcane tops are being utilized for animal feeding in many areas in India, in spite of its poor nutritive value and palatability. Chopped sugarcane tops are mixed with wheat straw and water soaked concentrate mixture/oilcake and this mixed ration can be offered to animals to overcome the palatability problem. Sugarcane bagasse is another crop residue which is available in plenty. Cottonseed hulls can also be utilized for the feeding of ruminants as a source of roughage as it is having low protein and high fibre contents. Amongst other hulls available, groundnut hull is abundant in India. Groundnut shells are fibrous and poor in protein and mineral contents. Unlike cottonseed hulls, these are very brittle, hence can be turned into power form and can be utilized for animal feeding at lower level in the ration, when fibre is not a limiting factor.

Improving feed value by processing or treatment

The nutritional value of these cheaply and readily available feed resources is  generally poor. Simultaneously, various strategies such as chemical pre-treatments, use of different supplements and physical processing were identified to be of considerable advantage in improving the feeding value of these fibrous feeds. However, the application of these improvement strategies either in the form of managing the crop residues themselves or by seeking for suitable forms of feeding systems that complement deficiencies of the residues is still unsatisfactory, especially in developing countries and under the small scale farmer’s condition. Ruminants despite their unique and highly efficient digestive system, are not able to extract sufficient energy to grow and produce milk from low quality or highly lignified residues. These must be properly processed or treated in some way to make them useful for production.

Traditional processing and feeding methods: The processing and feeding methods include particle reduction and reconstitution of roughage, and are still included in university textbooks and scientific publications. Chopping and water soaking are popular practices for crop residue feeding throughout the country. Although they do not always result in consistent improvements in animal performance, they definitely result in reduced diet wastage and diet selection.
Physical treatment: Numerous physical processing techniques to enhance the utilization of crop residues by ruminants have been used, with varying degrees of success. The most studied physical treatments for enhancing crop residue use by ruminants are grinding and pelleting. Grinding, or fine chopping, decreases particle size, increases surface area and bulk density of both leaf and stem fractions, and hence raises rumen microbial accessibility or feed intake. The increase in intake due to grinding is generally higher with low quality than with high quality residues, and with small and young animals rather than with older and larger animals. The critical feed particle size to exit the rumen is smaller in sheep than in adult cattle and therefore a greater degree of grinding is necessary before they leave the rumen.

Chemical treatment: Attempts have been made to improve the digestibility and nutritive value of crop residues. A major breakthrough was chemical treatment to remove encrusting substances (cellulose, hemicellulose and lignin). Many chemicals have been screened in laboratory experiments for their potential to enhance digestibility. However, only three are being routinely used in animal research: sodium hydroxide (NaOH), ammonia (NH3), and calcium hydroxide (CaOH). The modes of action of chemical treatment on crop residues, especially with alkalis, can be:

•  Hemicellulose solubilization,
•  Increases in cellulose and hemicellulose digestion, and
•  Increases in digestion rate for cellulose and hemicellulose.

Sodium hydroxide treatment: Sodium hydroxide treatment of crop residues has been investigated by many workers across the whole world. The procedure basically followed the dry method, where NaOH is applied at 3-5 % and the moisture content is
20-30 % of DM. Alkali treatment may saponify the ester bonds between lignin and carbohydrates or the phenolic acid-carbohydrate complexes in plant cell wall. Through these effects, structural carbohydrates in both lignified and unlignified plant tissues become more digestible, with consequent increases in rate and digestibility. The  treatment with NaOH results in increases in crop residue palatability and digestibility, and in animal performance. Although NaOH treatment works effectively in improving the nutritive value of crop residues, NaOH is expensive, corrosive and its use may result in significant excretion of sodium ions in animal excreta. Long-term accumulation of sodium may lead to soil fertility problems and environmental pollution. Thus, application of NaOH treatment of crop residues is not popular with the farmers at present.

Ammoniation: Ammoniation of crop residues has drawn a great deal of attention due to several advantages: effectiveness in improving digestibility, addition of non- protein nitrogen to treated residues, and absence of sodium accumulation in soils. Most data have shown a decreased NDF content, but little change in ADF and ADL contents of crop residues due to ammoniation. The results suggest that ammoniation can break the linkage between hemicellulose and lignin and make the hemicellulose fraction partially soluble to NDF solution. The soluble hemicellulose would be highly digestible by ruminal micro-organisms. After being ammoniated, treated crop residues have an increased N content relative to untreated residues. Many in vitro and in vivo digestion trials have been conducted to evaluate the effect of ammoniation on digestibility of different crop residues. Improvements in the feeding value of crop residues due to ammoniation have been observed in many feeding trials.
Urea-ammonia: Urea is generally the most practical and often the cheapest chemical available for treating straws in many areas of tropics. This treatment is most suitable in tropics because of its ready availability, familiarity of farmers with its transport, storage and application in addition to its good effect on intake, digestibility, growth and milk production. The straw is treated with urea (4kg urea dissolved in 65 litre of water and sprayed or sprinkled on 100 kg straw) and stored in the form of kup/bonga/ dhar for three weeks. The digestibility was found to be improved by 40-45% and the voluntary intake increased by 86-100%. Crude protein content of straw increased from 3.5 to 7.5%. It gave higher growth rate (200-250g/day) than wheat straw supplemented with urea (100-200g). Urea treated straw supplemented with 1 kg concentrate mixture or a supplement of 400g cottonseed cake can support a growth rate of about 350-
400g/day in crossbred cattle, indicating thereby that feeding of urea-treated (5%) chaffed paddy straw ad lib. with 4 kg berseem and concentrate mixture can support 9- 10 kg of milk/head/day in crossbred cattle.

Biological approach

Ensilage: For small-scale family farms, ensiling dry cro p residues after reconstitution of moisture is usually the best way for preserving feeds, since farmers do not have suitable equipment to quickly harvest their cereal plants. Another reason is that they have to sow promptly the next crop in most regions with a double-cropping system. Ensiling dry crop residues involves actions such as chopping, reconstitution of moisture, pressing and mixing with certain additives, including micro-organisms such as lactic acid producing bacteria, cellulolytic bacteria, for proper fermentation followed by pressing to remove air from silo.
 A large number of dry crop residues have been successfully ensiled with addition of microbial products in recent years. The effect of microbial ensilaging of dry crop residues is probably to hydrate and weaken plant structures so that less energy is expended on rumination. Ensiled crop residues usually have good palatability for ruminants, and thus have high intake. In comparison with ammoniated straw, microbial ensiled residues give higher intake, faster rate of passage and therefore better performance. Other advantage of microbial ensilage is its low input cost for acquiring microbial products and accessories, e.g. plastic sheets, and therefore microbial ensilage is considered a better method to enhance the feeding value of dry crop residues. However, microbial ensilage generally results in lower digestibility than ammoniation. Another disadvantage of microbial ensilage includes substantial loss (usually 5-10 % of DM) of organic material that would otherwise be rapidly fermented in the rumen. As a result, it is still argued academically whether the anaerobic ensilage of such ready digestible materials within crop residues is economically beneficial to the animal. Further in-depth research is required to select bacteria strains that selectively degrade cell wall fractions, especially lignocelluloses.

Exogenous enzymes for ruminants: The exogenous fibrolytic enzymes might be the key to improve the utilization of crop residues for ruminants. The use of enzymes is biological as well as environment friendly. These enzymes mainly contain cellulase and hemicellulase activities and are mainly of fungal origin. These enzymes can be in powder, granular or liquid forms. The mode of action of enzymes in ruminants is not fully understood. The enzyme addition increases feed intake due to pre and post ingestive enzyme effects such as increased palatability due to increased release of sugars, increased rate and extent of digestion or increased passage rate. Exogenous enzymes are most effective when applied by spraying enzyme solution directly on to the forages prior to ingestion. Adding exogenous enzymes to the diet increases the hydrolytic capacity of the rumen mainly due to increased bacterial attachment, stimulation of rumen microbial populations and synergistic effects with hydrolases of ruminal microorganisms. These enzymes in the rumen are generally more stable if applied prior to ingestion. They also appear to survive in the small intestine for a time sufficient to have an effect on target substances. However, postruminal effects of exogenous enzymes on digestion are likely only if enzymes are infused into rumen or added to feed in a manner that allows for easy solubilization from feed and rapid passage from the rumen.