Ruminant livestock are typical herbivores with multi-compartmental stomach having unique ability to utilise fibrous plant feedstuffs by means of fermentative digestion brought about by an array of reticulo-rumen microbial enzymes. However, in order to increase productivity from dairy animals we have made good progress through selective breeding for the genetic upliftment of best dairy traits in cows. As a result of that, nutritional requirements to match high dairy merit demands feeding of more concentrate-based ration rather than relying only on forages. Such a system is most successful in the developed world under intensive rearing. In developing country like India, the progress that could be potentially be achieved is partly constrained by a huge deficit in feed resources, both, the green forages as well as concentrate ingredients.
Types of by-product roughages produced in India
Worldwide total crop residue availability has been estimated to be 3.8 billion metric tons with cereals contributing 74%, sugar crops 10%, legumes 8%, tubers 5% and oil crops 3%. Asia alone contributes to more than 45% of global straw production. Availability of crop residues in India has shown a constant increasing trend since last 10 years i.e. from 467.50 million tonnes in 2005 to 517.65 million tonnes in 2010 and 574.06 million tonnes in 2015. It is further projected to increase up to 637.53 million tonnes by 2020. Simultaneously, it has been projected that crop residues will contribute to >70% of feed budget for Indian livestock by the year 2020. The major agro-residues in terms of volumes generated in India (in million metric tons, MMT) are found to be rice straw (112), rice husk (22.4), wheat straw (109.9), sugarcane tops (97.8) and sugarcane bagasse (101.3). This shows the relevance of crop-residue based by-product feeding for sustainable livestock production.
Theoretically, if 100% energy of these byproducts (mainly cereal straws) is utilised, it can yield up to 10 MJ ME/kg DM. Unfortunately, only 60% of this energy is available to the animals as the digestibility of straw varies between 40-60% depending upon the variety and type of straw. This poor digestibility of straw accompanied with poor availability of energy from untreated straw is due to covalent bonding between lignin and carbohydrates (cellulose and hemicellulose). Lignin shields the carbohydrates from microbial and enzymatic attack in the rumen, thus acting as cementing material in the cell wall. In addition to poor digestibility of straw, the lower content of crude protein and the absence of other essential nutrients (minerals and vitamins) limit its use in the ration of productive animals. However, by-products from legumes (legume straw, haulms etc.) have excellent fodder value and are superior in terms of protein content, digestibility and hence nutritive value. These are also rich sources of minerals mainly calcium and can be considered as maintenance type of feeds.
The available crop residues can be classified into following four categories (see Table 1 for compositional details):
1. Fine (slender) straws: Rice, wheat, oat and finger millet straw
2. Coarse straws: Sorghum (jowar), pearl millet (bajra), maize and small millet straw
3. Haulms of legumes: Pulses and groundnut haulms
4. Others: Sugarcane tops
How to improve feeding value of fibrous roughages?
Fibrous crop-residues also known as lignocellulosic feeds though available in plenty, are not used to their fullest potential mainly because of poor voluntary intake (1.5-2% of body weight) and digestibility (40-45%). These are characterized by high fibre, low protein (3-6%) and low energy (45%) contents. A small increase in digestibility by 3-5% units has been observed to bring about almost 20% improvement in animal performance. Therefore, different treatment methods have been tried with varied degree of success, which are as follows.
Methods to treat crop residues for improved utilisation
Among the several methods of physical treatment, soaking in water, chopping, grinding, steaming and densification are common. Chopping can be achieved by hand-operated or motor driven chopping machines (toka). Chopped straw or threshed wheat straw mixed with green fodder and little concentrates and soaked in water is a popular farmers’ practice commonly known as ‘sani’ preparation, which is an effective way of delivering almost all required nutrients similar to that of total mixed ration (TMR) feeding.
Fine grinding and pelleting of straws may decrease digestibility due to faster passage through gut and may not be economical. Coarse grinding is recommended for utilisation of maize cobs for cattle feeding. Steaming is particularly useful for improving quality of sugarcane bagasse in the sugar factory. However, densification of straw along with green fodder and concentrates in the form of ‘densified complete feed blocks’ is useful in increasing bulk density, ease in transporation (decreases volume by five folds) and can be stored for future use particularly during drought situation. Several crop residues can be utilised effectively by incorporating them in complete feed blocks (Table 2). There is a need to design the simple feed block machines suiting the requirements of small to medium scale farmers of the country.
This involves use of chemicals like strong (sodium and potassium hydroxide) and weak alkali (calcium hydroxide), urea-ammoniation and other oxidising chemicals. Though sodium hydroxide (NaOH at 2-4% level) is more effective in increasing digestibility of straws up to 75%, its use is associated with serious environmental pollution besides increasing kidney load to excrete excess sodium ions. Calcium hydroxide (4% at 50% moisture for 90 days), on the other hand is a safe alternative which poses no threat to the environment. Lime (5% at 60% moisture for 3 weeks) treatment is also effective in enhancing digestibility up to 60%. However, the most useful and widely accepted treatment method is urea-ammoniation technology. Fertiliser grade urea at the rate of 4 kg is dissolved in 40 L of water and sprayed on 100 kg of chopped straw spread over polythene sheet, mixed uniformly and stacked air-tightly with another polythene sheet for a period of 3-4 weeks. This has the additional advantage of enriching straw CP from 3-4% to 11-14% besides increasing intake and digestibility. Despite such benefits, majority of farmers are skeptical in adapting this simplest yet beneficial technology.
It involves combination of physical and chemical treatments. Many methods are tried but none has seen successful implementation at the farm level. A comparison of influence of different physical treatments along with digestibility is presented in Table 3.
As about 70-80% of cell walls of cereal straws are unavailable to rumen microbial digestion, biological treatment aims at selectively degrading lignin by various microbes mainly basidiomycetes white-rot fungi. Though there is an improvement in digestibility by 10-15% with many fungi, inherent organic matter loss as well as lack of field level transformation of cumbersome laboratory cultivation of fungi through solid-state fermentation limits its adaptation by farmers.
Supplementation of critical nutrients
Majority of cereal straws are deficient in many nutrients and are unable to serve even for maintenance of livestock. For instance, on a complete straw ration, a growing calf may lose body weight of about 150 g per day while upon feeding urea treated straw, it can gain a daily weight of 150 g. Therefore, straw-based diets need to be supplemented with other feedstuffs to obtain maximum productivity.
Following types of supplementation are commonly done in straw-based feeding system.
1. Catalytic supplementation
Eg. urea molasses mineral blocks
2. Moderate supplementation
Eg. slowly degradable proteins
3. Substitutional supplementation
Eg. green forages (legumes and non-legumes)
Catalytic and moderate supplementa-tions can also be done by means of ‘protein banks’ through good quality tree leaves (Leucaena, Gliricidia, Moringa etc.) at the farm level. Critical nutrient supplementation has the benefit of improving rumen function by fulfilling CP needs (7-9%) and thus increasing microbial protein synthesis.
Alternatively, straws can be mixed with animal organic wastes to form wastelage that can be a valuable feed resource during scarcity periods. About 40 kg of cow dung or poultry droppings, 10 kg molasses, 1 kg mineral mixture and 0.5 kg salt are mixed in 22 L of water, sprayed over 48 kg of chopped straw, and ensiled for 6 weeks anaerobically.
Recently it was reported that ground maize cobs mixed with jaggery, mineral mixture, salt and other ingredients improved milk performance of cows in a Co-operative Milk Producer Society, thereby economizing the cost of production (The Hindu, July 26, 2015).
Future perspectives in utilising crop residues
Plant breeding and selection of superior cultivars having highest straw/stover/haulm quality with respect to digestibility, nitrogen content without affecting grain yield could be a potential area to be considered in improving feeding value of crop residues (Table 4). Several recent experiments (ILRI, Hyderabad) have clearly demonstrated variation in quality of crop residues as affected by the type of cultivar. For example, commonly grown cultivars of groundnut was demonstrated to differ in haulm quality in terms of nitrogen content by two folds, in vitro digestibility (10% units) and metabolisable energy (3 MJ/kg) which resulted in nitrogen balance of 6-11 grams per day in growing sheep. This shows that high quality haulms can improve animal performance by two folds compared with their low quality counterpart. More and more of such experiments on various crops duly considering quality of by-products (nitrogen content, digestibility, fibre and lignin content etc.) could actually bring about an improvement in livestock productivity.
Dairy farming in India thrives largely on crop residues and other fibrous by-products, as it helps in keeping the input feed cost on the lower side. Improved utilisation of crop residues will have a great implication in enhancing dairy production by Aproviding improved feed security and thus, partially mitigating mismatch between requirements and availability of feds for livestock. Furthermore, quantitative estimates of various crop residues and other fibrous by-products should be made up- to- date, both at state level as well as at country level. Besides that, suitable methods to improve their feeding value must be popularized as per regional availability, for improved ruminant production in India.
References and tables are available on request
M.S. Mahesh and Dr S.S. Thakur, Dairy Cattle Nutrition Division, ICAR-National Dairy Research Institute