Biomass Management for Fodder & Energy

Lesson 11. Processing of straw/ fodder for animal use

One of the most abundant lingo-cellulosic wastes on earth is paddy straw. Annual production of rice is about 136.5 MT. About 1 to 1.5 kg of straw is produced per kg of grain harvested and thus, 136.5 – 150 MT of paddy straw is estimated to be produced annually. In India, approximately 70-80 MT of paddy straw is disposed off by burning. Although burning leads to organic matter loss, it is quick and overcomes the many problems associated with slow decomposition of straw. It reduces the incidence of pests and diseases, facilitates soil preparation for a second rice crop or for other off-season crops, and it returns minerals to the soil.

11.2. Rice straw as an animal feed

Rice straw is traditionally fed during periods of feed shortage, but it does not provide adequate nutrients for maintenance. The use of straw for ruminant feeding is cellulose and physically make the structural fibers swollen constrained by its low digestibility due to high silica and lignin as well as low protein and energy contents when given as the only feed to animals. With straw, the level of intake is governed by the amount of material in the reticulo rumen, its rate of digestion and the rate of passage of digesta out of the reticulorumen. Thus, mechanical processes associated with digestion, namely, particle size reduction during eating and rumination and rates of microbial fermentation are important in relation to the level of intake. Where rice straw constitutes 100% of the diet, then all its limitations, in terms of physical characteristics, chemical characteristics and low contents of essential nutrients, will manifest themselves and the animals will lose weight. However, sometimes even small contributions from other feeds in a mixed diet can produce markedly improved results through effects at the rumen and/or tissue level.

11.3. Challenges in feeding rice straw

Palatability - Cattle are more likely to eat rice straw if the time between rice harvest and straw baling is short. In addition, rice straw has a slight pubescence (small hairs) that cows may take some time to adapt to.

Low digestibility - Rice straw has very high silica content (8 to 14%). Silica is indigestible and decreases digestibility of the feed. This high silica level combined with other mineral compounds produces an average ash content of 17%.

Low protein - The crude protein of 2 to 7% on a dry matter basis in rice straw requires protein supplementation to meet the nutritional requirements of most cattle.

High in oxalates - Oxalates in rice straw decrease the absorption of calcium.

11.4. Feeding value

The major components contribute to the feeding value or nutritive value of straw/forages are (a) chemical composition, representing the gross amounts of nutrients available; (b) level of voluntary intake, indicating the amounts of nutrients consumed; (c) digestibility, indicating the proportions of nutrients that are digested and absorbed and become available for metabolism; and (d) efficiency of metabolism at the tissue or cell level.

11.5. Improving the Feeding Value through Pretreatments

Pretreatment processes can improve the feeding value of straw by increasing its digestible energy content, by increasing feed intake or by a combination of these effects. Maximal increases in digestibility are only achieved with an adequate balance of energy and other essential nutrients, such as nitrogen and minerals. Further, limitations in the supply of these essential nutrients can limit the intake of straw independently of their effects on digestion and consequently, they are also necessary to allow expression of potential intake responses. In addition to the effects on feeding value, other factors are also important. Pretreatments can range from simple chopping and soaking procedures to elaborate chemical pulping operations or carefully controlled fermentation processes. Many of these processes are technologically feasible, but economic constraints limit their use to particular situations or in fact, rule out their use under practical conditions. Pollution effects of some of the pretreatments need to be considered and evaluated before promoting their use.

11.6. Physical pretreatments

Some physical processing methods such as chopping or grinding are unlikely to affect the chemical composition of straw. However, others such as soaking, steaming under pressure and gamma irradiation do have effects on the chemical composition of fibrous residues ranging from losses of cell soluble to alterations in the structural carbohydrates of the plant cell wall.

11.6.1. Soaking and wetting

Rice straw soaked for 3 days results in 8-14% dry matter losses, indicating removal of soluble cell contents and reduced feeding value.

11.6.2. Chopping

Straws are sometimes chopped to reduce wastage and to facilitate feeding, but this does not alter the cell wall structure in such feeds.

11.6.3. Grinding and pelleting

In the rumen, degradation of cellulose requires direct association of microbial cellulases with the substrate and, hence, the rate of hydrolysis would be expected to be affected by the cellulosic surface area accessible to the enzyme. The extent of any increase in cellulosic surface area is likely to be determined by the fineness of grinding. A process such as ball-milling results in extreme reduction in particle size to the point of the physical separation of cell wall components  and this results in marked increases in in vitro digestibility.

11.6.4. Steaming under pressure

Steaming exerts physical effects through the separation of cell wall structures and chemical effects including the cleavage of bonds between cell wall constituents, the degradation of hemicellulose, and a hydrolytic action of the acids resulting from these processes.

11.6.5. Gamma irradiation

Ionizing radiation has been investigated as a means of increasing the availability of nutrients in plant cell walls for microbial digestion as this pretreatment may reduce the resistance of fibrous feeds to physical degradation without the necessity for fine grinding.

11.7. Chemical pretreatments

The objective of chemical pretreatments of fibrous feeds is to increase their digestibility and intake through solubilization of some of the cell wall components or disruption of complexes of lignin and cell wall carbohydrates. Some such treatments solubilize lignin, while others such as extreme pH conditions, whether acid (below pH 4) or alkaline (above pH 8) increase the solubility of hemicelluloses. The chemicals which have been most extensively used for the purpose of increasing cell wall degradation can be broadly classified into three groups: alkalis, oxidative reagents and acids. Of these alkalis have been the most commonly investigated chemicals for improving the feeding value of fibrous residues. Chemical treatment of straw with alkalis such as ammonia and sodium hydroxide, has been commonly used for improving both apparent digestibility, bacterial colonization on cellulose and voluntary intake of straws.

11.7.1. Alkali spray treatment

The alkali spray treatment of straws has been shown to improve digestibility and intake. Sodium hydroxide is generally regarded as the most effective alkali for improving digestibility. The straw is sprayed or sprinkled with a dilute solution of NaOH at the rate of 1 litre per kg and the moist straw is immediately fed to animals. The optimum concentration of the NaOH solution appears to be about 5 percent where straw is to be fed with limited supplements, although where treated straw forms only about 50 percent of the diet, 7 to 8 percent appears to be better.

11.7.2. Ammonization of rice straw

By processing rice straw with urea its quality and digestibility can be improved. It can also increase the protein content of rice straw. It would seem that with adequate moisture content and suitable temperature conditions, microbes which produce urease are capable of degrading urea with the formation of ammonium compounds such as ammonium carbonate, bicarbonate or hydroxide which then permeate through the straw.

Procedure for ammonization of rice straw

Two plastic bags are taken and one bag is put inside the other to make a double thickness. Then 15 kg of air-dried rice straw is put inside the bag. The rice straw is compacted by pressing it down with hands. 870 g urea dissolved in 5 liters of water is poured into the plastic bag of rice straw. The inner plastic bag is closed by binding the mouth and then the opening of the outer plastic bag is tied separately. The bag of straw is put in a safe place and can be opened after one month. The mouth of the bag should be left open to expose the straw to the air for two days. It can then be used as livestock feed. Sweet soy sauce or molasses may be added to increase the palatability.  Once the cattle become used to the ammoniated rice straw, it can make up 60-100% of their feed intake.

11.7.3. Calcium hydroxide treatment

Calcium hydroxide is a relatively cheap chemical reagent for treating crop residues as its precursor lime is cheaper than sodium hydroxide. However, to be effective the calcium hydroxide needs to be used at reasonable concentrations to compensate for its low alkalinity and it needs to be applied by a soaking method to allow for its low solubility.

11.7.4. Pretreatment with oxidative reagents

Oxidative agents are known to reduce lignin content and to break bonds between lignin and carbohydrates of straw. However, pretreatment with oxidative chemicals, such as sulphur dioxide, ozone and chlorinated compounds involves the use of sealed reaction vessels and the processes are generally carried out under controlled conditions.

11.7.5. Pretreatment with acids

Acid pretreatment of fibrous feeds hydrolyses the hemicellulose in cell wall material thereby releasing sugars. Also some lignin-carbohydrate bonds are acid-labile and may be broken during this treatment and with concentrated acid cellulose can be hydrolysed. Treatment with hydrochloric or sulphuric acids followed by periods of storage was found to increase the rumen fluid digestibility of fibrous materials and this was associated with increases in water-solubility of the feeds.

11.8. Physico-chemical pretreatments

When physical and chemical pretreatment processes are used in combination they might be expected to be more effective in increasing the nutritive value of fibrous feeds. In this regard the effectiveness of alkali treatments might be increased if the surface area exposed to chemical action was increased by chopping or grinding or by steaming under pressure.

11.9. Biological treatment

The use of fungi and/or their enzymes that metabolize lignocelluloses is a potential biological treatment to improve the nutritional value of straw by selective delignification problems are considered and should be overcome as fungi may produce toxic substances. It is also difficult to control the optimal conditions for fungal growth, such as pH, temperature, pressure, O₂ and CO₂ concentration when treating the fodder. With recent developments in fermentation technology and alternative enzyme production system, the costs of these materials are expected to decline in the future. Hence, new commercial products could play important roles in future ruminant production systems.

11.9.1. White-Rot Fungi Treatment

White-rot fungi, belonging to the wood-decaying basidiomycetes, as lignocellulolytic microorganisms are able to decompose and metabolize all plant cell constituents (cellulose, hemicellulose and lignin) by their enzymes. Many species of white-rot fungi which are effective lignin degraders have been used to assess their ability to improve the nutritive value of fodder for ruminant nutrition but fungal metabolism can dissipate carbohydrate.

11.9.2. Treatment with Enzymes

Enzymatic hydrolysis of rice straw is an interesting way to produce sugars from cellulosic wastes because of its mild operating conditions, regarding pH and temperature and the absence of by products. Xylan-rich cell walls which contain significant amounts of lignin, are also generally resistant to enzymatic hydrolysis and require severe physic-chemical pretreatments such as steaming, radiation, acid hydrolysis and alkali digestion before the polysaccharides become accessible to enzymes and can be hydrolysed to monomeric sugars in high yield.

Although several treatments have been used to improve the degradability and voluntary intake of rice straw, such as physical or chemical treatments, the practical use of these treatments is still restricted in terms of safety concerns, costs and potentially negative environmental consequences. Moreover, the application of ligninolytic fungi or their enzymes combined with chemical pre-treatments to rice straw may be an alternative way to decrease the amount of chemicals affecting some synergy.

11.10. Forage value of rice straw

Rice straw varies greatly in its forage value with protein from 2 to 7 % and 44 to 56 % Acid Detergent Fiber (ADF). ADF is a laboratory method of determining the fiber content that can assist in predicting the digestibility of a feed. Crude protein is estimated by determining the total nitrogen and multiplying by 6.25. The lower the ADF per cent, the digestibility of the feed will be more.

Rice straw of 2 to 3 percent crude protein on a dry matter basis should not be used for cattle feed, as its forage value is generally less than the cost of baling and hauling, and it may cause poor animal health or death. The number of days that baling occurs after harvest will greatly impact palatability and forage quality. Forage that is baled 1 to 3 days after harvest maintain the best palatability (smell, flavor and colour) of the forage. Forage chemical quality starts to decrease at 6 to 10 days after harvest.

11.11. Supplementation of formulation of practical diets

11.11.1. Feeding rice straw with forage supplements

The most common feeds provided with rice straw are roadside grasses, while other important forages are cassava, gliricidia, leucaena and sesbania. In specific areas forages from many other trees, crops and water weeds including acacia, banana, jackfruit, pigeon pea, neem, sweet potato vines and water hyacinth are utilized. It is known that small quantities of green forage can improve the utilization of straw diets through increases in intake and digestion. These beneficial effects are apparently due to influences on rumen function. The leaves of these plants remain green during dry periods when the availability of and quality of roadside grasses are low and hence they can be valuable supplements. Within small farm systems these plants can have other beneficial effects in that they often provide fuel and in the case of leguminous types they may help to improve soil fertility by fixing atmospheric nitrogen. However, care should be taken to ensure they are palatable and that they do not contain toxic constituents. The value of forage supplements in feeding systems based on rice straw has advantages additional to their nutritional effects occur where they are available on the farm and hence are easily accessible and relatively cheap and in that they reduce requirements for other supplements.

11.11.2. Feeding rice straw with concentrate supplements

Concentrate supplements which are used may be home-grown or by-product feeds from mills, such as rice bran, coconut cake, cassava chips and palm kernel cake. Purchased concentrates from feed manufacturers have advantages in that they may be formulated feed mixtures designed to provide limiting nutrients in balanced amounts or they may be individual feeds required for specific purposes, like coconut cake, fish meal or molasses. Costs of the various formulated feeds have been increasing due to rising prices of ingredients and this has acted as a major deterrent to their use as the profit margin becomes small.

 

References

1. Doyle, P. T., C. Devendra and G. R. Pearce. 1986, Rice straw as a feed for ruminants, International Development Program of Australian Universities & Colleges Limited publication.
2. Garret, W.N., Walker, H.G., Kohler G.O. and Hart, M.R. 1976,  NaOH and NH₃ treated rice straw for ruminants, J. Anim. Sci., 43: 322. (Abstract)
3. Jackson, M.G. 1977 Review article: The alkali treatment of straws. Anim. Feed. Sci. Technol., 2.
4. Hanafi, E.M., H.H. El Khadrawy, W.M. Ahmed and M.M. Zaabal, 2012. Some Observations on Rice Straw with Emphasis on Updates of its Management, World Applied Sciences Journal 16 (3): 354-361