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Methods of treating cereal straws

Physical Methods

There are two main physical methods of treatment: grinding and pressure-cooking.

Grinding increases the voluntary intake of straw with or without an increase in digestibility, but it often leads to an increase in DE intake. Jackson ( 1 ) quoted an increase of up to 30 per cent in DE by grinding.

Pressure-cooking can increase digestibility, but is generally costly. It may merit further study as an industrial process. Both these physical methods are less effective than alkali treatment.

Soaking of straw in water before feeding is being practiced in many parts of India. It helps to remove soluble oxalate. Though the digestibility of the fibre is depressed, one to two hours of soaking increases voluntary consumption (3).

Biological Methods

Attempts have been made to decompose lignin by microbial and enzymatic means to increase digestibility of lignocellulosic material. Organisms that degrade cellulose and hemicelluloses are of no use, since they deplete the straw of valuable nutrients that the animal itself can digest. Large increases in in vitro digestibility have been recorded by employing white rot fungi, but farm-scale treatment methods have yet to be designed, and feeding trials with animals must be conducted to evaluate the usefulness and practicality of this method of treatment.

Chemical Methods

Attempts to increase the feeding value of poor-quality roughages date back to the early 1900s, when Kellner and Köher in Germany observed that, after sodium sulphite treatment in the paper-making industry, straw pulp was highly digestible (88 per cent) for cattle. Further developments led to the Beckmann method of treatment that had application in Germany for some years beginning in 1921. It is recorded that since the Second World War more than 2 million tons of straw have been treated by this method in Norway alone. Since the early 1960s there has been a revival of interest in straw treatment and several methods of this have been developed. They vary in cost of treatment, effectiveness, and suitability under different conditions.

Many chemicals have been tested, but sodium hydroxide has been found to be the most effective. The effect of sodium hydroxide is to dissolve lignin, silica, and hemicelluloses. Cellulose is not dissolved. The degree of solubilization of cell wall material increases with higher concentrations of the alkali (table 3). Either "wet" or "dry" methods of alkali treatment can be applied to straw.

Wet Methods

TABLE 3. Effect of Varying Levels of Sodium Hydroxide on Cell Wall Material of Wheat Straw (Grams per 100 Grams of Original Untreated Straw Dry Matter)

Source: Sharma (4).

Beckmann method. Straw is soaked in 10 or more litres of a 1.5 per cent NaOH solution per kilogram of straw (12-15 kg NaOH/100 kg straw in a volume of 800 1,000 litres of water) and washed in a closed system with extra water after an appropriate period of soaking (18-20 hours). In this way a wet straw is produced with a sodium content of 2 per cent. The method increases the organic matter digestibility by about 20 percentage units for the expenditure of 4-6 kg NaOH/100 kg straw. Straw treated by this method has been fed to livestock on farms in Norway for the past 40 years. One-third to one-half of the total roughage requirement (15-20 kg/day) is given as treated straw. This method, however, has the following disadvantages:

• it requires a large quantity of NaOH; about 8 kg of NaOH are used for every 100 kg of straw;
• a large volume of water is required - a total of about 50 litres per kilogram of straw for treatment and washing;
• 20 to 25 per cent of the original dry matter is lost because of leaching;
• it creates a pollution problem because wash water has to be discarded.

Torgrimsby method (modified Beckmann method). In view of the pollution problem created by the Beckmann method, Torgrimsby in 1971 suggested a closed system in which the amount of water added to the system is equal to the amount of water removed in the treated straw. The method was further developed by Wethje in 1975, and the straw treated by this method is now being evaluated by the Agricultural University of Norway. The easiest way to visualize the Torgrimsby method is to follow the daily sequence on a small-scale farm. Three tanks are needed, each with an attached drain board. Tank A contains 1,000 litres of a 1.5 per cent NaOH solution in which 100 kg of straw, in bundles or bales, are soaked. The first washing tank, B. is twice as long as A and contains 2,000 litres of water. The second washing tank, C, the same size as A, contains 1,000 litres of water.

Straw treated by the Torgrimsby method has a dry matter content of about 20 per cent and a sodium content of about 2 per cent. In vitro dry matter digestibility is about 70 per cent, an increase of 32 per cent over the untreated digestibility of 38 per cent (5). The method has the following advantages:

• it uses less NaOH than the Beckmann method;
• it uses less water than the Beckmann method;
• dry matter loss is reduced;
• there is no pollution problem because it is a closed system.

It appears to be ideal for small-scale farms in Asia unless field trials prove otherwise.

Dry Methods

Spray treatment. With a view to eliminating the disadvantages of the Beckmann method, Wilson and Pigden (6) evolved a dry process of treatment in which straw is treated with a small volume of concentrated solution of NaOH. The straw is sprayed or sprinkled with the NaOH while being mixed. Research has shown that 4 to 6 kg of NaOH dissolved in 200 litres of water is adequate to wet 100 kg of straw. The quantity of solution required is less (100-120 litres) if a pressure sprayer is used. In a small-scale operation for feeding a few animals, one worker could apply the NaOH solution using a sprinkling can while another turns the straw with a fork. The efficiency of treatment would be less when using a sprinkling can because of poor wetting of straw.

For treating large batches of straw, a screw auger with spray nozzles inside it could be used quite effectively. Another possibility is a horizontal mixer with an overhead spraying device. Both these simple pieces of machinery could be worked either manually or with farm power.

Straw treated in this way is moist and has a pleasant yellow colour and a pleasing odour. Treated straw has a pH of 10 to 11. Animals eat this straw readily, often 20 to 30 per cent more than untreated straw. Digestibility is often increased by 10 to 15 percentage units. Sodium content increases by approximately 0.6 percentage units for every kilogram of NaOH per 100 kg of straw added. When treated with 4 kg NaOH/100 kg straw, the titratable alkalinity is equivalent to 0.5 kg NaOH.

Bulk treatment and stacking. One way to increase the effectiveness of the alkali is to heat the treated straw to a temperature of 80 to 90 C. A practical way of doing this would be to spray the straw with the minimum amount of sodium hydroxide solution and stack the damp, treated straw (10-20 litres per 100 kg straw). If the stack is big enough (3-4 tons), the treated straw will heat up to a temperature of 80 to 90 C. The heating is caused by the chemical reactions between the NaOH and straw. The temperature reaches a peak during the first 3 days and then declines for another 15 days or so to ambient temperature. As a result of heating, moisture evaporates, leaving the straw sufficiently dry for storage. The stack must be made at a well-ventilated site and must not be covered. The initial moisture content of straw should not exceed 17 per cent before treatment.

Though this method looks attractive, it may not be suitable for small-scale farm situations, because a specially designed straw treater is required for the proper penetration of NaOH in a small volume of water (10-20 litres per 100 kg straw). (Such a machine has been produced in Denmark and has shown good results.) The digestibility of straw treated by this method is increased by 10 to 15 percentage units.

Bulk treatment and ensiling. Alkali spray-treated straw can be ensiled satisfactorily for up to one year. There is no microbial fermentation, and the straw remains stable because of its high pH. The optimum requirement of moisture in the final product for satisfactory ensiling may vary according to climatic conditions. Trials in Sri Lanka (Jayasuriya and Somasunderam, 1979, unpublished data) have shown that a moisture content of 55 per cent after treatment is ideal for ensiling treated straw under tropical conditions.

After six months, the treated material (4 per cent w/w) had an in vitro organic matter digestibility of 65 per cent (untreated digestibility 43 per cent). Only about 6 per cent of the straw developed moulds. The pH of the treated straw remained between 9 and 10, and the ensiled straw had a pleasant odour. Trials in the United Kingdom (Owen, personal communication) have shown that 40 per cent moisture in the final product is more suitable for ensiling straw in temperate climates.

Ensiling of straw can also be done with calcium hydroxide. If straw is spray-treated with calcium hydroxide and fed to animals on the same day or the day after, it has little or no effect on digestibility. In fact, we recorded a slight depression in in vitro digestibility with calcium hydroxide, possibly because of its low solubility. Trials have shown that calcium hydroxide can be as effective as NaOH if the treated straw is ensiled for five to six months. This, in fact, could be an answer to achieving a more economical method of straw treatment.

The optimum level of alkali for treating straw has varied from experiment to experiment. It appears that the level of alkali required is different for different roughages, diets, and animals. In general, however, digestibility and voluntary intake increase proportionately up to 3-6 kg NaOH/100 kg straw. In Sri Lanka (7), we found that for paddy straws the optimum level of treatment was around 4 kg/100 kg straw, although there were slight differences among varieties. Evidence also suggests that the optimum level of NaOH may vary with the amount of concentrate supplement given to animals. With high amounts of concentrates in the diet, the digestibility of treated and untreated straw often becomes the same. Under such circumstances, high levels of alkali - e.g., 8 kg/100 kg straw - appear to be beneficial, as the high alkalinity of the straw can counteract the fall of rumen pH and ensure favourable conditions for the activity of cellulolytic micro-organisms.

Ammonia Treatment

Treatment of straw and other roughages with ammonia (gas or solution) has great appeal because (i) it does not leave residual alkali as NaOH, and (ii) it increases the nitrogen content of the material by 0.8 to 1.0 percentage unit.

The standard method of ammonia treatment is not practical under most farm conditions in South Asia. Ammonia should be easily available at low cost with suitable facilities for transport and storage. Furthermore, ammonia application should be carried out by trained personnel.

Methods based on ammonia released from urea have been suggested as suitable for our conditions. One such method is now being evaluated by M.G. Jackson at the College of Agriculture, G.B. Pant University of Agriculture and Technology, in India. Chopped wheat straw is sprayed with a urea solution using a sprinkling can so that the final product has a moisture content of 30 per cent. The treated straw is ensiled immediately. Laboratory trials have shown that after three weeks of ensiling with 4 per cent (w/w) urea, straw digestibility increased by 10 to 12 percentage units (from 53 per cent to 63-65 per cent). An in vivo trial with growing calves is now under way.

Urea treatment seems to increase the dry matter intake by 40 to 50 per cent. The treated material has a pleasing yellow colour and is highly palatable for the animals. The ensiled material has to be exposed to air for one to two hours before feeding to rid it of excess ammonia.

Feeding value of alkali-treated straw

Alkali-treated straw can replace hay and silage in the diet of ruminants if the difference in the protein content between the treated straw and the hay or silage can be made good with an appropriate supplement. In Asia, where straws are widely used as a livestock feed, alkali treatment with supplemental nitrogen and minerals could boost productivity more economically than the feeding of cereal-based concentrate supplements. Results of many experiments have shown the possibilities in this direction.

As early as 1968, Donefer showed the digestible energy intake of sheep could be increased from 84 to 188 kcal/kg wt^0.75 by alkali treatment (8 per cent w/w) of oat straw supplemented with 2.5 per cent urea. Many feeding trials in South Asia have clearly indicated that weight gain in growing cattle and buffaloes can be increased by 0.2 to 0.3 kg/day by feeding treated straw supplemented with nitrogen. Trials by Pitchchiah in India have shown that treated straw can effectively extend the limited quantities of high-quality fodder. Our trials (8) have also indicated that, by treatment with 4 per cent (w/w) NaOH, the feed value of rice straw can be made equivalent to that of a medium- to high-quality fodder. The few trials that have been done with milch animals suggest that alkali-treated straw can be successfully incorporated even into the diets of high-yielding cows without altering the milk yield or milk composition. There is much evidence to indicate that, in general, alkali treatment increases the feed value of low-quality roughages.

Urea is an accepted source of non-protein nitrogen (NPN) for ruminants. Feeding trials in Sri Lanka (9) and elsewhere have clearly shown that urea is a suitable source of NPN for supplementing alkalitreated straw. Urea levels up to 2-2.5 per cent of the dietary dry matter have resulted in increased digestibility and greater voluntary consumption of dry matter. Non-traditional industrial by-products high in protein could also play a major role in making alkali-treated straw more complete nutritionally. Spent tea leaf (STL), a by-product of the instant tea industry in Sri Lanka, contains 30 per cent crude protein in the dry matter. Trials have shown that STL (about 7 per cent of the total dry matter intake) can be satisfactorily used as a nitrogen supplement for NaOH-treated rice straw (Jayasuriya, 1979, unpublished data). It appears to be just as good as urea as a source of nitrogen.

High levels of concentrate - levels greater than 30 per cent - should not accompany alkali-treated straw diets. Such levels appear to lower the digestibility of the treated straw, and therefore no benefit can be derived from treating straw used such diets.

The pH of treated straw is generally around 10, and every 1 per cent of the alkali used in the treatment increases the sodium content of the straw by about 0.6 percentage unit on a dry-matter basis. While the animal's body does have mechanisms to deal with high-sodium and high-pH feeds, an intake of high levels of alkali-treated material could lead to a certain degree of physiological stress. At lower levels of treatment (4 per cent and below) the stress is more or less avoided.

Extra sodium ingested is almost entirely excreted in the urine, and blood serum sodium levels are not increased. Milk composition is not affected. Animals often drink more water and excrete larger volumes of urine, but no apparent ill effects have been recorded.

The economic feasibility of feeding alkali-treated straw

Cost analyses have been done on data from many feeding trials conducted in various countries to assess the feasibility of feeding alkali-treated straws to ruminants. It is claimed that in Europe substitution of treated straw for hay or silage may be profitable, especially if the straw has no value other than the cost of collecting it from the field and treating it on the farm.

Trials in India have shown that feeding alkali-treated straw can result in substantial gain in terms of feed cost per kilogram gain in live weight and early maturity of animals (table 4).

TABLE 4. The Effect of Dry Treatment of Wheat Straw on the Performance of Dairy Heifers Fed a Wheat Straw-Berseem Diet

Source: Naik and Singh, 1977, cited in Jackson (5).
a. Spray-treated with 9 kg NaOH/100 kg straw.

In Sri Lanka, because of the high cost of NaOH, feeding treated straw under normal conditions may not be profitable. However, during periods of scarcity of good-quality fodder, treated straw could be of considerable value, especially in saving animals from starvation and death.

It is now felt that the rising cost of NaOH may curtail the use of NaOH-treated straw in ruminant diets in many farm situations. Time is now being devoted to a search for cheaper methods of treatment. Ensiling straw with calcium hydroxide and the use of ammonia released by urea have given promising results under experimental conditions. More information is required before these methods can be applied in the field.

Another hindrance in the use of treated straw, as with any new product, is the lack of sufficient information, in physical and economic terms, on the benefit of alkali treatment under real farming conditions at the village level. Animal husbandry in South Asia is more of a family affair in the village; very few large-scale farms are found in these countries. Therefore, the use of treated straw has to be demonstrated with the farmer's animals. Although the economics are favourable, alkali treatment of straw has not been adopted by many farmers in India because it has not been sufficiently demonstrated on farms to convince farmers of its usefulness.

Present use of fibrous residues in India and Sri Lanka

India. The livestock industry in India is mainly geared to produce milk and farm power. Because of the present export policy and high population density, only about 7 per cent of the total cultivated land is devoted to forage crops. Extraction and milling offals are limited in relation to the number of bovine animals. Of the 20 million tons of offal produced annually, about 2 million tons are exported. Thus, the availability of concentrate feed per animal is in the order of 0.2 kg/day. Statistics show that, excluding grazing, dry roughage - mainly wheat and rice straws - constitutes over 50 per cent of the total forage fed to cattle and buffaloes in India (10). This trend will have to continue as higher inputs of high-quality feeds such as forage and concentrate will not be available for future improvement in livestock production. Suitable treatment of straws and other fibrous residues supplemented with nitrogen and minerals will no doubt play an important role in the future of the animal industry in India.

Sri Lanka. Approximately 2 million tons of rice straw are produced annually in Sri Lanka as a by-product of the grain industry. Very little is currently used in the feeding of livestock. Much of the straw is not harvested but is ploughed in or burned directly on the field. Most of the harvested straw (but only 2 to 3 per cent of the total) produced in the northern, central, and southern parts of the island is used in the paper industry. At present it appears to be more profitable to supply the paper industry than to feed livestock, especially when green fodder is available in abundance during rainy seasons. However, with the completion of the diversion scheme of the largest river in the country within the next few years, many uncultivated areas of the country will come under the plough for rice production. This will increase straw production many-fold. Thus, there is a tremendous potential for upgraded straw as a livestock feed in Sri Lanka in the future.

References

1. M.G. Jackson, "The Alkali Treatment of Straws" (review article), Animal Feed Sci. Technol., 2: 105 (1977).

2. W.J. Clawson, W.N. Garrett, and S. Richards, Microbial Utilization of Straw: A Review (California Agric. Ext. Serv. Publ. MA-1, 1970).

3, M.L. Chaturvedi, U.B. Singh, and S.K. Ranjhan, "Effect of Feeding Water-Soaked and Dry Wheat Straw on Feed Intake, Digestibility of Nutrients, VFA Production in Growing Zebu and Buffalo Calves,"J. Agric. Sci., 80: 393 (1973).

4. S.D. Sharma, "A Study of Roughage Silica Solubility" lM.Sc. thesis, G.B, Pant University, Pantnagar, India, 1974).

5, M.G. Jackson, Treating Straw for Animal Feeding, Animal Production and Health Paper No. 10 (FAO, Rome, 1978).

6. R,K. Wilson and W.J. Pigden, "Effect of NaOH Treatment on the Utilization of Wheat Straw and Poplar Wood by Rumen Microorganisma," Canad. J. Animal Sci., 44: 122(1964).

7. M.C.N. Jayasuriya, "Sodium Hydroxide Treatment of Rice Straw to Improve Its Nutritive Value for Ruminants," Trop. Agric. (Trinidad), 56: 75 (1979).

8. M.C.N. Jayasuriya, "Alkali Treatment of Paddy Straw: Effect of Energy and NPN Supplementation on Digestibility and Intake by Sheep," J. Nat Sci. Council, Sri Lanka, 1 980.

9. M.C.N. Jayasuriya, "Urea as a Source of NPN for Ruminants: 11. Effect of Urea as a Source of NPN on Digestibility and Voluntary Intake of Sheep Fed Alkali-Treated Rice Straw," J. Nat. Sci. Council, Sri Lanka, 1980.

10. V.N. Amble, V.V.R. Murthy, K.V, Sathe, and B.B.P.S. Goel, "Milk Production in Bovines in India and Their Feed Availability," Indian J. Vet Sci. Animal Husb., 35: 221 (1965).

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