W.J. Pigden* and F. Bender**
Millions of tons of lignocellulosic material are going to waste every year. Much of it collects as unavoidable by-products around industrial sites such as sugar mills and sawmills, or comes from cereal production. Additional millions of tons could be made available from the world's forests, which are in areas generally considered unsuitable for agricultural crops.
Table 1. Typical lignocellulose content of some plant materials
|Plant material||Hemicellulose||Cellulose||Lignin||Total lignocellulose|
|Orchard grass 1 (medium maturity)||40.0||32.0||4.7||76.7|
|Rye straw 2||27.2||34.0||14.2||75.4|
|Birch wood 2||25.7||40.0||15.7||81.4|
1 Van Soest, 1964.
2 Solo, 1965.
Most of these carbohydrates which are high in lignocellulose have low digestibility for ruminants, low nitrogen (N) content and, consequently, low animal production potential. Simple inexpensive treatments which increase lignocellulose breakdown in the rumen, and supplements of nonprotein nitrogen could contribute significantly to increased animal production.
* Research Branch, Department of Agriculture,
** Formerly Research Scientist, Department of Forestry and Fisheries, Ottawa, Canada.
The combined cellulose/hemicellulose content of grasses, straws, hardwoods and bagasse ranges from 60 to 75 percent and the lignin content from 5 to 25 percent. From a nutritional point of view lignocellulose consists of three fractions: the lignin which is essentially unavailable to the rumen microflora, the digestible energy (DE) fraction which is readily available, and the potentially digestible energy (PDE) fraction which is very resistant to bacterial attack but which can be made available by special processing. If this can be done, it will also allow a much more effective use of nonprotein nitrogen (NPN) supplements to ruminant rations. Also, where readily available energy is lacking lignocellulose can be transformed to the status of a supplement by appropriate processing.
The limiting factors for cellulose breakdown are lignification, particle size (bulk) and the levels of nitrogen and minerals. (Lignification means the complex of factors which inhibit further lignocellulose breakdown when other limiting factors are absent.) Table 1 gives the typical lignocellulose content of some plant materials.
Table 2. Digestion ceiling of lignocellulosic material
|Experimental material||In vitro percentage dry matter digestibility at various time intervals (hours)|
|Wheat straw (alkali treated)||30.0||45.0||-||67.0||-|
|Ground lodgepole pine wood||6.0||6.0||6.0||6.0||6.0|
|Ground aspen wood||3.0||5.0||12.0||19.0||23.0|
|Bleached spruce sulphite pulp||8.0||31.5||72.0||91.0||100.0|
Each lignocellulose feed has its own “digestion ceiling” where the rate at which it is used by the rumen microflora slows down so that essentially no further energy is available to the animal. This is illustrated by Pigden and Heaney (1969) as follows (Table 2).
The in vitro values in Table 2 represent the maximum DE available and would be modified somewhat in vivo by the rate of passage through the reticulo-rumen. The data show:
Lignocellulose of low N content requires N supplementation proportionate to its digestion ceiling. In addition, reducing the bulk by grinding increases the N requirement.
Figure 1. In vitro digestibility of steamed aspen wood
Bulk factor and particle size
Above about 65 percent digestibility, bulk no longer controls the intake of forages. Below 65 percent bulk becomes a major but variable factor which can be modified by mechanical comminution. Grinding a lignocellulose:
While grinding may markedly reduce the time during which feed particles are exposed to rumen microflora and thereby lower the digestion ceiling, this loss can be more than compensated by the increase in the daily DE intake as shown by Heaney et al. (1963) in Table 3.
Table 3. The effect on digestibility of grinding lignocellulosic materials
|Physical form of timothy grass||Stage of maturity|
|Digestibility of crude fibre|
|Ground and pelleted||54||31|
|Intake of digestible energy|
|Ground and pelleted||250||175|
Grinding is thus an effective way to upgrade low quality forages as it allows the animal to consume nearly as much DE as with higher quality, unprocessed forage. Since most of these materials are low in crude protein, N frequently becomes the factor limiting the effectiveness of ground materials.
Nitrogen availability and requirements for lignocellulose digestion
The availability of N for the breakdown of lignocellulose in the rumen is affected by both feed and physiological factors. Nitrogen is immediately available to the microflora from the feed and also in the form of urea which is recycled via the parotid saliva, and from the blood through the rumen wall. Thus, over short feeding periods of a few days the ruminant can compensate, to a degree, for lack of feed N and maintain N levels in the rumen adequate for lignocellulose breakdown (Clarke and Quinn, 1951; Minson, 1967).
The minimum N requirement of roughages fed as the sole diet appears to range from 0.6 to 0.8 percent. Small additions of urea to such roughages result in greatly accelerated cellulose breakdown, increase in dry matter intake and an increase in the digestion coefficient for crude fibre (Burroughs, 1950; Campling, 1962; Minson and Milford, 1968). In cases where low N forages, such as wheat straw and oat straw, do not respond to N supplementation, lignification is likely to be the limiting factor (Minson and Pigden, 1961; Head, 1953; Louw and Van der Wath, 1943; Williams, 1959).
Recently Donefer (1969) showed that raising the digestion ceiling of straw with caustic soda (NaOH) treatment did not increase intake unless it was also supplemented with N. Supplementation with 2.5 percent urea led to greatly increased DE intake, thus showing that N became the second limiting factor. Grinding and pelleting also increase the N requirement (Milford and Minson, 1968). One percent N appears adequate for cellulose digestion in the reticulo-rumen for materials up to 50 percent DE; for higher levels the N requirement may be increased to 1.5 percent. Starch or sugar supplements may raise it to 2 percent.
Materials with inadequate mineral contents such as wood, straw and corncobs require supplementation with macro- and micronutrients. In particular sulphur (Moir, 1968) and phosphorus could become limiting factors in the use of NPN.
Processing of lignocellulose
In the early work, alkalis and acids were used to improve the digestibility of forages. These treatments related to pulping were costly and in many cases wasteful (due to loss of hemicelluloses). Soaking with subsequent washing (Beckmann process) is impractical in most instances. “Dry” processes using NaOH, such as proposed by Wilson and Pigden (1964), increase the dry matter digestibility of straws and aspen. Improved digestibility (Tarkow and Feist, 1969) is probably due to the saponification of uronic acid esters associated with the zylan chains. By increasing the lignocelluloses' capacity to swell, alkali treatments enhance diffusion and make the enzymes more accessible (Smith et al., 1970). Utilization of urea N by the rumen microflora for the synthesis of cellular protein, with NaOH-treated fibrous polysaccharides as the energy source, has been demonstrated by Lampila (1963) and Donefer et al. (1969).
Table 4. The effect of steam processing on the digestibility of lignocellulose
|Lignocellulosic material||Digestion ceiling|
High velocity electrons and gamma rays, although capable of increasing digestibility, require economically prohibitive quantities of energy.
Much work has been done in the past on the steam treatment of lignocellulose with widely varying degrees of success. Recently Bender et al. (1970) with more systematic studies established practical conditions of pressure and steaming periods under which in vitro digestibilities of low grade forages and deciduous woods could be considerably increased, as shown in Table 4.
By using ammonia to neutralize the organic acids set free by the steam hydrolysis, these workers added 1-1.5 percent N to steamed poplar, mainly as ammonium acetate. So far coniferous woods have not responded to steam treatments. The economics of steaming followed by ammonia injection appear favourable. Good results have been obtained in feeding trials with steamed aspen.
Utilization of untreated wood by ruminants
Chipped and ground poplar, ensiled (i.e., stored in the wet state) and supplemented with minerals, urea and a small amount of energy (molasses), can serve as an emergency ration for cows and sheep (Melfort Research Reports, 1967 and 1968; Enzman et al., 1969). Ground mesquite, mixed with molasses and concentrates, has given satisfactory results with beef cow herds in Texas (Marion et al., 1967). Untreated wood, to be of use in ruminant feeds, must be of relatively small particle size and supplemented with N, minerals and some (readily available) energy. If a low cost processing technique can increase further the PDE fraction this can favourably influence wood utilization in maintenance rations for beef cattle.
Figure 2. Piles of waste bagasse outside a sugar mill
Since large quantities of bagasse are potentially available at sugar mills, bagasse deserves special attention. Traditionally most bagasse has been burned in old inefficient boilers to operate the mills. Modern oil-fired boilers would be of greater economic value and would release large quantities of bagasse for feed. Where bagasse is being used as a fibre source, the pith has to be removed. This fraction — about one third of the total bagasse — is nutritionally valuable, due to its small particle size, and has been successfully fed to cattle in combination with molasses. Both alkali and steam treatments can be used to increase bagasse and pith digestibility (Stone et al., 1965a, 1965b; Bender and Heaney, private communication).
Suggestions for further research
The intake of DE from low quality forages and wood is limited by lignification, the bulk factor, and levels of nitrogen and minerals. The effectiveness of these materials as feeds can be improved by grinding, alkali treatment, steaming and N supplementation.
Grinding minimizes the bulk factor and increases the rate of fermentation, rate of passage through the rumen and DE intake; but N supplementation is required. Alkali (3–6 percent dry matter basis) or steaming which can increase digestibility and intake, with no subsequent washing, are considered optimum. Ammonia is the preferred alkali.
Figure 3. Sheep on digestibility trials with steamed poplar wood
Increased N requirements, caused by grinding, alkali and steam treatments, can be met by urea or other NPN supplementation. The requirements for N go up to 1 percent by grinding and to 1.5–1.75 percent by chemical and steam processing. If readily available carbohydrate constitutes 20 percent of the diet, N requirement increases to 2 percent. Some of the N should be in the form of urea or other readily available sources of ammonia. The optimum NPN supplementation for low lignocellulose feeds may be determined by establishing in vitro digestion ceilings and relating these to the N content of the feed in question.
Lignocellulosic materials such as bagasse and sawmill residues are concentrated in areas of established industrial activity and offer the best prospects for significantly increasing the supply of DE. Cereal straws and corn stover are potentially available at low cost in sufficient volume. Forages such as elephant grass and pangola grass could be harvested from small acreages. There appears to be little possibility for grazed forages.
The economics of all the treatments depend on availability, composition and concentration of the raw materials, cost of steam, labour and chemicals, type of animal production required, levels of DE in the original feed and susceptibility to treatments.
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