by
Prof. Heinrich Scholler
Munich, Federal Republic of Germany
| Summary | Résumé |
| During the recent past, hydrolysis processes have been developed for the production of chemical and pharmaceutical products from the Ligno-cellulose fraction of annual and perennial plants. The main products obtained through these processes are xylite, alcohol, fermentation yeast, organic acids of the oxy-range and finally furfurol. Xylite can efficiently replace dietary cane sugar or beet sugar (saccharoses), reducing the likelihood of coronary heart disease. It also is recognized as a successful sweetener in diets for diabetics. Another end product from Ligno-cellulose hydrolysis is lignin, which also may be used by the pharmaco-chemical industry, for instance in products against diarrhoea. The scholler process, which is being used throughout Europe (including the U.S.S.R.) has a dominant position among the hydrolysis processes presently available. Some of its important attributes are low capital investment, high productivity, high purity standards of the products and relatively few problems with regard to construction and environmental pollution. For hydrolysis the following raw materials may be considered: sawdust, all timber from tropical rain forests, low-quality roughages such as cereal straws, bagasse, stover from maize and sorghum, cotton stalks and hulls, groundnut haulms, sunflower hulls, olive-pulp, esparto, reed, beechnut shells, etc. Depending on the number of phases, the hydrolysis process yields residues of up to 75% of the original material that are suitable as animal feed. The main products are sugar molasses containing xylose and dextroses, waste liquor of xylite crystallization, and ligno-cellulose. Wood sugar molasses and other crystallization residues contain some 55% of soluble carbohydrates. Their calcium content is significantly higher than that of sugarbeet molasses, but its sodium content considerably lower. Digestibility coefficients of the nitrogen-free extract (NFE) fraction were 89 for sheep and 77 for pigs. In a feeding experiment with pigs, no significant differences in performance were observed between animals on a ration containing wood molasses and waste liquors and the control group receiving sugarbeet molasses. Guantitatively, ligno-cellulose represents by far the largest proportion of the by-products obtained, some 60 to 75% of the original material. Compared to untreated material its value as feed for ruminants is greatly enhanced through the hydrolysis required for production of xylose in general, and even more so following hydrolysis by the Scholler process. Experiments are under way for the production of bacterial protein from a mixture of hydrolysed ligno-cellulose and animal excrements. Continuing experiments are expected to demonstrate the usefulness of creating an integrated system for industrial and animal agricultural production. In experiments with pigs and chickens, lignin was included in diets at a rate of 2 to 8% and produced remarkable antibiotic effects. Absorption of nutrients and protection against infection were greatly improved and the incidence of digestive disorders decreased. It is concluded that the addition of some 2% of lignin to diets of young animals reduces the mortality rate by 30 to 50%, has a marked sparing effect on requirements for antibiotics and improves daily liveweight gains. | On a récemment mis au point des méthodes de fabrication de produits chimiques et pharmaceutiques par hydrolyse de la fraction lingo-cellulosique de plantes annuelles et persistantes. Les principaux produits ainsi obtenus sont les suivants: xylite, alcool, levure, oxacides organiques et furfurol. La xylite peut remplacer avantageusement le sucre de canne ou de betterave de régime (saccharose) en réduisant les risques de maladies des coronaires. Elle est aussi connue comme édulcorant efficace dans les régimes pour diabétiques. Un autre produit final de l'hydrolyse de la ligno-cellulose est la lignine, qui peut également être utilisée dans l'industrie chimico-pharmaceutique, par exemple pour fabriquer des antidiarrhéiques. Le procédé Scholler qui est utilisé dans toute l'Europe, y compris l'U.R.S.S., est la plus importante de toutes les méthodes d'hydrolyse actuellement employées. Parmi ses qualités majeures, on peut citer le faible investissement nécessaire, sa productivité élevée, un haut degré de pureté des produits et le fait qu'elle ne pose que des problèmes mineurs dans le domaine de la construction et de la pollution de l'environnement. Pour l'hydrolyse, on peut envisager d'utiliser les matières premières suivantes: la sciure, toutes les essences de forêts tropicales humides et les fourrages grossiers de qualité médiocre tels que les pailles de céréales, la bagasse, les tiges de maïs et de sorgho, les tiges et les capsules de cotonnier, les fanes d'arachide, les téguments des graines de tournesol, les grignons d'olives, le sparte, les roseaux, les coques de faines, etc. Selon le nombre de ses phases, l'hydrolyse peut produire jusqu'à concurrence de 75 pour cent de résidus à partir des matériaux originels qui conviennent pour l'alimentation animale. Les principaux produits sont les mélasses qui contiennent du xylose et des dextroses, la solution résiduaire de la cristallisation de la xylite, et la ligno-cellulose. Les mélasses de bois et autres résidus de cristallisation contiennent quelque 55 pour cent de glucides solubles. Leur teneur en calcium est nettement plus élevée que celle des mélasses de betteraves, mais leur teneur en sodium beaucoup plus faible. Les coefficients de digestibilité de l'extractif non azoté étaient de 89 pour les ovins et de 77 pour les porcs. Une expérience d'alimentation effectuée sur les porcs n'a révélé aucune différence de performances sensible entre les animaux qui recevaient des rations contenant des mélasses de bois et des solutions résiduaires et le groupe témoin dont l'alimentation comprenait de la mélasse de betteraves. La ligno-cellulose est de loin le plus abondant (60–75 pour cent) des sous-produits tirés du matériau de départ. Sa valeur pour l'alimentation des ruminants est, par rapport au matériau non traité, considérablement accrue par l'hydrolyse nécessaire à la production de xylose en général, et plus encore par l'hydrolyse selon la méthode de Scholler. On a entrepris des expériences en vue de produire des protéines bactériennes à partir d'un mélange de ligno-cellulose hydrolysée d'excréments animaux, et l'on espère qu'elles prouveront l'utilité de la création d'un système intégré pour la production agricole animale et industrielle. Dans certaines expériences effectuées sur des porcs et des poulets, on a introduit 2 à 8 pour cent de lignine dans l'alimentation et obtenu des effets antibiotiques remarquables. L'absorption de nutriments et la protection contre les infections sont considérablement accrues et les risques de troubles digestifs diminués. D'où on en conclut que l'addition de 2 pour cent environ de lignine dans le régime des jeunes animaux réduit le taux de mortalité de quelque 30 à 50 pour cent, permet de réduire sensiblement les besoins en antibiotiques et améliore le gain quotidien en poids vif. |
Resumen
En los últimos tiempos se han utilizado procedimientos de hidrólisis para fabricación de productos quimicos y farmacéuticos a partir de la fracción ligno-celulósica de plantas anuales y perennes. Los productos principales obtenidos mediante estos procedimientos son xilita, alcohol, levadura de fermentación, oxi-ácidos orgánicos y, por último, furfurol. La xilita puede suplir con eficacia la alimentación con azúcar de cańa o azúcar de remolacha (sacarosa), reduciendo asi la probabilidad de la incidencia de enfermedades coronaries. Tiene también valor reconocido como edulcorante en dietas para diabéticos.
Otro producto final de hidrólisis de ligno-celulosa es la lignina, que puede utilizarse también en la industria farmaco-química, por ejemplo, en productos contra la diarrea. El proceso Scholler que se utiliza en toda Europa, incluida la URSS, es el más empleado de los procesos de hidrólisis actualmente disponibles. Algunos de sus méritos más importante son el exigir poco capital, la elevada productividad, el alto nivel de pureza de los productos y el hecho de que plantea problemas relativamente pequeños en lo que respecta a la construcción y a la contaminación ambiental.
Para la hidrólisis pueden servir las siguientes materias primas: aserrín, toda clase de madera de bosques higrofíticos tropicales, forrajes bastos de mala calidad como pajas de cereales, bagazo, tallos y hojas de maíz y sorgo, tallos y cáscaras de algodón, planta de maní, cáscaras de semillas de girasol, pulpa de aceituna, esparto, carrizo común, cáscaras de hayuco, etc.
Según el número de fases, el proceso de hidrólisis llega a obtener hasta un 75 por ciento de la materia prima para poder utilizarlo como pienso. Los productos principales son azúcarmelazas, que contienen xilosas y dextrosas, jarabe residual de cristalización de xilita y ligno-celulosa.
Las melazas de azúcar de madera y otros residuos de cristalización contiene un 55 por ciento de carbohidratos solubles. Su contenido de calcio es notablemente superior al de las melazas de remolacha azucarera, pero su contenido de sodio es considerablemente inferior. Los coeficientes de digestibilidad de fracción de extracto libre de nitrógeno fueron 89 para el ganado ovino y 77 para el porcino. En un experimento de alimentación de cerdos no se observaron diferencias notables de rendimiento entre los animales, que recibían una ración que contenía melazas de madera y jarabes de residuos, y el grupo testigo al que se suministraban melazas de remolacha azucarera.
Cuantitativamente, la lignocelulosa representa con mucho la mayór proporción de los subproductos obtenidos, es decir, un 60–75 por ciento de la materia original. En comparación con el material sin tratar, su valor como pienso para rumiantes se refuerza mucho gracias a la hidrólisis en general necesaria para la producción de xilosa pero llega a reforzarse más, si se realiza la hidrólisis según Scholler.
Se están haciendo experimentos para producir proteína bacteriana de una mezcla de ligno-celulosa hidrolizada y excrementos de origen animal. Se espera que la continuación de los experimentos demuestre la utilidad de establecer un sistema integrado de producción industrial y agropecuaria.
En experimentos realizados con cerdos y pollos se incluyó en la alimentación una dosis del 2–8 por ciento de lignina, produciéndose notables efectos antibióticos. La absorción de nutrientes y la protección contra infecciones mejoraron mucho y disminuyó la incidencia de trastornos digestivos. Se concluyó que la adición de un dos por ciento de lignina a la alimentación de los animales jóvenes reduce el índice de mortalidad de 30 a 50 por ciento, contribuye a disminuir considerablemente las necesidades de antibióticos y mejora los aumentos diarios de peso.
by
W.J. Pigden
Research Co-ordinator
Planning and Evaluation Directorate
Research Branch
Agriculture Canada, Ottawa, Canada
| Summary | Résumé |
| Forest product wastes are primarily sources of energy, being high in carbohydrates and low in protein, minerals and vitamins. The major sources are slash from logging operations, tree foliage, solid and liquid effluents from pulp mills and fibre-board plants, stands of species not desired or under-utilized by the forest products industry and discarded paper products (newspaper, paperboard, etc.). The major component of these wastes is Ligno-cellulose, most of which requires external processing to allow the efficient extraction of the feed energy. The key to efficient exploitation of Ligno-cellulose is low-cost processing, which will allow the rumen microflora to extract this energy, combined with the unique ability of the ruminant to convert available cellulose to milk and meat when appropriate supplementation is used. A number of processing methods are under development. The most feasible and economical appears to be steaming at high temperatures and pressures. Until recently the method had been successful only for hardwoods, but recent data indicate that softwoods can also be successfully processed, thus tremendously expanding the potential use of the method. This method gives sufficiently high digestibility values and is sufficiently inexpensive to permit the processed product to enter feeding systems at moderate to high levels for milk and meat production. Only limited numbers of tree species, mainly temperate-zone hardwoods, have been tested. Many others, especially tropical species, require testing. The complete hydrolysis of ligno-cellulose to provide substrates for protein production and for direct use by non-ruminants is considered too expensive and energy-consuming for general use. Wood pulp is also too expensive. Accessibility of wastes for processing and feeding is an important economic factor, as the cost of long-distance transportation is generally prohibitive. Linked with this is the vital need to develop mobile or “on-farm” processing equipment as part of the system. Surveys to determine accessibility and amounts of waste are essential for the evaluation and development of these resources. Efficient use of non-protein nitrogen (NPN) is important in ruminant feeding systems utilizing forestry wastes. Gaseous ammonia has been successfully incorporated into steamed wood during processing and is effectively utilized by ruminants, presumably as ammonium acetate. Wood molasses can be substituted for cane molasses in ruminant rations and has unique qualities for improvement of NPN utilization. Many facets of NPN supplementation for forestry wastes remain unexplored. The development of new technology for whole-tree harvesting operations requires the evaluation of tree foliage as a feedstuff. Foliage has potential primarily in ruminant feeding systems. Pulping and fibreboard effluents can be employed to produce singlecell protein (SCP) such as Torula yeast and Pekilo protein. These proteins are suitable as protein supplements for rationing swine, poultry and ruminants. The technology of production and use has been well demonstrated. In areas where plant proteins thrive, cost of producing plant protein is much lower than for SCP, but because of problems of pollution, balance of payments, and protein scarcity, the use of effluents for SCP production is likely to increase rapidly. Research needs include further development of methods and equipment for processing ligno-cellulose: more information on supplementation, especially the use of NPN; surveys of true availability of supplies of forestry wastes and economic evaluation of waste utilization in feeding systems. | Les déchets des produits forestiers sont essentiellement des sources d'énergie étant donné leur taux élevé en hydrates de carbone et leur faible concentration en protéines, en minéraux et en
vitamines. Il s'agit surtout de déchets d'abattage, de feuillages, d'effluents solides et liquides des fabriques de pâte et de panneaux de fibres, de peuplements d'essences indésirables ou sous-utilisées par l'industrie des produits forestiers et de vieux papiers, tels que journaux, cartons, etc. La principale composante de ces déchets est la lignocellulose qui, généralement, nécessite un traitement mécanique rendant possible l'extraction efficace de l'énergie des aliments. La clé d'une exploitation rentable de la lignocellulose consiste en un traitement économique qui permettra á la microflore du rumen d'extraire cette énergie, grâce á l'aptitude unique des ruminants à convertir la cellulose assimilable en lait et en viande au moyen d'une supplémentation appropriée. Un certain nombre de méthodes de traitement sont á l'étude. La plus pratique et la plus économique semble être l'étuvage à des températures élevées et sous de fortes pressions. Jusqu'à ces temps derniers, la méthode n'a été fructueuse que pour les bois durs, mais les données recueillies récemment indiquent que les bois tendres peuvent également être traités avec profit, ce qui accroît considérablement l'utilisation potentielle du procédé. Cette méthode donne des taux de digestibilité suffisamment élevés et elle est assez peu coûteuse pour permettre aux produits traités d'entrer dans les systémes d'alimentation en concentrations tant modérées que fortes pour la production de lait et de viande. Les essais n'ont porté que sur un nombre limité d'essences, surtout des bois durs de la zone tempérée. Maintes autres essences, surtout tropicales, doivent être soumises à des tests. On estime que l'hydrolyse totale de la lignocellulose en vue de fournir des substrats pour la production protéique et pour l'utilisation directe par les non-ruminants est trop coûteuse et absorbe trop d'énergie pour convenir à l'usage général. La pâte de bois est également trop coûteuse. L'accessibilité des déchets aux fins de traitement et d'alimentation est un facteur économique important, étant donné que leur transport sur de longues distances est généralement d'un coût prohibitif. A ce facteur, il faut joindre la nécessité vitale de mettre au point un équipement mobile ou utilisable dans l'exploitation comme élément du systéme. Il importe d'effectuer des enquêtes pour déterminer l'accessibilité des déchets et les quantités dont on doit disposer pour évaluer les ressources et en tirer parti. L'utilisation efficace de l'azote non protéique est importante dans les systémes d'alimentation des ruminants qui tirent parti des déchets de l'industrie forestiére. L'ammoniac, incorporé en cours de traitement au bois, étuvé, est utilisé efficacement par les ruminants, sans doute sous forme d'acétate d'ammonium. L'on peut substituer à la mélasse de canne dans les rations la mélasse de bois, qui posséde des qualités uniques pour améliorer l'utilisation de l'azote non protéique. Maints aspects de la supplémentation des déchets de l'industrie forestière par cette substance restent à étudier. La mise au point d'une nouvelle technologie de l'exploitation par arbre entier nécessite l'évaluation du feuillage comme aliment des animaux. Il offre surtout des possibilités dans les systémes d'alimentation des ruminants. Les effluents de l'industrie de la pâte et des panneaux de fibres peuvent servir à produire des protéines monocellulaires telles celles de la levure Torula et du Pekilo. Ces protéines conviennent à la supplémentation des rations destinées aux porcins, aux volailles et aux ruminants. La technologie de leur production et de leur utilisation a été parfaitement démontrée. Dans les régions qui conviennent aux protéines végétables, leur coût de production est trés inférieur à celui des protéines monocellulaires, mais par suite de divers problèmes- pollution, balance des paiements, et pénurie de protéines - l'emploi des effluents pour la production de protéines monocellulaires fera vraisemblablement des progrés rapides. La recherche doit porter sur les secteurs suivants: mise au point de méthodes et d'équipements pour le traitement de la lignocellulose, recueil de renseignements relatifs à la supplémentation, notamment en ce qui concerne l'utilisation de l'azote non protéique, prospection des disponibilités réelles en déchets de l'industrie forestiére et évaluation économique de l'utilisation des déchets dans les systémes d'alimentation. |
Resumen
Los desperdicios de los productos forestales son esencialmente fuentes de energía, con un alto nivel de carbohidratos, y pobres en proteínas, minerales y vitaminas.
Las fuentes principales son la broza de las actividades de corta, el follaje de los árboles, los efluentes sólidos y líquidos de los molinos de pulpa y de las fábricas de tableros de fibras, masas de especies no empleadas o subutilizadas por la industria de productos forestales, y residuos de papel, por ejemplo, de periódico, cartones, etc.
El componente principal de estos desperdicios es la lignocelulosa, que en su mayor parte exige una elaboración externa que permita la extracción eficaz de la energía forrajera. El elemento esencial de la explotación de la lignocelulosa es un bajo costo de elaboración que permitirá que la microflora del rumen extraiga esta energía, junto con la capacidad especial del rumiante de convertir la celulosa “disponible” en leche y carne utilizando elementos complementarios adecuados.
Se está preparando cierto número de elaboración. El más factible y económico parece ser la vaporización a temperaturas y presiones elevadas. Hasta hace poco este método habia tenido éxito solamente para las maderas duras, pero los últimos datos indican que también pueden tratarse con éxito las maderas blandas, lo cual aumenta enormemente sus posibilidades de utilización. Este método permite una capacidad bastante alta de digestibilidad y es suficientemente barato como para permitir que el producto elaborado sea utilizado en los sistemas de alimentación animal en niveles moderados o altos, para la producción de leche y de carne. Hasta ahora sólo se ha ensayado un número limitado de especies de árboles, en especial maderas duras de la zona templada. Muchas otras, especialmente las especies tropicales, necesitan nuevos ensayos.
La hidrólisis total de la lignocelulosa para obtener sustratos para la producción de proteínas y para su uso directo por animales no rumiantes se considera muy costosa y exige demasiada energía como para poder utilizarla generalmente. La pulpa de madera es también demasiado costosa.
Las posibilidades de acceso a los desperdicios para su elaboración y su utilización como piensos constituye un factor económico importante, ya que el transporte a grandes distancias es, en general, prohibitivo. Otro factor afín es la necesidad vital de desarrollar, como parte del sistema, un equipo de elaboración móvil o de elaboración “en la granja”. Es esencial efectuar investigaciones para determinar las posibilidades de acceso y las cantidades de desperdicio a fin de poder hacer una evaluación eficaz y explotar estos recursos.
El empleo eficiente del nitrógeno no proteico es importante en los sistemas de alimentación de rumiantes en los que se utilizan los desperdicios forestales. Se ha tenido éxito en la incorporación de amoníaco gaseoso en la madera vaporizada durante el proceso de elaboración, y los rumiantes lo aprovechan de manera eficaz, probablemente como acetato de amonio. En la alimentación de los rumiantes la melaza de madera puede sustituir a la melaza de caña de azúcar, y tiene cualidades excelentes para mejorar la utilización del nitrógeno no proteico. Todavía no se han estudiado muchas facetas de la complementación de Los desperdicios forestales con este nitrógeno.
El desarrollo de una nueva tecnología para las actividades de exploitación forestal exige la evaluación de las calidades del follaje como alimento para los animales. El follaje ofrece posibilidades sobre todo en los sistemas de alimentación de rumiantes.
Se pueden emplear efluentes de los molinos de pulpa y de las fábricas de tableros de fibra para producir proteínas unicelulares tales como las de levadura Torula y Pekilo. Estas proteínas sirven de complemento proteico para alimentación del ganado porcino, de las aves de corral y los rumiantes. La tecnología de su producción y de su utilización está muy desarrollada. En las zonas donde abundan las proteínas vegetales, el costo de la producción de estas proteínas es muy inferior a la de la producción de proteínas unicelulares, pero debido a problemas de contaminación, balanza de pagos, y escasez de proteínas, probablemente aumentará rápidamente el empleo de efluentes para la producción de proteínas unicelulares.
Entre las investigaciones que es necesario efectuar podemos citar las siguientes: perfeccionamiento de los métodos y del equipo para la elaboración de la ligno-celulosa; más información sobre factores complementarios, especialmente la utilización de nitrógeno no proteico; estudios sobre la disponibilidad efectiva de suministros de desperdicios forestales y una evaluación económica de la utilización de los desperdicios en los sistemas forrajeros.
Introduction
Feed energy is by far the largest and most expensive feed ingredient for animal production. Forest product wastes, with minor exceptions, are high-energy products, high in carbohydrates but low in proteins, fat, minerals and vitamins.
Figure 16.1 shows that enormous amounts of highly lignified wood waste are available for feed if economical processing methods can be found.
The major sources of waste are under- or non-utilized species and the tops, foliage and stumps in wood harvesting operations; solid and liquid wastes from wood manufacturing industries (sawmills, pulp mills and fibreboard plants); discarded paper and paperboard products (newspapers, cardboard, etc.).
In harvesting operations 40 to 50% of the wood of the species utilized is left as tops, foliage, stumps, etc., in addition to about an equal amount of non-utilized species. In the developing countries, especially the tropics, the percentage left behind is much higher because of the emphasis on selective exploitation and export of logs from high-value species.
Most wood processing industries are located in the developed countries (Virkola, 1975), so that most of the mill waste products are also produced in those areas. (Hence, of necessity, most of this paper is concerned with wastes in developed countries.) Processing losses are very high: 30 to 70% for chemical pulps, 40 to 60% from wood manufacturing and dressing operations, some of which is recycled. However, total wood harvesting and processing losses are enormous, representing a much higher percentage of the total than the percentage utilized.
Species of tree has an important bearing on the quality and cost of feed produced from the waste and has important regional implications. Some hardwood species, such as aspen, are relatively easily converted to good feed. Softwoods are much more difficult and costly to process, and technology for this purpose is less advanced. Testing of tropical species has hardly been touched. In northern countries such as Canada, about 80% of the trees are softwoods; in warm temperate zones the percentage is much higher; in the tropics most forest species are hardwoods. The very large number of species in the tropics appears to present a more complicated problem of utilization than in temperate zones.
A serious economic problem is that much of the wood waste is generated in areas remote from animal feeding operations, and transportation costs dominate the economics of utilization. Conversely, these wastes represent opportunities to develop animal production in areas where feedstuffs are scarce and animal products are imported.
A major advantage of feeds derived from forest sources is continuity of supply, in that annual yields of forest products are not affected by drought, pests, etc., as are agricultural crops.
Products such as pulp, lumber and plywood are of much higher value than animal feed, so that feed cannot compete directly for wood supplies where the wood is needed for conventional products. However, because of recent high feed prices and anticipated feed shortages, increased energy costs, new methods of processing wood for feed, development of methods of wood processing to obtain high-value products (e.g. xylitol, furfural) with associated by-products for feed and new environmental regulations, a strong new interest has developed in further evaluation of forest waste products in feeding systems.
Partially Hydrolysed Wood
The partial hydrolysis of wood is one of the most promising processing techniques. The objective is to increase the accessibility of the complex carbohydrates to the rumen microflora, increase palatability and intake and provide a milieu favourable to non-protein nitrogen (NPN) utilization. There are basically two approaches: to aim at producing a feed at the lowest possible cost with certain by-products as possible cost offsets, or high-value products such as xylitol and furfural with processed wood as a feed by-product.
Stake Process (Steam + pressure)
This is essentially a limited hydrolysis and physical modification process carried out with direct steam at high pressure, utilizing in part the wood acetyl groups to break up the ligno-cellulose complex. It evolved from the original Bender et al. (1968) steaming process, developed in Canada, which has been further developed and is being commercialized by Stake Technology Ltd. The raw wood (chips or sawdust) is processed through continuous-flow equipment at high temperatures and pressures, yielding a brown, dispersed fibre product containing about 50% moisture and 1.5 to 2% crude protein; the product is about 50 to 65% digestible, depending on species of wood. It is highly palatable, the rate of rumen digestion is rapid, and intakes in rations containing up to 80% are high. It is very similar to haylage or silage in physical characteristics and can be handled in mechanical systems and fed in the same way as silage. It is slightly acid and does not appear to mould or deteriorate on standing. There has been no indication that any toxic or deleterious substances are produced. The method is applicable only to hardwoods, but progress has been made in adapting it for softwoods.
Hexagons indicate major potential sources
Circles indicate major feedstuff sources
Percentages indicate proportions of total forest products as wastes in a developed country.
Figure 16.1. Major forest waste products and their feed potential.
It has been demonstrated under experimental and commercial conditions that, adequately supplemented, the Stake feed can be used at high levels for winter maintenance for beef cows, at the 50% level of DM intake in feedlots (1125 g/day gain) and in general as a replacement for medium-quality hay or silage. Carcass quality of beef cattle has been found to be satisfactory.
This material is highly compatible with NPN and can be enriched with gaseous ammonia during processing to produce ammonium acetate, a safe source of NPN, which is readily utilized by rumen microorganisms; otherwise, urea can be fed.
The cost of processing is estimated at $10 to $15/ton DM. The cost of the raw material ranges from zero to $45/ton depending on source and competition, but large quantities are available at less than $20/ton. The cost of supplementation for protein, minerals and vitamins is estimated at $4/ton DM. Thus, from a cost viewpoint the product can enter into a large number of rations in competition with classical feeds, depending on cost of raw material, transportation and processing. It should be emphasized however that feed cannot compete directly with high-value forest products, and that raw materials for feed production are generally available at prices far below those paid for pulp or other forest products.
The overall economic cost of such a processed-wood feed product is determined more by the cost of collecting and delivering the raw materials and the processed product than by the processing cost. Since this process is continuous-flow, the equipment is a small, mobile unit which can be easily moved from place to place, thus eliminating a large part of transportation costs, a major advantage. Power requirements can be met by normal electrical supplies or petrol-fuelled power units.
Development has progressed to the point where the product has been demonstrated to have excellent potential in several experimental studies, some field trials have been successfully completed and large-scale commercial tests are being developed.
Low-acid Hydrolysis
A number of experimenters have tested low-acid (1% or less H2SO4) hydrolysis of wood under pressure to produce a fibrous wood product which is highly palatable to ruminants and varies in digestibility from 32% (Butterbough and Johnson, 1974) to around 60%. Two of these processes show promise.
The Jelks process is an acid hydrolysis of wood (under pressure) assisted by oxidation and some catalysis. The product is a degraded poly-dispersed fibre which differs from the Stake product in that it contains many more lower molecular compounds. The process was developed in the USA and is being developed by the East Perry Lumber Co., Frohna, Missouri. It is claimed that this product is a very satisfactory feed for ruminant production, can be fed at high levels if adequately supplemented, and is very palatable. It seems probable that the product is batch-produced, requiring large-scale non-mobile equipment. Most of the information on economics of processing is a trade secret, but an educated guess would put it at about twice that of the Stake process.
Claims have been made that the material can be used for non-ruminant feeding.
Insufficient information is available for an adequate evaluation of the nutritional quality and the economics of production of feed by this process, but it appears to have commercial possibilities. A number of large-scale feeding projects are apparently under way in the US. From a nutritional viewpoint it seems evident that this material can enter a large number of ruminant production rations. From a cost viewpoint there is less certainty, depending to a large extent on the cost of other feeds.
The Groves process is an acid digestion process, in which low levels of acid (1% or less of H2SO4) are added to softwood that is then cooked under pressure for about 15 minutes to produce a dark, moist end-product, smelling like molasses. The product appears to be similar to that from the Jelks process. This feed is highly palatable and can certainly supply a major part of the maintenance requirement for beef cattle, as shown in tests at Kamloops Research Station, B.C. It is claimed that over 900 g gain/day were obtained in trials with beef heifers where acid-hydrolysed sawdust replaced the hay in feedlot rations (Bates, 1975). There were no indications of any toxic problems.
Cost of this treatment is estimated at $15 to 25/ton, but this is a preliminary estimate.
This process is of special interest because it is oriented toward softwoods, which are very difficult to transform into a low-cost feed. Development has progressed to the point of evaluating the product at research stations and in field trials.
Alkali Hydrolysis
Studies by Wilson and Pigden (1964) showed that the treatment of poplar (aspen) wood (Populus tremuloides) with 6% NaOH produced a five-fold increase in in vitro digestible dry matter, to a level comparable with average hay.
In in vivo studies with goats, Mellenburger et al, (1971) concluded that alkali-treated aspen had a dry-matter digestibility coefficient of 52% and could replace roughage of average quality. While the “dry” alkali treatment has been commercialized for straw, it does not appear to have developed beyond the experimental stages for wood.
Wood pulps
Wood is converted to pulp by one of three main processes. The first is a mechanical process to produce fibre for lowgrade pulp, used extensively for newspapers, without removing the lignin. The pulp has the same digestibility as the original material, i.e. zero or slightly above. The other two pulps are chemically processed by cooking the wood chips with either alkali (kraft process) or acid (sulfite process); this dissolves the lignin, hemicellulose and other organics and liberates the cellulose fibres. These pulps are highly digestible by ruminants.
The spent cooking liquors contain 35 to 65% of the total wood in a dilute solution of about 10% solids, depending on the process. Some of the liquors are concentrated and burned to recover chemicals and for heat energy, but large quantities of sulfite liquor are dumped into water-ways. In Europe, sulfite pulp-mill liquors are utilized for production of feed, energy or chemical recovery. In North America, and especially in Canada, there are many sulfite pulp mills without recovery systems, 36 in Canada alone. In this respect, Canada is clearly an underdeveloped country, since it is estimated 500 000 tons of SCP could be produced annually from these sulfite effluents (Stone, 1975).
It was established many years ago (Hividsten, 1940; Hividsten & Homb, 1947), and confirmed many times since (Saarinen et al., 1959), that chemical pulp is an excellent energy source for ruminants, with a generally high digestibility and a very low protein, mineral and vitamin content. Properly supplemented with crude protein, minerals and vitamins, it served in Europe during two world wars as a commercial production ration, alone or mixed with forage, to produce milk and meat. However, since chemical pulp is normally worth $200 to 350/ton and comparable feeds cost well under $100/ton, the economics of use are very poor indeed, except in emergencies.
Pulp mills generate a waste fibre product, clarifier sludge, containing very fine and broken fibres that escape the recovery systems. This is quite digestible (40 to 65%) and has received considerable attention. More efficient recovery systems are reducing losses, so future supplies are likely to be much lower (Elgee, 1975).
Waste Paper and Paperboard
These products originate from chemical pulps that are highly digestible by ruminants, mechanical pulps that are poorly digested, and semi-chemical pulps of intermediate digestibility.
Printing and writing paper and paperboard are the two largest waste materials in developed countries, and only about 20% are recycled back into paper and paper-board products (see Figure 16.1). Annual USA production is as follows:
| Waste | Millions of tons |
| Printing and | |
| writing paper | 21 |
| Packaging paper | 6 |
| Paperboard | 26 |
Source: American Paper Inst, (1971)
The chemical analysis of paper is shown in Table 16.1.
The major components of newspaper are crude fibre (68.9%) and lignin (17.4%). The lignin content of office paper is of course much lower (1.6%). Newspapers, which account for about 50% of the printing and writing paper, consist chiefly of mechanical pulp, with some (20 to 25%) chemically pulped fibre. Their digestibility is 25 to 30%, almost directly proportional to their chemical pulp content. They can be shredded, ground, etc., and incorporated into a wide variety of ruminant rations, but their low available-energy value limits their intake and productivity. For example, in beef production rations 5 to 10% of newsprint were satisfactory, but 16 and 24% seriously reduced feed intake and daily gains (Dinius, 1973). An inexpensive partial hydrolysis of news-print would upgrade it to a useful valuable feed. The Stake process would be suitable for paper from hardwood pulps.
Paperboard and office paper are much more digestible (45 to 80%) than newsprint and can replace substantial proportions (50% or more) of conventional roughages in ruminant rations.
All paper and paperboard waste must of course be adequately supplemented with crude protein and micronutrients. Heavy metals from printing processes (especially lead, polychlorinated biphenyls, adhesives, fillers, sealants, coatings, additives, etc.) are of concern. Only some of the toxicological aspects of waste paper use have been satisfactorily resolved.
| Office Paper1 | Newspaper2 | ||
| organic matter | % DM | 94.5 | 99.1 |
| ash | % DM | 5.5 | 0.9 |
| crude protein | % DM | 0.7 | 0.7 |
| ether extract | % DM | 1.9 | 3.7 |
| crude fibre | % DM | 83.8 | 68.9 |
| N-free extract | % DM | 8.1 | 25.8 |
| lignin | % DM | 1.6 | 17.4 |
| Ca | % DM | <0.1 | <0.10 |
| P | % DM | <0.07 | <0.07 |
| K | % DM | <0.30 | <0.30 |
| Mg | % DM | 0.05 | 0.63 |
| Na | % DM | 0.03 | |
| Si | % DM | 2.0 | |
| Mn | ppm | 15.0 | 10.00 |
| Fe | ppm | 259.0 | 206.00 |
| Cu | ppm | 9.0 | 12.00 |
| Zn | ppm | 180.0 | 138.00 |
| Mo | ppm | 2.6 | 8.00 |
| Gross energy | Kcal/gDM | -- | 4.77 |
1 Source: Nishimuta et al. (1969)
2 Source: Sherrod and Hansen (1973)
Research and development requirements include a processing method for partial hydrolysis of softwood newsprint and a thorough study of toxicological aspects of paper utilization.
Untreated Wood Waste
Untreated wood waste is defined as a wood waste product either finely comminuted and already in a physical form suitable for feeding (e.g. sawdust), or physically processed only to the point where it can be fed. No chemical or biological treatments are involved.
Sawdust from most species of wood is almost indigestible. However, various types have been found to be satisfactory substitutes for other roughages in high-concentrate high-energy rations for beef and dairy cattle in areas where such roughages are not readily available at moderate prices. The primary purpose is to improve the rumen function with little expectation of energy input, although some sawdusts with digestibilities over 40% have been reported (Mellenberger et al., 1971). At many locations where there are no alternative uses for the sawdust the cost is low ($ 5 to $25/ton), if it is not needed as a fuel energy source on site. Generally this material is fed at fairly low levels (5 to 15 % of ration) which do not seriously restrict the energy intake of high-producing animals. Coarse sawdust appears to be better than fine. Some sawdusts appear to have beneficial associative effects on feed utilization.
Certain woods, such as trembling aspen, are suitable for “survival” rations if comminuted and adequately supplemented. A feed for wintering of beef cattle was made from chipped and coarsely ground whole poplar trees, stored in a pit silo. When supplemented with a small amount of cereal energy, minerals and NPN this “wood silage” was 25 to 35 % digestible and was about equal to supplemented wheat straw (Research Report, 1968). Very large quantities of aspen are located in cattle production areas in Western Canada.
Similarly, in Texas, Marion et al. (1957) found that chipped, ground mesquite wood mixed with molasses and concentrates was suitable as a feedlot maintenance ration for beef cows at lower levels of supplementation.
The effective use of such waste materials requires that each be tested and applied under local conditions.
Wood Molasses
Wood molasses is a term used to describe a group of products produced by direct hydrolysis of wood, by concentrating sulfite liquor, or from the waste effluents of particle and fibreboard processing. The products very considerably in their chemical composition depending on species of wood and commercial process. They are generally high in sugars, especially pentoses, and in dissolved lignin. Some contain such useful ingredients as ammonia and minerals.
Their major feed use is in ruminant rations. Hydrolysed wood products have some application to non-ruminants because their sugar content is lower in pentoses than most of the other products.
Acid-hydrolysed Wood (Molasses)
This material is a suitable substitute for cane or beet molasses in ruminant feeding systems. It has a beneficial effect on utilization of NPN by ruminants greater than that of normal cane molasses. It is rated as having 60 to 80 % of the value of cane molasses on an equal dry matter basis. It can also be used at lower levels for swine or poultry feeding.
The cost of producing this product is far too high to enable it to compete with cane or beet molasses under normal economic circumstances, but in emergencies it can supply feed energy on a large scale. If cheaper methods of hydrolysis can be found, it could become a major energy source.
Lignin Sulfonates
Lignin sulfonates are produced by concentrating sulfite pulping liquors to a point where they contain about 50 to 55 % solids; they are then handled like molasses or spray dried. Ammoniun calcium, magnesium or sodium sulfonates can be produced, but the ammonium lignin sulfonates (ALS) are probably the most important, being good sources of both crude protein and energy. The composition of ALS ranges as follows, the proportions depending on the species of wood and the pulping process:
| %DM | |
| Lignin | 40–50 |
| Sugars | 10–25 |
| NPN (as crude protein) | 12–24 |
| Acetic acid | 5–10 |
| Other organics | 7–10 |
| Inorganic (mainly Ca, S and K) | 5–10 |
In commercial use in North America, ALS is mixed with cane or beet molasses, employed as a binding and pelleting agent, or fed as a low-level source of energy.
ALS has been tested extensively in ruminant feeds as energy and protein supplement, generally as a replacement for molasses. At 3 to 10 % levels of replacement and on an equal DM basis, it has about 80 % of the feeding value of cane molasses, considering both its energy and CP content; it is readily accepted by cattle and sheep. There is evidence that its hemicellulose content leads to better utilization of NPN than that of cane molasses. The NPN in ALS is well utilized and appears to be released more slowly than that from urea. These sulfonates can also be readily used up to the 4 % level for poultry and swine as binders for pelletted rations. In the USA, the FDA restricts use of the lignin sulfonates in feeds to the 4% level, except that 11 % are permitted in molasses. The cost of producing ALS is probably substantially higher than the selling price: the difference, it can be assumed, is made up in the price of the major products, pulp and paper. A major stimulus to developing and marketing these products has been new anti-pollution regulations. Substantial amounts are being marketed in the USA and Canada, current prices being about $ 100/ton in Montreal. It would appear that the main future for these products is at low levels, either for energy and protein or for binders in both ruminant and non-ruminant rations.
Supplies of sulfite liquor are likely to be large for a long time. The technology of producing these materials is very simple, but the economics of production appear to be poor. Future prices of feedstuffs and alternatives for pulping liquors will determine the extent of their further development and use.
Fibreboard Liquors
Liquors from particle and fibreboard production can be concentrated to give a liquid high in hemicelluloses, with interesting properties for improved utilization of NPN. These are relatively minor in terms of total volume but have local importance. Masonex is an example of one which has been tested extensively in the USA.
Single-Celled Protein (SCP) from Spent Sulfite Pulping Liquors and Hydrolysed Wood Waste
Two main methods for producing SCP are either commercially proven or in an advanced stage of development. These are Torula yeast and Pekilo protein.
Torula yeast is a well-proven commercial process, giving a product that averages about 50 % crude protein. As a medium-quality protein (deficient in methionine), it has been shown to be valuable for supplementing swine, poultry and ruminant rations; it has been extensively used in Europe, but less so in North America and other parts of the world.
Spent pulping liquors are normally available at no cost, but for other wood waste a hydrolysis process is a major cost factor.
Cost and price estimates for production of Torula yeast protein from sulfite pulping liquors, wood and paper wastes are shown in Table 16.2.
The Pekilo process, as recently developed in Finland, produces a high-quality mycelial protein from waste sulfite pulping liquors. The product is a light, fluffy, cream-coloured powder, containing 55 to 60 % protein and rich in vitamins and minerals. Feeding tests with chickens, pigs and calves indicate this is a safe, high-quality prote in supplement that can replace large quantities of high-quality protein (fishmeal, soybean meal, etc.) and has wide application in animal feeding systems. A very promising protein source, it appears to have several advantages over Torula yeast; in particular, the quality of the protein is higher. It could be especially useful to countries which must import a large part of their supplemental protein requirements.
| Substrate | Plant size in tons/yr | Manufacturing costs $/ton | Total est. price (50 % protein) $/ton | Total est. price (100 % protein) | |
| $/ton | ¢/lb. | ||||
| Sulfite | 20 000 | 315 | 475 | 950 | 48 |
liquor | 10 000 | 430 | 610 | 1220 | 61 |
| Wood | 100 000 | 340 | 500 | 1000 | 50 |
hydrolysates | 10 000 | 530 | 850 | 1700 | 85 |
| Newspaper | 20 0001 | 465 | 625 | 1270 | 63 |
hydrolysates | (2 million people) | ||||
1 Appropriate to a population centre of 2 000 000
Source: Wayman (1975)
Commercial production was initiated at Jamsankoski, Finland, by the United Paper Co. in 1974, and a production level of 10 000 tons/ year was expected in 1975. The process is said to be economically viable in Finland (Romantschuk, 1974).
The cost of Pekilo protein is estimated to be one-half that of Torula yeast or even less (15 to 25¢/lb, 100 % protein) because of lower plant capital requirements and operating costs, greater utilization of liquor substrate and higher quality of protein (Private communication).
The cost of 50 % soybean meal may be taken as a standard for comparison. In North America, over the past three years, it has fluctuated between $120 and $500/ton or 6 and 15¢/lb or, on a 100 % protein basis, 12 and 50¢/lb. The present price of 50 % meal is about $200/ton, or 20¢/lb on a 100 % protein basis.
With the very large increases in plant protein coming onto world markets from such countries as Brazil, and the high relative cost of SCP production from pulping liquors or hydrolysed wood or waste paper, Torula yeast SCP from forest waste does not appear to have a very bright future under normal economic conditions. Pekilo protein appears to offer much more promising possibilities and may compete with soya. In addition, in regions where normal supply/demand economics are drastically modified by severe protein shortages and balance of payments problems; by new, tough, pollution control legislation; or by state control of the economy, SCP is likely to increase in importance to satisfy part of the regions' feed protein requirements. For example, Russia produces very large quantities of SCP (100 000 to 200 000 tons) from hydrolysed wood and pulping liquors. It is emphasized that pulp-mills also have alternatives, such as the production of wood molasses, requiring less costly equipment than SCP, or the use of spent liquors for fuel coupled with recovery systems for pulping chemicals.
Feed from Tree Foliage (“Muka”)
The needles of conifers and the leaves of broadleaved trees can be made suitable for animal feed with limited processing. Essentially, all that is required is a heat treatment at 210°C for a few minutes, to drive off moisture and unpalatable essential oils, followed by milling. The major development of this process has taken place in Russia, where about 100 000 tons/year are produced and fed; the product is called “Muka” (Keays and Barton, 1975; Stone, 1975).
Muka is somewhat similar to alfalfa, being rich in cellulose, carotene and minerals and containing one half to two thirds as much protein. It is fed as a supplement to poultry, cattle, milking cows and pigs at 5 to 8 % levels. Its chemical composition is shown in Table 16.3. There is less carotene and protein in winter than in summer and more carotene in hardwood foliage than in conifers.
| Component | Spruce muka | Birch muka | Alfalfa meal |
| Protein, % | 8.79 | 8.0 | 18.3 |
| Fats, % | 6.54 | 8.2 | 3.2 |
| Cellulose, % | 35.6 | 18.0 | 26.2 |
| Nitrogen-free extractives, % | 34.0 | 56.8 | 41.8 |
| Ash, % | 4.4 | 4.2 | 9.6 |
| Carotene, mg/kg | 139.0 | 380.0 | 172.0 |
| Riboflavin, mg/kg | 6.0 | 4.0 | 13.2 |
| Calcium, % | 0.72 | 0.78 | 1.13 |
| Phosphorus, % | 0.17 | 0.26 | 0.31 |
| Potassium, % | 0.44 | 0.73 | 1.34 |
| Magnesium, % | 0.59 | 0.3 | 0.2 |
| Iron, mg/kg | 158.5 | 101.0 | 212.0 |
| Manganese, mg/kg | 292.0 | 30.0 | 29.0 |
| Copper, mg/kg | 5.6 | 8.0 | 9.9 |
| Zinc, mg/kg | 31.5 | 121.0 | 16.0 |
| Cobalt, mg/kg | 158.0 | 90.0 | 360.0 |
Claims for feeding Muka in Russia include reduced susceptibility to disease, increased egg, milk and meat production and increased vitality. The type of Muka (species of tree), kind of animal and type of animal production are related to recommended feeding levels, which should be closely regulated.
In addition to Muka the U.S.S.R. produces a chlorophyll-carotene paste extracted from fresh foliage with petroleum ether. This material is used in very small percentages (0,002 to 0.4 %) in diets for poultry and cattle, with beneficial effects.
Nearly all of the research and development have been carried out in the U.S.S.R., and most of the literature is in Russian. Output per plant is low, averaging 650 tons/year; present tonnage is expected to double by 1985. The major impetus to development of Muka in Russia appears to have been the large-scale production of essential oils with Muka as a by-product, the shortage of animal feed ingredients and the development of whole-tree harvest methods.
A recent Canadian report by Keays and Barton (1975) contains 288 references.
Canada has initiated some limited research on Muka. It is estimated that about 6 million tons of foliage are wasted each year from normal forest harvest operations. If whole-tree logging operations are developed on a major scale, a substantial part of this tonnage could be available for feed. It is expected that this will stimulate further research and development, since in such processes the foliage becomes a contaminant which must be removed from pulp chips produced from the branches. The cost of producing Muka in Canada is estimated at about $100/ton.
| Fungus | Approx. decay time (days) | Average weight loss (%) | Loss of lignin (%) | Loss in total carbohydrates (%) | Rumen fluid digestibility (%) |
| None (control) | 0 | 0 | 0 | 0 | 46 |
| Fomes ulmarius | 77 | 13 | 34.6 | 10.2 | 64 |
| Ganoderma applanatum | 71 | 51 | 42.2 | 57.6 | 46 |
| Polyporus berkeleyi | 88 | 21 | 57.8 | 22.0 | 77 |
| P. frondosus | 64 | 20 | 47.2 | 19.3 | 71 |
World potential for Muka also depends chiefly on application of whole tree harvest methods.
Bark
Bark amounts to 10 to 15% of the total wood, and the large amounts produced on mill sites create a major disposal problem. Most barks do not appear to have been tested for their nutritional value, but in general bark appears to have little potential as a large-scale feed source because most barks are not very digestible and moreover do not respond favourably to treatments, such as the Stake process, for improving digestibility. There are exceptions, such as aspen bark, which was shown to contain about 36% TDN and 2 to 3% of crude protein (Enzmann et al., 1969) and to have up to 50% digestibility in roughage rations (Mellenburger et al., 1971). Barks from species of ash, elm and basswood were shown by Millet et al. (1970) to have in vitro digestibilities ranging from 25 to 45%.
In specific local situations a few of the barks have application in maintenance or emergency rations for ruminants.
Fungal Degradation of Lignin
This process is based on the ability of white-rot fungi to utilize lignin with minimal degradation of cellulose and hemicellulose. The product is a high cellulose-hemicellulose fibre which has potential as a roughage for ruminants.
Japanese, American and Swedish workers have taken a leading role in experimental studies with the objective of producing a feed or biological pulp or simply preparing cellulose for further enzymatic digestion. The studies by Kirk and Moore (1972) on aspen clearly indicate potential for producing digestible wood as shown in Table 16.4.
The fungal degradation process is slow compared to steaming or chemical treatment, as it requires several weeks, but this is not necessarily a serious drawback. The work of Karl-Eriksson (1974) on biological pulps is noteworthy.
Research needs include screening of the many thousands of fungi with possible potential and developing ways of optimizing conditions for decaying wood and methods of reducing degradation of the carbohydrates.
References
American Paper Institute, 1971, The Statistics of Paper, New York, N.Y.
Bates, David, 1975, What's cooking in wood rations. Canadian Cattlemen, p. 24, December.
Bender, F., D.P. Heaney, and A. Bowden, 1968. The potential of steamed wood as a feed for ruminants. Forest Prod. J. 20 (4): 36–41.
Bender, R. 1975.
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Dinius, D.A. 1973, Wood Waste Materials as Feedstuffs for Ruminants, (ANRC Papers) Feedstuffs 45 (14); 22, 1972.
Elgee, Harold, 1976, Cellulosic Wastes - Feed or Fibre, Conference Proceedings, “Waste Recycling & Canadian Agriculture”, AERC, Toronto. April 1975.
Enzmann, J.W., R.D. Goodrich and J.C. Meishe. 1969. Chemical Composition and Nutritive Value of Poplar Bark. J. Animal Sci. 29: 653– 660.
Hividsten, H. 1940. Fodder Cellulose, its use and nutritive value. Nord. Jordbrugsforskning 22: 186.
Hividsten, H. and T. Homb. 1947. Survey of cellulose and Beckman treated straw as feed. Proc. 11th Int. Cong. Pure & App. Chem. 3:-113.
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Keays, J.L. and G.M. Barton. Recent Advances in Foliage Utilization. Information Report VO-X-137. Environment Canada, Western Forest Products Laboratory, Vancouver, B.C.
Kirk, T.K. and W.E. Moore. 1972. Removing lignin from wood with white-rot fungi and digestibility of resulting wood. Wood and Fibre 4(2): 72–79.
Marion, P.T., C.E. Fisher and E.C. Robinson. 1957. Ground mesquite wood as a roughage in rations for yearling steers. Progress Report 1972, Cattle Series 141, Texas Agr. Exp. Sta.
Mellenburger, R.W., L.D. Satter, M.A. Millett and A.J. Baker. 1971. Digestion of Aspen, alkali-treated aspen and aspen bark by goats. J. Anim. Sci. 31: 756.
Millett, M.A., A.J. Baker, W.C. Feist, R.W. Mellenburger and L.D. Satter. 1970. Modifying wood to increase its in-vttro digestibility. J. An. Sci. 31:781–788.
Nishimuta, J.F., L.B. Sherrod, R.D. Furr, and K.R. Hanson. 1969. Nutritive value of sheep rations containing various levels of paper. J. An. Sci. 29:642. (Chem. anal. of office paper).
Pritchard, G.I., W.J. Pigden and R.K. Wilson. 1962. The effect of gamma irradiation on the utilization of wheat straw by rumen microorganisms. Can. J. An. Sci. 42: 215.
Research Report, 1967. Melfort Research Station, Canada Department of Agriculture, Research Branch.
Romantschuk, Hakan. 1974. Feeding Cattle at the pulp mill. Unasylva, P. 15–17 (Autumn).
Saarinen, P., W.J. Jensen and J. Alhojärvi. 1959. Digestibility of high yield chemical pulp and its evaluation. Acta Agral. Fennica 94:41.
Sherrod, L.B. and K.R. Hansen. 1973. Newspaper levels as roughage in ruminant rations. J. An. Sci. 36: 592.
Stone, J. 1975. Forest products as commercial feed. Conference proceedings, “Waste Recycling in Canadian Agriculture”, Agric. Econ. Res. Council of Canada.
Virkola, Nils-Erik. 1975. Available Cellulosic Materials. Symposium on Enzymatic Hydrolysis of Cellulose, Aulanko, Finland.
Wayman, Morris and Timothy I. Obiaga. 1975. Single Cell Protein - Food or Feed. Conference Proceedings, “Waste Recycling and Canadian Agriculture”, Agr. Econ. Res. Council of Canada. Toronto.
Wilson, R.K. and W.J. Pigden. 1964. Effect of a sodium hydroxide treatment on the utilization of wheat straw and poplar wood by rumen microorganisms. Can. J. Animal Sci. 44: 122–123.
