|Keywords:||International Livestock Research Institute (ILRI); Feeding (animal); Biosafety/Foodsafety; Socio-economic impact.|
|Correct citation:||MacMillan, S. (1996), "Improving the Nutritional Status of Tropical Ruminants." Biotechnology and Development Monitor, No. 27, p. 8-9.|
Cattle, sheep, goats, buffalo and other ruminant animals in developing countries feed mainly on poor-quality plant material, on natural rangelands or pastures as well as on bushes, trees and crop residues. The efficient utilization of such materials depends upon symbiotic micro-organisms in one of the digestive compartments of these animals. A research project at the International Livestock Research Institute (ILRI), based in Kenya and Ethiopia, focuses on the ability of ruminants to utilize microbial organisms to produce good-quality protein. Understanding the rumen fermentation system will allow scientists to modify it to improve ruminant production.
The major cause of poor livestock productivity in tropical regions of
the world is inadequate nutrition. In Africa, for example, where grain
is too scarce and valuable as human food to be given to animals, the principal
source of animal feed is unimproved pastures as well as edible parts of
shrubs and trees. Many ruminants kept by small farmers on the continent
feed on nothing but natural pasture throughout their lives. Across the
semi-arid regions of the world, that natural pasture is dry and of little
nutritive value for many months of the year. In the dry season proper,
when the standing grass is mature and dormant, the grazing comprises mostly
hard-to-digest fibre with low protein content. In periods of drought, even
this material is in short supply. Following the harvesting of grain, two
to three months into a dry season, livestock in mixed crop-livestock farming
systems might also feed on cereal straws, stubble or other leftovers such
as maize stover, which have a nutritional value similar to that of mature
natural grass. If available, smaller quantities of residues from leguminous
crops, which have a greater nutritional value than cereal residues, may
be used as protein supplements.
Ways to better utilize such low-quality feed are now being intensely investigated by research groups around the world. These groups are focusing on a special characteristic of ruminant livestock, particularly those indigenous in the tropics, that enables them to utilize poor-quality cellulosic feed as an energy source.
The rumen system
All ruminant livestock possess a complex stomach system comprising three or four compartments, the first and the largest of which is the rumen. Continuous anaerobic fermentation takes place here, before digestion in other parts of the stomach and the intestines. This fermentation is accomplished by communities of symbiotic micro-organisms: protozoa, bacteria and fungi. When ruminant animals are born, their rumen is microbe-free; the unique micro-flora and fauna start to appear only after birth. Once established, the rumen microbial community is relatively stable and will change only with changes in the nutrients available. The quality and quantity of products of this fermentation process depend on the types and activities of the micro-organisms in the rumen. Fibre-digesting bacteria, for example, are of primary importance for ruminants fed on poor-quality material of which cellulose and hemicellulose, in roughly equal proportion, may comprise about 80 per cent of the total.
Traditional supplements. In an animal whose diet is made up solely of low-quality forages, the rumen environment does not provide its microbial inhabitants with sufficient nutrients to grow or to digest fibrous materials efficiently. Feed deficiencies are traditionally corrected by adding to the animal diet fodder (dried food, hay, straw, etc.), herbaceous legumes (forage from cowpea, lablab and groundnut hay) and/or the leaves and pods of leguminous fodder trees, where these are available. These nutritional supplements may not only increase protein supply to the animal but also create a favorable environment in the rumen for fermentation of basal roughage and microbial protein synthesis. Furthermore, when dead, the increased numbers of rumen micro-organisms themselves become a source of increased protein for the animal. Feed supplements can thus act to increase the total food intake, the amount of protein over the basal poor-quality diet and the digestibility of the diet. These increases will in turn improve milk and meat production by as much as 200 to 400 per cent.
Fodder trees and shrubs. In many areas of Africa, the leaves and fruit of indigenous fodder trees can be used to supplement pasture grazed in the dry season. However, many trees and plants protect themselves against stresses such as drought, insect predators and microbial infections by producing toxic compounds that adversely effect the host animal on their ingestion or the micro-organisms of its rumen. Several deleterious compounds have been identified in forages and fodder trees. Many of the African browse species, for example, contain high levels of polyphenolics and insoluble condensed tannins. Some phenolic compounds are toxic to rumen bacteria, fungi and protozoa. Fodder trees, shrubs and other plant material must therefore be examined for such anti-nutritional factors before their use as feed supplements can be recommended.
ILRI aims to develop chemical, microbiological and animal/tissue techniques that can be used to screen plant materials for anti-nutritional factors and toxins. With these new methods, ILRI will assess the potential of fodder trees and shrubs to enhance the diets of tropical livestock, and also develop strategies to overcome the effects of anti-nutritional factors in plant accessions of otherwise high agronomic merit.
Crop residues and by-products. In regions such as Asia, where land available to small farmers for forage production is declining rapidly, ruminants will have to depend more and more on fibrous crop residues and by-products for their energy sources. Theoretically, up to 90 per cent of ruminant feed could comprise agricultural by-products. In Southeast Asia, for example, rice cultivation and oil palm production produce the largest quantity of residues and by-products in the form of rice straw, palm press fibre, oil palm trunk and fronds and palm kernel cake. Given the great quantities of by-products available in Asia and that tropical ruminants are excellent converters of low-quality feed into milk and meat, these non-conventional feeds are fast emerging as an alternative source of nutrients for intensive ruminant production. Many of these by-products, however, will have to be modified in some way and some will need treatments to detoxify them. The nutrient energy in most such non-conventional feeds is neither readily available nor sufficient, even for the ruminant digestive system. Researchers are thus investigating ways of modifying these potential feeds to improve their digestibility. They are also looking for ways to manipulate rumen microbes to accelerate the rate of digestion of cellulosic feeds.
Results of ILRI studies in Niger and Nigeria to define optimal feed supplementation regimes show that rumen environmental conditions and digestive microbial activity vary with the seasons in cattle, goats and sheep grazing year-round on fields of crop residues or Sahelian rangelands. This seasonal variation appears to be associated with the different diets selected by these animals. Moreover, as stock grazed crop residues, high levels of intake of a quality diet were observed only during the first three or four weeks of the season, when the amount of above-ground leaf exceeded 400 kg/ha. After the first month of grazing, the amounts of protein- and energy-yielding nutrients appear to be insufficient for efficient microbial activity and maintenance of the animals.
Modifying the rumen ecology
Understanding the factors controlling rumen microbial activity may allow scientists to modify the rumen ecology in order to create conditions that will optimize the use of poor-quality feed by ruminant livestock. The fastest way to improve rumen function in an animal is to introduce digestion-enhancing bacterial species from other animals or to selectively increase populations of species that inhabit the rumen only at low levels. Bacteria from one ruminant species have been experimentally shown to colonize others successfully. It has been further demonstrated that the cross-inoculation of rumen fluid from wild to domestic ruminants alleviates tannin toxicity and enhances the productivity of livestock browsing tannin-containing shrubs.
The transferral of microbes between animal species is being facilitated by the precise molecular methods now available to track individual organisms within complex mixtures. Without precise and quantitative tracking systems, the effectiveness of any particular organism within the microbial community in the rumen cannot be clearly demonstrated.
Rumen ecology may also be modified by altering the function of bacteria using genetic engineering techniques. In the past ten years, many approaches have been taken in attempts to transform rumen bacteria by introducing a foreign gene or genes into the bacteria. Research in sheep conducted at the Institute of Biology and the Institute of Biotechnology, University of New England, both in Armidale, Australia, has proved that genetic manipulation of rumen bacteria is feasible. Bacteria grown in the laboratory were shown to recolonize the rumen and genetically modified bacteria did so as efficiently as their unmodified counterparts. The altered bacteria neither died out, as some scientists suspected they would, nor affected the health of the sheep. Although diet influenced the population levels of the introduced bacteria, as expected, the competition for nutrients under poor dietary conditions did not appear to disadvantage the modified bacterium.
Although ILRI is not applying this approach in its research, ILRI scientists will follow the Australian work with interest for indications of when use of recombinant rumen bacteria may become practical and safe in developing countries.
International Livestock Research Institute (ILRI), PO Box 30709, Nairobi, KENYA. Phone (+254) 2 630743; Fax (+254) 2 631499; E-mail ILRI@cgnet.com
This article is based on the proceedings of a workshop on "Rumen Ecology Research Planning", organized by ILRI in 1995. The aim of the workshop was to help ILRI identify the most promising areas of rumen ecology research for improving the nutritional status of tropical ruminants. For a copy of the proceedings, edited by R.J. Wallace and A. Lahlou-Kassi, please write to Head of Publications, ILRI-Ethiopia, P.O. Box 5689, Addis Ababa, Ethiopia. ISBN 92-9146-005-2.
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