With the present state of knowledge, the manipulation of plants is likely to improve the utilisation of feed resources by livestock with lesser investment of efforts and money compared to the manipulation of rumen microbes. The limitations associated with engineering of rumen microbes, establishment of engineered microbes in the rumen and their functional contribution have been highlighted in the Background Document posted on 8 June. In addition, exploitation of the benefits of engineered microbes and animals will require considerably more time, including a lengthy process to obtain regulatory approval because of animal welfare, ethical and safety concerns.
Plant biotechnologies to improve the nutritional quality of plant feedstuffs and by-products offer many more opportunities and potential in animal nutrition. Tremendous strides have been made in the recent past. The genetic engineering of a golden rice with high levels of beta-carotene and iron (likely to have vast implications for developing countries) has demonstrated that it is now possible to transfer not only a single gene, but the entire genetic pathway for producing a nutritionally advantageous constituent in a plant.
There are several examples where the composition of oils, proteins and carbohydrates in seeds of corn and soyabean, and other crops has been modified to produce grains with enhanced value using plant breeding and molecular technologies. Traditional plant breeders have focused on improving the agronomic characteristics of crops, including yield, disease resistance and quality characteristics for human food. More dialogue, interactions and collaborative projects between plant breeders and animal nutritionists are required so that trait modification will benefit both crop and livestock industry.
Improving feed quality through genetic manipulation holds great promise, e.g.,
1) increase in digestibility of
existing nutrients especially of fibre for tropical forages,
2) decrease in fibre content and increase in cell solubles,
3) addition of beta-glucanase, arabinoxylanase and phytase in barley, rye and other grains for use in diets for monogastrics,
4) increase in soluble carbohydrate in roughages,
5) increase of protein in tropical forages and decrease in degradability of protein in the rumen for temperate forages,
6) increase in sulphur amino acids in leguminous forage,
7) regulation of protein and carbohydrate contents and their degradation to achieve maximum microbial protein synthesis in the rumen.
Transgenic expression in plants of edible antibodies and other novel proteins of industrial applications are at a very early stage of development.
The reduction or elimination of plant secondary metabolites by plant breeding and molecular technologies might not be a right approach, especially for developing countries where the plants are faced with various environmental vagaries and the plant secondary metabolites have a defensive role for the plant to ensure survival by protecting it against insect predation or by restricting grazing of herbivores. Transfer of rumen microbes from resistant ruminants to susceptible ruminants has been shown to be promising for mitigating problems associated with the presence of plant secondary metabolites (antinutrients) such as mimosine, flurocaetate, to some extent for tannins, and a toxic accession of Acacia angustissima (toxic principle has not yet been identified). The establishment of microbes converting these toxins/antinutrients to innocous compounds in the rumen and enumeration of oxalate-degrading microbes when plants containing oxalate is gradually introduced into the diet, can be monitored using competitive PCR method and 16S rRNA-targeted oligonucleotide probes which do not require culturing of microbes.
Probes designed for ruminal methanogens would enable us to understand localization of the methanogens and to take into account their ecological control to reduce methane production. These probes also have the potential to evaluate factors influencing the rumen ecology and to devise better feeding strategies, to exploit fully the known additive and to identify new additives. These probes and other molecular techniques such as restricted fragmented length polymorphism, PCR, monoclonal antibodies have also opened up the possibilities to evaluate risk of dissemination of transgenes into the environment and detection in human food chain, and detection of pathogens and trace residues of drugs and other undesirable compounds in animal products.
Harinder P.S. Makkar, PhD
Animal Production and Health Section
Joint FAO/IAEA Division
International Atomic Energy Agency
Wagramerstr 5, A-1400 Vienna
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