Genetically modified crops as animal feed

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Friday, 10 July 2009 06:08

Genetically modified crops, products derived from them and enzymes derived from genetically modified micro-organisms are widely used in animal feeds. The global animal feed market is estimated at some 600 million tonnes. Compound feeds are principally used for poultry, pigs and dairy cows and are formulated from a range of raw materials, including maize and other cereals and oilseeds such as soybeans and canola. It is currently estimated that 51 percent of the global area of soybeans, as well as 12 percent of canola and 9 percent of maize (used as whole maize and by-products such as maize gluten feed) is genetically modified (James, 2002a). Safety assessments of novel livestock feeds in Canada, the United States and elsewhere look at the molecular, compositional, toxicological and nutritional characteristics of the novel feed compared with its conventional counterpart. Considerations include the effects on the animal eating the feed and on consumers eating the resulting animal product, worker safety and other environmental aspects of using the feed. In addition, comparisons of nutritional composition and wholesomeness between animal feeds containing transgenic versus conventional components have been the subject of many studies. The major concerns associated with the use of GM products in animal feeds are whether modified DNA from the plant may be transferred into the food chain with harmful consequences and whether antibiotic-resistance marker genes used in the transformation process may be transferred to bacteria in the animal and hence potentially into human pathogenic bacteria. As the production process for the enzymes used in animal feeds takes place under controlled conditions in closed fermentation tank installations and eliminates the modified DNA from the final products, these products do not pose any risk to the animal or the environment. The enzyme phytase has particular benefits in feeding pigs and poultry, including a significant reduction in the amount of phosphorus released to the environment. Researchers have examined the effects of feed processing on DNA to ascertain whether modified DNA remains intact and moves into the food chain. It has been found that DNA is not fragmented to any great extent in raw plant material and silage, but remains partially or fully intact. This means that, if GM crops are fed to animals, animals would be likely to be eating modified DNA. In order to consider whether modified DNA or derived proteins consumed by animals have the potential to affect animal health or to enter the food chain, it is necessary to consider the fate of these molecules within the animal. Digestion of nucleic acids (DNA and ribonucleic acid, RNA) occurs through the action of nucleases present in the mouth, the pancreas and intestinal secretions. In ruminants, additional microbial and physical degradation of feed occurs. Evidence suggests that more than 95 percent of DNA and RNA is completely broken down within the digestive system. In addition, research carried out on the digestion of transgenic proteins in in vitro culture has shown nearly complete digestion occurring within five minutes in the presence of the enzyme pepsin. Of further concern is whether there can be transfer of antibiotic resistance from the marker genes used in the production of GM plants to micro-organisms in animals and thence to bacteria pathogenic to humans. A review commissioned by FAO has concluded that this is extremely unlikely to happen (Chambers and Heritage, 2004). Nevertheless, this paper concluded that markers which code for resistance to clinically significant antibiotics, critical for treating human infectious diseases, should not be used in the production of transgenic plants. MacKenzie and McLean (2002) reviewed 15 feeding studies of dairy cattle, beef cattle, swine and chickens published between 1995 and 2001. The feeds studied were insect- and/or herbicide-resistant maize and soybeans. The animals were fed a transgenic or conventional product for time periods ranging from 35 days for poultry to two years for beef cattle. None of these studies found any adverse effects in the animals fed the transgenic products for any of the measured parameters, which included nutrient composition, body weight, feed intake, feed conversion, milk production, milk composition, rumen fermentation, growth performance or carcass characteristics. Two of the studies found slight improvements in feed conversion rates for the animals fed insect-resistant maize, possibly because of lower concentrations of aflatoxins, antinutrients that result from insect damage. In summary, it may be concluded that the risks to human and animal health from the use of GM crops and enzymes derived from genetically modified micro-organisms as animal feed are negligible. Nevertheless, some countries do require labelling to indicate the presence of GM material in imports and products derived thereof.

Last Updated on Friday, 10 July 2009 08:56