Current Traits in Biotech Feed Crops
Much of our crop production is used as feeds for livestock, including 80% of the corn grown in the USA and approximately 70% of the soybeans grown worldwide. Byproducts from cotton & canola seed processing are also widely used as animal feeds. Biotechnology methods (http://sbc.ucdavis.edu/outreach/abc/new_tools_abc.htm) have been used to enhance all of these crops and the resulting cultivars are now being grown on over 100 million acres worldwide. This brochure describes biotechnology and conventional breeding for development of livestock feed crops, and the identity preservation systems needed to manage the commercial production and marketing of these crops. (References)
Biotech Crop Traits. Among the transgenic crop plants approved by USDA APHIS for commercial production in the US are canola, corn, cotton and soybeans engineered for herbicide compatibility and protection against insects. Development of crops that are tolerant to broad-spectrum herbicides enables more efficient weed control and improvements in soil conservation. Crop plants that are tolerant to herbicides such as glyphosate, bromoxynil and phosphinothricin have been developed using transgene biotechnology methods. Glyphosate inhibits an enzyme of aromatic amino acid biosynthesis; tolerance is based upon an altered enzyme that functions normally in biosynthesis even in the presence of glyphosate. Tolerance to phosphinothricin and bromoxynil is based on enzymes that inactivate the herbicide molecules via a covalent reaction. Tolerance to imidazolinone herbicides was developed through mutant selection in tissue culture (somaclonal variation) and plant regeneration methods. Transgenic crops can be protected against certain types of insects by expression of Bt genes. Bt genes encode Cry proteins that disrupt function of the digestive tract in many insects, but are nontoxic to other organisms. Mycotoxin contents are reduced in these Bt transgenic plants due to reduced insect feeding and fewer damaged sites at which fungal infection of the crop plant can occur. Thus, this transgenic approach provides considerable benefit in reducing the mycotoxin problem where there has been little progress via conventional breeding methods. Engineering these pest protection traits into the seed may reduce the need for pesticide chemicals in crop production, and may enable the grower to choose pesticides that are less harmful to the environment. Recent economic studies by USDA document benefits of transgenic crops including yield increases, cost savings to growers and reductions in chemical application. (References)
Feeding trials. In the USA, the FDA Center for Veterinary Medicine is responsible for regulation and licensing of biotechnology feeds and feed additives for livestock. The current biotechnology feed crops were engineered for "input traits," characteristics expressed during growth of the crops. The feed value of these biotech crops is expected to be unchanged from that of conventional feed crops. Trials with poultry, catfish, swine, cattle and sheep have demonstrated that feeds derived from transgenic crop plants are equivalent to conventional crops in safety and feed value. The Federation of Animal Science Societies has concluded that transgenic feeds are digested normally, so there is no effect on the safety of meat, milk or eggs produced by the livestock. (References)
Future Traits in Biotech Feed Crops
Potential future traits in transgenic crops include more "output traits" that will enhance food and feed qualities. Transgenic crop traits that may reach the marketplace within the next 5 years include traits such as reductions in toxins and antimetabolites, increased digestibility, and improved nutritional quality. (References)
Mycotoxins. Biotechnology approaches show promise for reduction of mycotoxin contamination in feed crops. Some mycotoxins such as fumonisins are virulence factors for fungal infection of growing crop plants. Other mycotoxins such as aflatoxins are produced by fungi that infect the crop after harvest. Mycotoxin contamination may make the crop unfit for use as livestock feeds. Currently, mycotoxins are partially controlled through breeding plants for resistance to fungal diseases, fungicide chemical sprays and various agronomic practices. Biotechnology may help to provide additional protection against mycotoxins. Reduction of mycotoxin contamination in insect-protected Bt transgenic crops was described above. Expression of antifungal peptides in transgenic crops may reduce fungal growth in crop plants and mycotoxin levels in feeds. These peptides specifically inhibit fungi by interfering with membrane structure or by inhibition of cell wall synthesis. Transgenes that detoxify mycotoxins or that cause mycotoxins to be eliminated from the plant cells have also demonstrated some ability to protect plants against mycotoxins. There has also been some success with biological controls. Certain harmless microbes can competitively exclude some mycotoxin-producing fungi from colonizing the crop. These approaches for mycotoxin reduction will be helpful in improving feed quality, animal health and the quality of meat, milk and eggs produced. (References)
Phytic acid. The content of antinutritional substances such as phytic acid can be reduced through biotechnology and conventional breeding approaches. Phytic acid binds phosphorus and certain other mineral nutrients, thereby reducing their bioavailability. Monogastric animals such as swine and poultry do not digest phytic acid efficiently, so much of the phytate and associated minerals passes into the feces. Phosphate in these wastes contributes to eutrophication of waterways. Mutations that reduce phytic acid levels throughout the plant have been isolated in maize, barley and rice. These mutants have yield reductions of 5 to 15%. Corn cultivars with reduced phytate and "high available phosphorus" are currently on the market. Phytase, an enzyme that digests phytic acid, can also be purified from transgenic microbes and used as a feed additive. Phytase has also been produced recently in transgenic canola, alfalfa and rice plants. Use of these plants as feeds provides the same benefits as adding the purified enzyme to conventional crop diets. Initial studies demonstrated no adverse effects of these phytase-enhanced feeds on animal health. This transgenic approach may be able to avoid yield penalties because phytic acid production throughout these plants is normal and phytase enzyme production can be targeted specifically to the grain. (References)
Lignin. The digestibility of crops can be improved via reduction in the lignin content of their cell walls. Lignin is a nondigestible polyphenolic component of the cell wall. Forage crop plants have been selected for altered cell wall characteristics that provide increased digestibility in livestock. "Brown midrib" mutants in corn and sorghum develop lower lignin contents and provide feeds with increased digestibility, but in corn there is a significant yield penalty relative to nonmutant varieties. A biotechnology approach for reduction in lignin content has improved digestibility in alfalfa, but further work will be needed to determine the yield and other agronomic properties of the transgenic alfalfa. (References)
Vaccination via feed. Edible vaccines delivered via feeds may also help to maintain the health of livestock in the future. Animals have been immunized against diseases through feeding of transgenic plants expressing antigens (i.e. subunit vaccines) from various microbes. These edible vaccines have been successful against diseases caused by transmissible gastroenteritis coronavirus, foot-and-mouth disease virus, rabies virus, swine diarrhea, avian influenza, bovine viral diarrhea virus, swine fever virus and rabbit hemorrhagic disease virus. Some of these are now being entered into veterinary trials, but it will be some time before any edible vaccine products are licensed for marketing. Nevertheless, this biotechnology strategy has great potential for providing benefits that could not be achieved through plant breeding approaches.
Biotech plants have also been used to produce chimeric plant virus particles expressing antigens from various animal pathogens. These chimeric plant virus particles have been purified from host plant tissue and used as vaccine injections. Antigen structures displayed on the surface of these virus particles are very effective in stimulation of immune responses in animals. These plant-derived vaccines have been successful for protection of animals against infectious diseases such as canine parvovirus, mink enteritis virus, feline panleucopenia virus and Staphylococcus aureus. (References)
Protein quality. Amino acid balance in proteins from all feed crops needs to be optimized for better animal nutrition because plant proteins typically do not provide the optimum amino acid ratios required for efficient protein synthesis in animals. Cereal proteins are deficient in lysine and tryptophan. Breeding with opaque-2 mutants has produced "quality protein maize" with improvements in the lysine and tryptophan contents of the seed proteins. Legume proteins are often deficient in methionine, cysteine and lysine. Wild soybean germplasm with improved contents of methionine and cysteine may be used to introgress this trait into the cultivated soybeans. Biotechnology may also be useful through expression of foreign proteins that are rich in the amino acids that are limiting in the crop plant. For example, expression of a high-methionine protein from sunflower in lupin seeds enabled increased weight gain and efficiency of wool growth when these transgenic lupins were used as feed grain for sheep. Expression of a feedback-insensitive aspartate kinase gene in transgenic alfalfa caused enhanced biosynthesis of threonine, another amino acid that is commonly limiting in livestock diet formulations. (References)
Digestive enzymes. A wide range of digestive enzymes currently added as feed additives for monogastric animals may eventually be supplied by expressing the enzymes directly in transgenic feed crops. Glucanase enzyme expression in transgenic barley enabled barley to equal the feed value of corn in diets for poultry. Many other feed additive enzymes are currently purified from microbes, including various cell wall degrading enzymes, starch degrading enzyme, phytase (see above) and protease. (References)
High oil grain. High oil corn developed via breeding is on the market. These varieties have seeds with larger embryos, producing increased content of oil, essential amino acids and vitamins in the seed. Feeds containing this energy-dense corn improve animal performance. These are sold as single cross hybrids or as blends. Blends are composed of a pollinator variety having a very high oil content together with a conventional corn variety. The hybrid seeds produced in the field have oil content midway between that of the parents. High oil grain developed via biotechnology may reach the marketplace within 5-10 years. (References)
Identity Preservation
Identity preservation (IP) is a system of crop management, crop testing and detailed record-keeping designed to document that distinct crop varieties have been kept separate from planting through harvest and on to the end user (Identity Preservation ABC, manuscript submitted, http://sbc.ucdavis.edu/outreach/abc/abc_series.htm). Some groups have called for all biotechnology products to be labeled and kept segregated from conventional food products. For example, USDA regulations for organic agriculture certification exclude the use of biotech crop plants or their feeding to organic livestock. In addition, feed crops enhanced with output traits also require IP systems to ensure that the value from the output traits can be delivered undiluted to the end-users. IP may also be done with conventional crops for endusers who wish to avoid biotechnology crop products. In the years to come agriculture will increasingly convert from the production of bulk commodity products to the production of many speciality products having distinct valuable traits.
The failure to maintain IP with StarLink corn in years 1999 and 2000 was a costly error that demonstrated some considerable procedural and infrastructural challenges that agriculture must solve in order for biotechnology output traits to be successfully commercialized. Growers, shippers and endusers are working to ensure that crops with output traits are kept segregated from conventional crops. Initially, many of these speciality feed crops will be grown on small acreages and used in domestic markets, perhaps not far from where they are grown. IP is especially critical for export of biotechnology crop products because some transgenic crops have not yet been approved for import into certain countries. (References)
Conclusions
The initial “proof-of-concept” transgenic feed crops have performed well in the field, providing agronomic and economic benefits to growers while reducing agricultural impacts on the environment. These transgenic feed crops have also provided a safe and nutritous supply of feeds for livestock. Many new biotechnology crop traits under development will provide additional improvements in livestock feeds in the future. Government licensing of biotech crops will continue to play an important role in order to ensure that these crops are as safe as conventional crops. Identity preservation systems will be increasingly important to ensure that the desired biotech or non-biotech crop traits are delivered in the feed products.
Additional Sources of Information
Livestock Feeds - Biotechnology Resource Series http://sbc.ucdavis.edu/Outreach/Livestock_Feeds.htm
ABC Series, Agricultural Biotechnology in California http://sbc.ucdavis.edu/Publications/Agricultural_Biotechnology_in_California_(ABC)_Series.htm
UCBiotech, University of California, Berkeley http://ucbiotech.org/index.html