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Are Genetically Modified Feeds Safe for Horses?By Dr. Joe Pagan · July 22, 2016

The safety of genetically modified (GM) foods for human consumption is a contentious and controversial subject. Much of the information fueling this debate is based on little or no scientifically sound evidence. This has spilled over into the horse industry with many feed manufacturers capitalizing on this angst by offering “GM-free” horse feeds. What are genetically modified feed ingredients, and are they really safe to feed to horses?

Genetically Modified Traits

The most common form of plant genetic engineering is called transgenic modification. A transgenic plant contains one or more genes that have been artificially inserted using recombinant DNA technology. The inserted gene sequence may come from another unrelated plant, or from a completely different species such as a bacterium. Extensive safety testing is required by government agencies with this type of modification.

Scientists use genetic engineering to instill desirable characteristics in plants. For example, tobacco, corn, rice, and many other crops have been engineered to express encoding for insecticidal proteins from Bacillus thuringiensis (Bt). Others have been modified for herbicide resistance, which allows the herbicides, such as glyphosate and dicamba, to be used on weeds without injuring crops. Other engineering efforts boost the nutritional value of crops, such as increasing the beta-carotene, or vitamin A, in rice and improving the fatty acid profile in soybeans.

Insect Resistance

The classic example of genetic engineering for insecticide resistance involves Bt corn. Pesticide-resistant corn has been engineered to produce a protein, known as Cry, isolated from the bacterium Bacillus thuringiensis (Bt).

The Cry protein is activated by an enzyme found in the gut of specific insects. Cry then binds to the surface of cells in the digestive system, killing the insect. These enzymes are not found in the guts of animals, so Cry is not toxic to animals. The gene used in Bt corn produces a protein that kills Lepidoptera larvae, in particular European corn borer. Bt spores have been sprayed on crops for decades and are still widely used to control pests by organic farmers.  

Interestingly, as more and more Bt corn is planted, farmers use less insecticides. In fact, overall insecticide use in the U.S. has declined 0.6% per year since the introduction of Bt corn into the marketplace.

Herbicide Tolerance

Certain plants have been modified so that producers can routinely use herbicides without fear of destroying crops. These are called herbicide tolerant (Ht) crops. An example of this is Roundup Ready soybeans, a series of genetically engineered varieties of glyphosate-resistant soybeans produced by Monsanto, a multinational agrochemical and agricultural biotechnology corporation.

Glyphosate kills plants by interfering with an enzyme (EPSPS) needed by plants for the synthesis of the essential amino acids phenylalanine, tyrosine, and tryptophan. EPSPS is not present in animals, which instead consume these aromatic amino acids from their diets. Ht plants contain a bacterially derived form of EPSPS that is not affected by glyphosate.

On the downside, however, plant herbicide resistance is a mounting problem, as more and more weeds are becoming immune to the effects of glyphosate. New GM crops have been created to resist other herbicides, namely 2,4-D and dicamba, and genes are being stacked against multiple herbicides. To exacerbate the problem, no new herbicides are being developed.

Prevalence of GM

Genetically modified (GM) crops have been part of the agricultural landscape since 1994, when the first genetically modified food, the Flavr Savr tomato produced by Calgene, was introduced into the marketplace and licensed for human consumption. The tomato was genetically modified to have a longer shelf life by inserting a specific gene that delayed ripening. A year later, the first pesticide-resistant crops were approved in the U.S., including Bt potato, corn, and cotton varieties. Not long afterward, Ht cotton and Roundup Ready soybeans were developed.

What crops grown in the U.S. are genetically engineered? Figures from 2013 speak of the country’s use of GE crops: 93% of all corn planted was GE, 90% of all cotton, 93% of all soybeans, 95% of all sugar beet, and 90% of all alfalfa seeds. Much of this GM harvest is exported to the European Union (EU). In fact, about 80% of all livestock feed in the EU is imported. Specifically, 98% of all soybean meal is imported from the U.S., Brazil, or Argentina, and 80% of this imported soybean meal intended for animal use is GM.

Certain crops grown in the U.S. are not genetically engineered, and these include oats, barley, wheat, rice, sorghum, and millet.

Are Genetically Engineered Feeds Safe for Horses?

Though no studies have been completed on horses specifically, multiple studies have been performed on animals to gauge the effects of diets that contain GM ingredients.

In 2012, researchers published a review article in the Journal of Consumer Protection and Food Safety that assessed the nutritional effects and safety of animal feeds made from GM plants (Flachowsky et al.). More than 135 studies were taken into consideration in multiple types of animals, including beef cattle, dairy cattle, pigs, poultry, and other animals and fish. The authors found “no unintended effects in composition (except lower mycotoxin concentrations in Bt plants)” among cattle and pigs, and “no significant differences in digestibility and poultry health as well as no biological relevant unintended effects on performance of animals and composition of food of poultry origin.”

A review article was published in the journal Food and Chemical Toxicology, titled “Assessment of the Health Impact of GM Plant Diets in Long-term and Multigenerational Animal Feeding Trials: A Literature Review” (Snella et al., 2012). The objective of this systematic review was to collect data concerning the effects of diets containing GM corn, potato, soybean, rice, or triticale on animal health. Of the 24 studies reviewed, 12 were classified as long term (more than 90 days and up to two years in duration), and 12 were multigenerational (two to five generations). Many parameters were examined, including biochemical analyses, histological examination of specific organs, hematology, and the detection of transgenic DNA. The authors summed up their findings, “The studies reviewed present evidence to show that GM plants are nutritionally equivalent to their non-GM counterparts and can be safely used in food and feed.”


GM crops represent an important segment of worldwide agriculture, as evidenced by the amount of land used to cultivate them and their widespread use in both human and animal industries. Transgenetic modification allows desirable traits to be imparted to plants, primary of which are insect resistance (Bt technology) and herbicide tolerance (Ht or Roundup Ready innovation). Because of these technologies, crop losses due to pests have been reduced and insecticide use continues to fall. Genetic modification of plants through Bt and Ht methods is heavily regulated by government agencies. While no studies have been conducted specifically in horses, the preponderance of scientific evidence suggests that GM feeds and foods are safe for humans and animals.


Flachowsky, G., H. Schafft, and U. Meyer. 2012. Animal feeding studies for nutritional and safety assessments of feeds from genetically modified plants: A literature review. Journal fur Verbraucherschutz und Lebensmittelsicherheit (Journal of Consumer Protection and Food Safety) 7:179-194.

Snella, C., A. Bernheim, J. Bergé, M. Kuntzd, G. Pascale, A. Parisf, and A.E. Ricroch. 2012. Assessment of the health impact of GM plant diets in long-term and multigenerational animal feeding trials: A literature review. Food and Chemical Toxicology 50:1134-1148.

See the KER Technical Bulletin.