Recombinant DNA technology has had a major effect on agriculture, where it is now used to create crop plants and domestic animals with valuable traits. For many years, plant pathologists had recognized that plants infected with mild strains of viruses are resistant to infection by virulent strains. Using this knowledge, geneticists created resistance to viruses in plants by transferring genes for viral proteins to the plant cells. A genetically engineered squash, called Freedom II, carries genes from the watermelon mosaic virus 2 and the zucchini yellow mosaic virus, which protect the squash against viral infections.

Another objective has been to engineer pest resistance into plants to reduce dependence on chemical pesticides. A gene from the bacterium Bacillus thuringiensis, which produces an insecticidal toxin, has been transferred into corn, tomato, potato, cotton, and other plants. These Bt crops are now grown worldwide. Other genes that confer resistance to herbicides have been introduced into a number of crop plants. During 2013, over 18 million farmers worldwide planted 175 million hectares of genetically engineered crops. In the United States, 93% of all corn, 96% of all cotton, and 94% of all soybeans grown in 2014 were genetically engineered.

The genetic engineering of agricultural products is controversial. One area of concern focuses on the potential effects of releasing novel organisms produced by genetic engineering into the environment. There is also concern that transgenic organisms may hybridize with native organisms and transfer their genetically engineered traits. Other concerns focus on health-safety matters associated with the presence of engineered products in natural foods. Some critics have advocated the labeling of all genetically engineered foods that contain transgenic DNA or protein. Such labeling is required in countries of the European Union but not in the United States.

On the other hand, the use of genetically engineered crops and domestic animals has potential benefits. Genetically engineered crops that are pest resistant have the potential to reduce the use of environmentally harmful chemicals, and research findings indicate that lower amounts of pesticides are being used in the United States as a result of the adoption of transgenic plants. Studies conducted in China show that when Bt crops are used, farmers spray less chemical insecticide, allowing more predatory insects to survive and provide natural pest control. Transgenic crops also increase yields, providing more food per acre, which reduces the amount of land that must be used for agriculture. Genetically engineered plants offer the potential for the greater yields that may be necessary to feed the world’s future population.