Animal husbandry is an extension of biotechnology which allows geneticists to introduce genes or a fragment of gene code of one organism into the full genetic code of another. This is done after birth mainly by feeding farm animals hormone laden foods, as well as genetic vaccinations. Prior to birth, foreign DNA can be injected into the nuclei of egg cells or early stage embryos.
Once an organism contains any number of genes from another, it is refered to as a transgenic organism. Today, more and more farmers in the agricultural society are more willing to experiment with transgenic animals because of their proven advantages as per their health, productivity, and overall growth. In general, it is far easier to manipulate plants in this manner rather than animals due to the fact that entire plants can be regrown from few cells, and that Ti plasmids are easily used as a genetic vector for introducing new genes to the plant.
A more traditional definition of animal husbandry has always been the prolonged care and breeding of domestic animals. The advent and growth of genetics as a respected field of science in the 20th century has led to a gradual evolution of the definition.
Source
Biology, Third Edition by Campbell, Chapter 19: DNA Technology