But the knife remains suspended in the air as the regulatory body, Food and Drug Administration (FDA) is yet to make up its mind on whether and how such meat or milk could be allowed to enter the nation's food supply.
Pigs whose manure causes less pollution and which make fatty acids, normally found in fish, and hence good for health, disease-resistant cows, goats whose milk might help ward off infection in children and catfish needing fewer antibiotics are among the wonders chiseled by US bioengineers.
Genetically engineered animals are often created by injecting the gene of interest into a single-cell embryo. A more recent technique that is more efficient is to put the gene into a skin cell and create an embryo from that cell by cloning.
In both cases, the embryo with the foreign gene is then implanted into the womb of a surrogate mother. After some transgenic animals are born, additional ones can be made by conventional breeding, because the foreign gene generally will be passed on to some of the offspring, as would any other gene.
But even 15 years after it started sifting the data available to it, the FDA will not say when its rules for this sector would be ready.
"We want to get it out, but we also want to get it right," insisted Julie Zawisza, a spokeswoman for the agency.
The FDA is turning to transgenic animals after having tentatively declared in December that milk and meat from livestock that is cloned — but not otherwise genetically manipulated — was safe for people to eat.
It considers clones to be less biologically radical than genetically engineered animals — which instead of being mere replicas of naturally occurring animals are given foreign DNA, usually from another species.
Larisa Rudenko, a senior biotechnology adviser in the FDA's veterinary drug division, said in a May presentation at the biotechnology industry's annual convention that each new type of genetically engineered animal would require approval for use in the food supply. That will be done, she said, under the umbrella of existing rules for drugs used in treating animal diseases.
The fast-growing salmon is the transgenic animal that has been swimming upstream the longest at the FDA. Its developer, Aqua Bounty Technologies of Waltham, Massachusetts, has been working to win agency approval for about 10 years.
Elliot Entis, Aqua Bounty's chief executive, said the company had already given the FDA. studies showing that the fish were healthy and that the implanted gene remained stable over generations.
He said the company also had tests done to show that its fish contained the same level of fats, proteins and other nutrients as other farmed salmon and would not set off unexpected allergic reactions in people who eat them. The fish also taste the same as other farmed Atlantic salmon, Entis said.
"Nobody has ever analyzed salmon as closely as we have had done," he said. But the FDA is asking for more data on safety and potential environmental effects on wild salmon.
Industry executives say the Enviropigs would be the next candidate for FDA approval.
Less far along in the approval pipeline are pigs that contain a gene from the roundworm allowing them to produce omega-3 fatty acids, a nutrient normally found in fish that is good for the heart. That, in theory, could make eating pork or bacon healthier, although that has yet to be tested.
If only the FDA finalizes its rules on the issue, big money would come in pouring for the transgenic livestock industry, experts say.
"Right now, it's very hard to get any corporate investment," said James D. Murray, a professor at the University of California.
But some experts caution that even if the FDA clears the regulatory path in coming months, investors and agribusiness companies might still shy away. Many fear that consumers would shun foods from transgenic animals, sometimes referred to as genetically modified organisms, or GMOs.
"The companies we have spoken to have gone organic, and they are very concerned, at least up to the present time, of having GMO associated with their name," said Cecil W. Forsberg, a professor at the University of Guelph in Ontario, Canada, who helped developed the "Enviropig" with cleaner manure. Smithfield Foods, for one, the world's largest hog producer and pork processor, says it is doing no research on genetically engineered animals.
But critics say changing the genes of animals could lead to potentially harmful changes in the composition of milk or meat, like the introduction of a protein that could cause allergic reactions. They say there could also be risks to the environment if, for example, extra-large salmon were to escape into oceans and out-compete wild salmon for food or mates. Some also say that some of the processes used to create transgenic livestock can harm the animals themselves.
Carol Tucker Foreman, director of the Food Policy Institute at the Consumer Federation of America, a consumer advocacy group in Washington, said regulations might not assuage consumers, many of whom object to the genetic engineering of animals on humane or ethical grounds, more than on safety concerns.
"The fact that the FDA has a powerful regulatory process for reviewing genetically engineered animals, far greater than they apply to genetically engineered crops, may not make any difference at all," Ms. Foreman said. "Because that's not what it's all about."