Frozen Assets

Ninety percent of the nation’s dairy cows—some 8.2 million animals—belong to a single breed: Holstein. Owing to the dairy industry’s extensive reliance on artificial insemination using semen from only the choicest bulls, this Holstein population is heavily inbred. “Today, it’s as if there were only about 35 unrelated cows [contributing genes to] our national Holstein herd,” explains geneticist Harvey D. Blackburn of the U.S. Department of Agriculture (USDA).

If a crippling disease emerged that selectively struck down these black-and-white ruminants, what would milk producers do? They’d turn to Blackburn.

At the National Center for Genetic Resources Preservation in Ft. Collins, Colo., he coordinates a 4-year-old stockpile of livestock “seed”—including enough Holstein semen and embryos to begin rebuilding a massive herd.

Frozen at –196 °C, semen should remain viable for many decades. Some samples in Blackburn’s collection date back more than a half-century. Therefore, he observes, “our collection would appear to be more [genetically] diverse than the actual Holstein population is today.”

If a disease wiped out Holsteins, breeders could rely on other cows to rebuild the Holstein line. Frozen embryos could be implanted into surrogate moms, or Holstein sperm could be used to inseminate, say, Jersey cows—and their offspring—to gradually convert a Jersey line into Holsteins. Barring crises, however, the bank’s frozen vials of sperm will serve as tools for research or improving existing breeding lines.

Other livestock need back-up protection, too. The center’s goal, Blackburn says, is to acquire reproductive material—principally semen—from “at least 50 unrelated animals for every breed [eaten as food].” In the United States, that includes almost 100 breeds. For some breeds, especially rare ones, the center currently stores semen for fewer than 10 contemporary individuals.

With the center’s deposits accruing exponentially—lately at a rate of about 14,000 half-milliliter samples of semen per month—the holdings now include some 150,000 units of semen and about 800 embryos. The collection represents 2,700 individuals from 65 breeds of cattle, swine, sheep, goats, chickens, and fish—including catfish, sunfish, and various trout. In contrast, the plant-gene bank in the Ft. Collins facility has samples from roughly 10,000 plant species (SN: 9/11/04, p. 170: The Ultimate Crop Insurance).

Collecting plant seeds since 1958, the facility accepted its first livestock-sperm deposits in 2000—and they’re requiring special handling. For example, animal germ plasm has to be chilled quickly, at a rate of 20°C to 40°C per minute, or roughly 10 times as fast as the rate for plant material. Even so, for some animal breeds, standard freezing and thawing methods don’t yield useful seed. For instance, turkey semen is, at best, marginally viable after freezing, Blackburn notes, so it isn’t being collected.

Last August, the depository had its first withdrawal. Researchers at the University of Missouri in Columbia ordered certain Holstein semen for a study on the genetics of milk production (see Learning from Studs).

Portfolio choices

In the January Reproduction, Fertility, and Development, Blackburn reported on his survey of foreign repositories of reproductive cells and tissue, including some of the biggest—in the Netherlands, France, and Brazil. All are equally young with similarly small portfolios. A couple-dozen cattle, swine, and sheep breeds account for the majority of their samples.

Some European facilities have taken a broader approach than Ft. Collins has—sampling horses, asses, and other draft animals that are used in farm work but not typically eaten, notes Irene Hoffmann of the United Nations Food and Agriculture Organization in Rome. A few countries even bank germ plasm of shepherd dogs because, in some environments, “you cannot herd sheep without them,” she notes.

Almost 20 percent of the breeds in the Ft. Collins collection are old-style breeds, some represented by no more than 50 to 200 surviving animals (SN: 10/4/97, p. 216: https://www.sciencenews.org/pages/sn_arc97/10_4_97/bob1.htm). For instance, the collection contains semen representing 17 Hereford boars, a breed for which the total population now numbers only about 500. “I’m quite excited at the recognition of the importance of including heritage or rare breeds,” notes Don Bixby of the American Livestock Breeds Conservancy in Pittsboro, N.C.

Even as the nascent depositories build their portfolios, the seed bankers see troubles ahead. Hoffmann, for instance, reports that there is a growing interest in screening germ plasm for disease. Indeed, she worries that seed bankers might exclude samples from “rare breeds held in developing countries, where you never find perfect hygienic conditions.”

Blackburn notes that banking livestock genetic material is expensive. However, he argues, the cost of not doing so can be even greater; it amounts to refusing an insurance policy against the loss of biodiversity in the animals upon which agriculture depends.