You're looking up at giant nostrils. Then the head, slightly bigger than a toaster oven, drops to your humble height to focus on you its unblinking brown eyes. Backing away to a safe distance, you keep in mind that the 2,500-pound body of this dairy bull could send you flying if it grazed your shoulder. Four other bulls line the pens to the right, five more on the opposite side of the barn languidly slap their tails, oblivious to your presence. You could easily be charmed by their brawny build, their stand-offish demeanor, their winsome docility; but they're not here to be charming, they're here for their semen.

"The dairy bull is a unique animal for studying male fertility," says Gary Killian, coordinator of Penn State's Dairy Breeding Research Center. In his office, two laboratories and a hallway down from the bulls, Killian speaks about artificial insemination centers, or "stud services."

"In the entire country there are a thousand or so bulls that are used on a regular basis to provide most of the semen for eight million dairy cows," he says. The situation is ideal for fertility research, he explains, because with "a single animal inseminating so many females, we can statistically say, based on thousands of inseminations, that this is a high-fertility male or this is a low-fertility male. For no other species can you make that kind of statement about a single, individual animal."

Drawing on this wealth of fertility data, Killian, along with senior research aide David Chapman and graduate student Aida Cancel, have found proteins in bull semen that may predict an animal's fertility level in advance of such statistical trials.

Analyzing a wide range of semen samples, they noticed that four proteins consistently showed up at fertility extremes: two appeared in large amounts in the semen of high-fertility bulls, while the other two were prominent in semen of low fertility. Semen dominated by the high fertility-related proteins had little or none of the low-fertility related proteins, and vice versa.

With this correlation in mind, Chapman devised an equation which uses the density of each of the four proteins in a semen sample to calculate a predicted fertility level. When tested against the artificial insemination centers' records, predictions based on the protein readings proved to be 90 percent accurate. Present methods of prediction, which rely on observations such as low sperm count or deformed sperm, says Killian, are much less reliable.

Using this strong link between the proteins and fertility, Killian hopes to develop a diagnostic test that could be used without complex laboratory equipment. With this protein-assay test, a semen sample, and 30 minutes, farmers and stud services would be able to tell from a color reaction whether a bull is a "dud" or a "stud." "In the past," Killian explains, "it hadn't been possible until after you'd inseminated many females to determine if you have a dud. Then time was lost, and potential profits were lost as well."

Chapman believes such a screening test could save large farm cooperatives and stud services millions of dollars; money that would otherwise be spent housing and feeding low-fertility bulls. If only high-fertility semen is used to inseminate milk cows, average dry spells would be shorter -- since each cow has to have calved to give milk. "Even in a herd of 50 cows it might be a difference of one or two more cows getting pregnant," Killian adds. "But one or two is still a lot of milk.

"Reproduction is the key to success in animal agriculture," Killian continues. "If we can improve the fertility of a herd by 5 or 10 percent by using bulls that have high fertility, we can significantly improve the profitability of the enterprise."

Killian, Chapman, and Cancel are now investigating how the proteins may be affecting sperm fertility. By comparing the proteins' amino acid sequences to known sequences of other proteins, they have identified them as common regulatory proteins. "The proteins are not novel discoveries," says Killian, "but they have never been described in the context of fertility," says Killian. He and his colleagues suspect that these regulatory functions of the bull proteins, says Killian, "affect sperm survival, the ability of the sperm to reach the fertilization site, or its ability to penetrate an egg."

Killian believes their future findings could have parallels in other animals -- even humans. Similar regulatory proteins are present in human tissue. These studies of studs could provide insights into developing cures for infertility in human males, or, if some of the proteins are found to inhibit fertility, new methods of contraception.

Gary Killian, Ph.D., is professor of reproductive physiology in the Department of Dairy and Animal Science, College of Agricultural Sciences, 109 Dairy Breeding Research Center, University Park, PA 16802; 814-865-5894. David A. Chapman, M.S., is a senior research aid at the Research Center. Aida Cancel is a recipient of the Minority Access to Research Career predoctoral fellowship from the National Institutes of Health. She graduated from Penn State with a M.S. in biochemistry on August 1991, and is currently pursuing a Ph.D. Chapman and Cancel are located at the Research Center; 814-856-5896. Funding for this project comes from the U.S. Department of Agriculture and artificial insemination cooperatives in Pennsylvania, New York, Ohio, and Wisconsin.