"We were working on a project involving
the effects of estrogen in roosters. Lack of sufficient levels of
estrogen can leave roosters sterile," explained Janice M. Bahr, a
professor of reproductive physiology. "As part of the research, we
were examining roosters' reproductive tracts and kept encountering
stones. We began looking into what was causing the stones, and at
first we thought the condition might result from too much calcium in
the feed. But we found out that calcium had nothing to do with the
development of these stones, which also lead to reproductive
The search for the cause of the stones
has unexpectedly indicated that the problem is connected to a
vaccine given chickens to prevent avian infectious bronchitis virus.
This very common virus affects only chickens and not other poultry
animals or humans. To prevent it, chickens are immunized several
times with a slightly altered version of the virus. And the vaccine
works well in protecting hens and roosters from the virus.
However, Bahr's research has turned up
a previously unknown side effect: the formation of stones in the
reproductive tract of roosters. A colleague at the University of
Arkansas first suggested looking at the vaccine after Bahr's
investigation of calcium in chicken feed proved it was not the
"We took fertilized eggs we knew were
pathogen-free for the experiment," Bahr said. "After the chicks were
hatched, we injected the vaccine in one group of roosters, based on
the industry-recommended vaccination schedule, and did nothing to
the other. Eight out of 10 immunized roosters had stones. None of
the roosters that were not injected with the vaccine had stones."
The fact that the vaccine can lead to
stones in the reproductive tract of roosters sheds light on a
fertility problem in broiler chickens. In the broiler industry, most
hens are fertilized by roosters, although there is some artificial
insemination. There is only a 60 percent fertility rate in
floor-mating and a slightly better 65 percent fertility rate for
"This is most likely a problem with the
sperm. We now know that vaccinated roosters have lower fertility
rates," said Bahr. "We found that vaccination with the avian
infectious bronchitis virus results in a reduced testosterone level
by as much as 50 percent."
Maximum hatchability is a prime concern
of poultry producers, she added. Each hatched egg translates into
profit. Today, the hatchability rate in the broiler industry is 83
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"If we could just raise the
hatchability level by 1 percent, it would mean $36 million more per
year for the broiler industry," she said.
In the past, Bahr said, the industry
has not worried too much about the low fertility rate because of the
"sheer number" of birds in a given poultry operation. "The solution
was usually seen as simply putting more roosters on the floor," she
"If roosters are producing sperm that
are only effective 60 percent of the time, that's a waste of
resources," Bahr said. "Simply putting more roosters on the floor is
not the answer."
The challenge now is to come up with a
vaccine against the avian infectious bronchitis virus that prevents
the disease without reducing rooster fertility.
While the vaccine has been used for
many years and the fertility rates have had the same challenge for
many years, researchers had not previously considered a correlation
between the two. In part, Bahr said, that is because research --
like the economics -- in the poultry industry, as well as with most
livestock, has an emphasis on the female of the species.
"You want to develop sows that can
produce more pigs, cows more calves, ewes more lambs, and chickens
more eggs or chicks," she explained. "That accounts for a reason why
this connection wasn't made sooner or more research focused on the
problem. Even though chicken is the world's number one source of
meat, there wasn't much attention paid to the rooster."
Bahr's research was initially supported
by the USDA's Animal Health and Disease Program. She has requested
support from the U.S. Poultry and Egg Association and the USDA.
One paper has been published on the
project, and another one will be in the near future. This novel
discovery linking vaccination with suppressed fertility in the
rooster will be presented at the World Poultry Congress in June in
Istanbul, Turkey, and at the Society for the Study of Reproduction
meeting in August in Vancouver.
many exciting discoveries, finding the connection between the
vaccine and the infertility of roosters was an example of
serendipity," said Bahr.
of Illinois news release]