erties. It is a testimony to these early studies that no superior alter-
natives to these biological products have been identified for cool
storage more than 50 years later.
However, the greatest breakthrough in sperm storage was the
accidental discovery that glycerol allowed sperm to survive freezing
and thawing. The ability to freeze sperm meant that genetic mate-
rial could be stored indefinitely and transported globally, creating
an international semen market where good genes could be shared
by all rather than owned by a few wealthy breeders. Under these
conditions one frozen ejaculate of about five billion sperm could
theoretically allow a bull like Freddie to inseminate 250 cows
anywhere in the world, at any time. In this way a few ejaculates from
Freddie would impregnate more cows then he could using his
own natural talents in his entire lifetime!
Artificial insemination was also crucial to dissect the interac-
tion of genes and the environment on phenotype (an organism’s
observable traits). Before AI it was hard to accurately identify
superior sires due to the confounding effect of each farm’s envi-
ronment on the progeny’s performance. If a bull’s daughters were
only measured on one farm then their superior performance traits
may largely depend on farm management and environmental
conditions rather than good genes.
The use of AI meant that individual bulls could be directly
compared over various environmental conditions. In this way AI
was not only crucial in disseminating superior genetics but in
developing advanced sire evaluation methods that allowed the
accurate identification of desirable genes.The Downside of Unrestricted Breeding
The intense selection for high milk-yielding daughters and an
almost unrestricted ability to breed from superior sires was not
without its problems. Walkway Chief Mark, a bull born in 1978,
produced daughters with high milk yields and sired more than
60,000 offspring via AI. (Compare this to the human situation,
where the number of births allowed from a single sperm donor in
NSW is currently five.) Many of Walkway’s sons were also popular
sires, and it is estimated that his genetics account for approxi-
mately 7% of the North American Holstein cow genome popu-
lation. These high rates of inbreeding in the Holstein cow
population, and the increasing genetic drive to turn feed into milk
at the expense of health, led to cows that were increasingly diffi-
cult to impregnate.
Some of these issues were overcome by the development of
novel breeding technologies. For example, the traditional method
of insemination after the detection of oestrus (a cow’s fertile
window) was abandoned as decreasing fertility meant that many
more cows underwent a silent oestrus that could not be detected
easily. To counteract this problem, fixed-time AI protocols, which
involve the hormonal stimulation and synchronisation of ovula-
tion, were introduced so that insemination could occur without
the need to detect oestrus.
Breeding programs were also refocused to take a more multi-
faceted approach to the design of the perfect cow. Breeding values
now contain multiple selection criteria that cover a range of health
and reproductive traits rather than solely focusing on milk yield
and composition. While such efforts have lifted the fertility of
dairy cows, vigilance is required to maintain a genetically diverse
and fertile population.Getting the Sex You Want
As milk production is a female-specific trait, the ability to skew the
population in favour of heifers has been a long-desired goal of the
dairy industry. In 1989 the ability to generate sex-selected offspring
was made possible through the modification of a flow cytometer
to separate sperm into X and Y chromosome-bearing popula-
tions based on their difference in DNA content.
In cattle, X-bearing sperm contain 3.8% more DNA than
Y-bearing sperm. This small difference can be detected by stainingJAN/FEB 2016|| 33Per Tillmann/Adobe