sperm with a DNA-binding fluorescent dye, exciting the indi-
vidual sperm with a laser and then measuring the intensity of the
light that the excited dye emits. Sperm with relatively higher or
lower levels of light emitted can then be assigned a positive or
negative charge and separated into X- or Y-bearing populations
using magnets.
This technology exposes sperm to several processing stressors,
and results in a moderate reduction in fertility (~20%) when
“sexed” semen is used compared with conventional samples. The
technology is now applied to a range of species including domestic
animals, humans and wildlife, and has produced several million
pre-sexed calves worldwide.
The ability to preferentially produce female calves has many
benefits, such as reduced birthing difficulties with heifer calves
(as they are smaller), while fewer unwanted male “bobby calves”
allows for more female replacement animals. Recent research even
suggests that lifetime milk yield is increased if the first offspring
produced by a cow is female rather than male.
These benefits provide compelling reasons to use sexed semen
in many situations despite the moderate reduction in fertility that
this technique brings.Disseminating Female Genetics
Artificial insemination is a highly effective tool to spread the genes
of highly valuable males, but what if you have a highly valuable
female? How do you disseminate her genetics?
In this case, a technology known as multiple ovulation embryo
transfer (MOET) is performed. MOET involves the adminis-
tration of hormones that stimulate ovarian follicles to grow and
ovulate (similar to what women go through in the stimulation
phase of an IVF cycle). Around the time of ovulation, these cows
are artificially inseminated and the numerous eggs within the
female are fertilised and become embryos. A few days later these
embryos are collected and transferred to surrogate animals that then
carry the offspring to term. In this way a female can pass on her
genes to many more offspring than she could naturally carry in her
lifetime.
The obvious limitation of the technology is that it is not as
easy to retrieve the genetic material of females (oocytes, embryos)
as it is to obtain the genetic material of males (semen). Accord-
ingly, MOET is more expensive and invasive than AI, and has
not been as widely adopted by the dairy industry.The New Era of Genetics
To finish we should return to the case of Badger-Bluff Fanny
Freddie. In 2009, many years before Freddie’s daughters revealed
him to be the best bull in the land, an analysis of Freddie’s genetic
code identified him as a superior sire. Genomic estimated breeding
values (GEBVs) are calculated from the sum of thousands of
DNA markers across the bovine genome that are linked to genesinvolved in the regulation of key traits (e.g. fertility, milk yield,
disease resistance). To perform such calculations, scientists compare
DNA markers from a large reference population with data recorded
for traits of interest.
The dairy industry is uniquely suited to this model because it
has highly accurate records of quantitative phenotypic traits.
Genomic evaluation was introduced in Australia in 2011 by the
Australian Dairy Herd Improvement Scheme. The Australian
reference population of genotyped cattle now contains more than
10,000 Holstein cows, and the genotypes of thousands more bulls
and heifers are added each year.
The reliability of GEBVs is slightly lower than that of a proven
bull. While the bull’s pedigree will have phenotypic data from
more than 50 daughters, GEBVs are estimated to contain about
the same amount of information as phenotypic data recordings
from 30 daughters.
The reliability of GEBVs will improve as the number of animals
in the reference population with both phenotypes and genotypes
increases. A recent project run by the Dairy Futures CRC and
Holstein Australia saw the number of cows in the reference popu-
lation increase by 30,000 at the end of 2015.
The main advantage of GEBVs is that animals with a desir-
able genetic profile can be used for breeding as soon as they become
sexually mature (2 years) rather than having to wait until progeny
testing is completed 3 years later. Additionally, genetically infe-
rior males and females can be identified at a very early age and
removed from the herd, reducing the costs associated with raising
less productive animals.
The early identification of Freddie’s superior genetics in 2009
allowed him to be bred from an early age. As of August 2015
Freddie boasted 27 sons in the top 200 proven sires of USA
Holsteins. Such a feat would not be possible if Freddie had only
started breeding in 2012.
Heifers can also be commercially genotyped in Australia from
$50 per animal to guide producers about which animals to retain
as replacements and to speed genetic progress through female
selection pathways.
The combined use of traditional genetic selection programs
with AI has irrevocably changed the dairy cow genome over the
past 40 years to produce more efficient cows. New genomic tools
are becoming cheaper and more reliable, and will no doubt have
an even more profound effect in the near future.
We will better understand the “Freddies” of the future and
will be able to breed from them with unheralded opportunity
due to past and present efforts of reproductive biologists and
geneticists. Such knowledge will be critical to address emerging
concerns, such as the selection of animals that can cope with more
extreme weather patterns or have lower methane emissions.
Tamara Leahy is a postdoctoral researcher and Simon de Graaf is a Senior Lecturer in The
University of Sydney’s Faculty of Veterinary Science.34 | JAN/FEB 2016