Australasian Science: Australia's authority on science since 1938

The ART of Milk Production

Per Tillmann/Adobe

Credit: Per Tillmann/Adobe

By Tamara Leahy & Simon de Graaf

Assisted reproductive technologies play an increasingly important role in the genetic improvement of the high-yielding dairy cow.

The most common rationale for the use of assisted reproductive technologies (ART) in humans is to increase the chance of conception. Terms such as in vitro fertilisation (IVF) or artificial insemination (AI) have entered the lexicon of the layperson as society has become more familiar with the use of technology to assist reproduction.

What is less familiar to the general public is that assisted reproduction is common in agriculture, not only to correct fertility problems but more commonly to drive genetic improvement and enhance the production efficiencies of the next generation of farm animals. Assisted reproductive technologies are used in almost all animal industries but have been particularly heavily adopted by dairy farmers.

Take the case of the bull known as Badger-Bluff Fanny Freddie (or simply Freddie). In 2012 Freddie was the top-ranked proven producer of cows in America. This ranking was derived from the production and health traits of Freddie’s daughters to determine an overall profitability score. In 2012, Freddie’s daughters were considered $792 more valuable over their lifetime than the average dairy cow, which made Freddie the most desirable bull in the country.

If left to his own devices, Freddie could pass on his valuable genetics to approximately 50 cows per year through natural mating. This would be beneficial to the one farmer who owned those 50 females, but would have minimal impact on the industry at large.

Genetic improvement relies on high selection intensity, good accuracy of selection and a short generation interval. What this means is that genes that are deemed to be superior need to be disseminated widely and passed on as quickly as possible. To do this the dairy industry employs AI.

Artificial Insemination

Artificial insemination is an assisted reproductive technique in which sperm are collected from the male and artificially introduced into the female reproductive tract for the purpose of conception. AI methods were developed at the turn of the 19th century and today 84% of dairies in Australia use this technology.

Such manipulation of the natural mating process has seen milk production in Victorian dairy cows more than double in the past 30 years despite cow numbers remaining steady and the number of hectares they occupy halving. Less inputs for greater outputs is not only better for the producer but is a more environmentally friendly and sustainable agricultural approach.

The ability to extend the fertile life of sperm was critical to achieving these advances. This was achieved by lowering the metabolic rate of sperm by storing them at cooler temperatures (5–15°C) in the presence of protective agents such as milk and egg yolk. Under these conditions, sperm could remain viable for transport for a few days so that semen can be delivered to many properties. It is a testimony to these early studies that no superior alternatives 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 material 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 interaction 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 environment 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 approximately 7% of the North American Holstein cow genome population. 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 difficult 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 ovulation, 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 populations 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 staining sperm with a DNA-binding fluorescent dye, exciting the individual 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 administration 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). Accordingly, 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 genes involved 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 population increase by 30,000 at the end of 2015.

The main advantage of GEBVs is that animals with a desirable 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 inferior 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.