饥饿游戏:奶牛育种未来

The United Nations Food and Agricultural Organisation have estimated that in order to feed more than 9 billion people by 2050, farmers will have to produce 70% more food1. And they need to do it using the same or less land, and despite increasing life expectancies and a rising per capita demand for livestock products. In this article, I am going to explore how...

As I see it we have three options: 1) we can optimise the current systems and resources we have; 2) we can seek out new edible options; or 3), we can wait for passenger services from Elon Musk’s SpaceX and exploit new planets, assuming they have sufficient, accessible protein from evolved systems and resident life forms universally known for their generosity. This is where I leave discussion of the third option, punctuating this line of thought instead with the more contemporary succinctness of Emmanuel Macon, "There is no planet B".

This is also where I reluctantly leave discussion of the second option, despite some fascinating things earmarked to appear on the menus of the future, including insect sausages, cultured (in vitro meat) burgers, and algae seasoning. Instead, I am going to focus on cattle and the dairy industry.

The cow was domesticated around the end of the last Ice Age (about 10,000 years ago) from a now-extinct wild ox species. The main "super power" possessed by these cows and other animals destined to become domesticated livestock was their ability to turn material not suitable for human food (meaning grass and forage) into highly nutritious meat, eggs and milk. Since this time, we have chosen which domesticated animals to breed, not just for those best suited to their environments (as imposed by natural selection) but for those which best suit our needs (as imposed by artificial selection). After generations of selection like this, the modern dairy cow has emerged as a larger, more finely honed and more productive athletic descendant of the bovine family tree.

自1900年代中期以来,动物育种者寻求objectively to measure the "genetic merit" of animals by measuring aspects of performance in many animals and overlaying this on a good family history or pedigree to trace the ancestry of such characteristics. A few statistical formulae later, and we get what is known as a ‘breeding value’ for that particular character in a group of animals. This is then repeated for other characters, and each of these characters are weighted depending on their importance to the breeder to arrive at an overall breeding goal.

If an animal is a country, what do I put the national budget into?

But why would we be interested in anything other than production, I hear you shout. Well, history has taught us that we can’t focus on one character when it comes to animal breeding, no matter how economically important that character is. In a recent lecture, Colorado State University animal scientist Temple Grandin describes the trade-off of energy a cow achieves thus: "If an animal is a country, what do I put the national budget into? If I put it all into the economy for production traits maybe I have no military left to fight off disease... but if you have a strong military, you don’t get very much meat."⁶

Indeed, by focusing almost exclusively on milk yield as an objective dairy cow breeding during the mid-1900s, yields doubled; but this came at the cost of impaired fertility, welfare and lameness. For meat it's no different: Belgium blue beef cattle bred for double muscle can’t give birth naturally anymore and pigs bred for reduced back fat inadvertently became aggressive. And dogs selected for aesthetics, like pugs, suffer breathing problems.

Characters are ‘linked’ like this because the instructions that code for them individually within the DNA often side side by side in the genome. So when you select for one trait, you also inadvertently get the other alongside it. So unless we investigate which traits are linked to those of interest (and account for these), it can lead to undesirable, unintended consequences.

Holistic lessons from history

History has taught us to be holistic, and more recent history is teaching us to err on the side of the animal. And in the same way that our animals have evolved, so too have our breeding goals: milk yield represents only about a third of the weight of contemporary dairy breeding goals. This is at the expense of importance given to health and fertility traits: we are comfortable to accept a cost to production because we realise that we need to preserve fertility and overall health too. Consequently, the animals have a life worth living.

In the pursuit of food security, and meeting the rising demand for food driven by a growing world population, research now also seeks to measure a cow’s efficiency, or channels of wasted energy. These words ‘efficiency’ and ‘waste’ sound much more abstract to measure than ‘production’ (milk yield or carcass weight). But the development of phenotyping technology, and the recent surge in genomics, is helping to revolutionise the accuracy with which we calculate breeding values and select animals to breed for future generations.

Within the dairy industry, genomic selection is expected to double the rate of gain in milk production². This is exciting, especially given a recent United Nations Food and Agriculture Organisation document suggests that agricultural growth could be three times more effective at reducing extreme poverty in low-income countries and 11 times more effective in sub-Saharan Africa than other sectors³. The cost, infrastructure and deployment of such technologies, alongside the collection of good phenotypic measurements, will determine the success of strategies like this. This is because, at a time where genomic information is becoming cheaper and more rapidly obtained, good quality phenotypes may increasingly become the rate-limiting step in continued genetic progress. This means that the need for good quality measurements of important traits on different breeds, in different systems and in different countries is becoming more important than ever.

The world’s longest-running breeding study on dairy cows

Reducing waste and improving efficiency, which are current breeding targets for future dairy cows, and the collection of genomic information, are priorities. At Scotland’s Rural College, research is being conducted on our research herd at Crichton Royal Farm in Dumfries; this is the world’s longest-running breeding study on dairy cows. Such longitudinal and comprehensive data are helping us to test new approaches to measure complex traits over the whole lifetime of the animal as well as to explore how genetics and diet interact. This work will underpin development and deployment of new breeding goals and indexes nationally.

浪费的一个主要来源是疾病,导致to animals dying or being culled, having depressed milk yields or requiring extra or more frequent attention. Selective breeding in disease control has been used to tackle mastitis in dairy cattle and, most recently, bovine TB. It can be favourable because it is a gradual rather than rapid front put up by drugs, which can lead to antiobiotic resistance. Further, advanced reproductive technologies allow us to transfer sperm and embryos rather than live animals, reducing opportunities for disease transmission. On the red carpet of livestock genetics, genomic selection and reproductive technologies have very much become the ‘power couple’ of genetic gain.

As part of their digestion, cows must produce methane because it is a by-product of breaking down the cellulose in grass. In fact, the livestock industry contributes about two thirds of the global methane emissions accountable to humans⁴. Therefore, methane represents another inefficiency (around 2 – 12 % of energy the animal takes in through its food is wasted on methane⁵) in dairy cow production and a main offender in the suite of greenhouse gases (around 25 times more potent than CO2). Research is, therefore, ongoing to see whether we can breed for reduced methane emission per unit of product (milk, meat), by measuring methane emissions on dairy cows (tools currently available range from lasers to bovine backpacks) to assess the genetic merit of some individuals to burp out more or less methane than others. Genetic research on the bacteria ("methanogens") that inhabit the animals' rumens (stomachs) and produce the methane in the first place is also on-going to see if we can select for cows with populations of gut microbes (the "microbiota") that produce fewer emissions.

Methane isn’t the only form of wasted energy, and research is now also addressing the question of "feed efficiency" itself as a trait and measuring energy in (through food), minus energy out (into agricultural product). Animal feed is important but expensive, so breeders are looking for ways to breed animals capable of turning less food into more meat and milk by extracting more nutrients from the food they consume.

The ambitious aim of all of this work is to produce 70% more food by 2050. And in the hunger games of the mid 21st Century, the dairy cow, beef cow, dairy goat, broiler chicken and the other members of the agricultural team, are all key players in the pursuit of future food security..

Steph Smith wrote this article after she met with the Naked Scientists at a workshop they were running recently for the Genetics Society to help scientists to develop enhanced skills in science communication.