HEIFERMAX

In a recent article, I wrote about feeding baby calves well enough to achieve high growth rates in the first eight weeks of life, so that later in life they can reach their genetic potential for milk production.

“Genetic potential” is one of many terms and concepts which are often used in discussions but which perhaps not everyone completely understands. This seems like a good time to explain a little about the complex issue of how characteristics or traits are inherited, what the term “genetic potential” really means and why heifers might not reach it.

Firstly, some definitions:-

1. DNA is found in almost all living organisms. It is the “instruction manual” by which hereditary characteristics are passed from one generation to the next.
2. A gene is a piece of DNA that carries information about a specific trait.
3. Genes can be either dominant or recessive.
4. A chromosome is a string of genes connected together.
5. Cattle have 60 chromosomes, 29 pairs of autosomes (a chromosome other than one that determines sex) and 1 pair of sex chromosomes.
6. All normal male mammals, including humans, have an X and a Y chromosome and females have two X chromosomes.
7. Children inherit one chromosome from each parent. The chromosomes in a chromosome pair are not identical, since one comes from each parent.
8. An allele is a gene that is a member of a set of genes that all belong to the same location, on a chromosome.
9. A genome is all the genetic material in an individual organism. Therefore, genomics is the study of all DNA of an organism. Genomics allows scientists to understand the structure and function of genes and how they are controlled and interact.
10. Heritability is the degree to which a specific trait is able to be transmitted from one generation to the next.
11. Genetic potential is the maximum level of a particular trait that an animal is capable of attaining.

There are two interdependent factors which influence what living things become. One is genotype and the other is phenotype.
All living creatures have fixed characteristics, which remain constant unchanged by environmental factors – this is their genotype.

Phenotype is the physical expression of this genotype, which can be influenced by external factors in the animal’s environment. To give an example that most farmers can relate to, genes within a river red gum seed contribute to a genotype which specifies that it will grow into an autotrophic organism with roots which derive nutrients from the soil, a solid trunk composed of cellulose and lignin which divides into smaller branches made of the same material supporting a canopy of green leaves containing chlorophyll which can use sunlight to convert CO2 and water into carbohydrates to be used as an energy source. No matter what the environment it grows in, a healthy tree will still have all these interacting characteristics in some shape or form. The redgum seed might have the genotype to become a tree of 35 metres high with a girth of 6 metres. However, if that seed germinates in 2 teaspoons of soil in a crack between two big rocks, the physical limitations of that seedling’s environment will alter its phenotype by preventing it reaching its genetic potential for height and girth. It will be lucky to reach a height of 35cm, and indeed this is exactly what bonsai trees are – trees which have had their phenotype manipulated by their environment, to the extent that they are unable to reach their genetic potential for size.

Photo - Gary Matthews

These river red gums have had their phenotype manipulated by the wind; instead of reaching a height of 35 m, they have only grown to about 3 or 4 metres and the normally upright trunks have been influenced by external factors to assume a size and shape different from that ordained by their genotype. The genotype which specifies the way the trees survive has remained unaffected by the environment.

Every cell of every living thing (plant or animal) contains sets of instructions called genes. The genes provide the instructions on what type of plant or animal it becomes, what it looks like and how it survives and interacts with its environment.Genes are linked together to form chromosomes and most animals have pairs of chromosomes which are made up of one chromosome from each parent. Humans have 23 pairs of chromosomes, cattle have 30 pairs. Sometimes, in the process of replicating or copying, the DNA which comprises the genes, a mistake occurs; this is called a mutation. Congenital defects such as Down’s Syndrome result from incorrect copying of the DNA.

Some genes have a quality known as dominant or recessive. A gene is said to be dominant when only one gene (rather than two) is sufficient for the expression of that trait to which the gene relates. There need to be two recessive genes (one from each parent) if a trait is to be expressed.

Dominant traits are more common than recessive traits due to this quality. To simplify this, if a child inherits a dominant gene from its father and a recessive one from its mother, the dominant one will prevail. In the simplified illustration below, where black is the dominant coat colour and white is recessive, a black cat (with 2 dominant genes) and a white cat (with two recessive genes) produce four kittens. Even though all the kittens carry a black gene and a white gene, they will all be black, as that is the dominant gene. The father’s genotype with two dominant genes (BB) is called homozygous dominant, the mother’s genotype with the two recessive genes (ww) is known as homozygous recessive, and all the kittens are heterozygous (Bw) because they have two different genes.

The illustration shows that the heterozygous kittens, which have only one gene for black colouring, can look the same as their homozygous black father which has two genes for black colouring. They look completely different from their homozygous white mother. In other words, the looks (or phenotype) of the animal are not necessarily an accurate predictor of what their progeny might be like.

Very few inherited qualities are so clear cut as the above illustration. There are many inherited characteristics which are polygenic, meaning that multiple genes decide the ultimate outcome. Human hair colour is an example where several genes interact to decide the ultimate hair colour.

Some genes are linked to sex; a well known example of this is the gene for colour blindness. Colour blindness affects about 8% of males but only 0.4% of women. This is because the recessive gene which causes the most common forms of colour-blindness is located on the X chromosome. Remembering that females have two X chromosomes and men have an X & Y chromosome, it is easy to understand that in women, the recessive gene on the X chromosome from one parent is normally over-ruled by a dominant gene for normal colour vision on the X chromosome from the other parent. In men, who lack the second X, the recessive gene may be the only one they inherit, as there is no over-riding dominant gene from the Y chromosome.

Heritability is a measure of the extent to which a particular trait is capable of being transmitted from parent to offspring. In other words it is a measure of how much of that trait observable or measurable in the parent animal (the phenotype) is caused by genotype & can therefore be transmitted to its progeny and how much caused by external influences. Heritability is measured as a percentage with very low in the range of 1% - 10%; low, 10% - 20%; moderate, 20% - 35% and high, 35 - 60%. The level of heritability of particular traits is used to establish EBVs, establish selection criteria for sires and plan breeding programmes. The degree of heritability of a certain trait is a good indicator of how useful the phenotype of an animal is in predicting its genetic merit or breeding value. Only in traits where heritability exceeds 40% is the phenotype of the parent is a good indicator of its genetic merit. This degree of heritability can explain why a mating between a really good working dog & bitch does not always produce pups which work well; in other words, the phenotypic working traits observed in the parents have low heritability.

Some traits have low heritability partly because there are many external factors which can influence the phenotype of the offspring. No matter how fertile a line of bulls has been, and how heritable that fertility might be, it will not ensure a high level of fertility in a bull calf after he has been castrated; this is a simplified example of the expression of an animal’s genetic potential being manipulated by its environment.

Dominant/recessive single gene traits are the most heritable and predictable; the heritability of polygenic traits such as milk yield is less predictable. In general, health, fitness, fertility and survival traits have low heritability. Milk or protein yield are moderately heritable and fat, lactose and protein percent have high heritability. The degree of heritability of a trait has no relationship to the economics of that trait. Many traits with low heritability have high economic value. The ability to predict the heritability of these traits can be improved by increasing the number of progeny measured for the trait, using information from previous generations or taking more measurements from the animal itself. Expression of these low heritability traits can be improved through better environmental conditions (i.e. manipulate the phenotype). Both these strategies will deliver economic benefit.

The main purpose of this article is to give farmers an understanding of the complexities of the genetic make-up of animals and of the possible interactions between the various genetic and environmental factors which influence calf health and growth rates and ultimately milk production and survivability. This understanding should include the fact that a heifer’s genetic potential for milk production is the maximum production level she is capable of attaining. Her phenotype (or ability to reach this genetic potential) is limited by many interacting factors, the effects of which start at or before birth and continue until she dies.

In a future article I will explain why the industry needs to re-examine the current recommendations for feeding baby calves, in the light of research which demonstrates that better nutrition and management prior to weaning can increase lifetime performance and efficiency of milk production and make it more likely that heifers reach their genetic potential for milk production.

I recently returned from working in Sabah, East Malaysia. The trip gave me the opportunity to gain an understanding of a dairy industry which is significantly different from many of the others I have seen in Europe, the U.S.A. & Canada.

Even the sub-tropical dairy industries in Florida and Queensland are significantly different from the industry in Sabah.

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Farmers often think that losing a few calves a year is not a big problem. In fact, dead calves are actually the tip of the iceberg. Related illness and poor feed conversion efficiencies & growth rates in surviving calves cost far more money than the value of the dead calves.

Heifers are going to cost money to rear; particularly when money is short, it needs to be spent where it will give the greatest return. Low milk prices often result in farmers attempting to save money by reducing the costs of calf feeds. Calves which are not adequately fed are more likely to become ill & die. Experience of large scale calf rearers would say that, in general, for every dead calf there are about 5 sick ones.

The death of only 2% of heifers reared is often viewed as a good result. The following illustration estimates the unseen costs and lost productivity associated with 2 deaths from a group of 100 calves. Using “middle of the road” figures based on a Holstein herd, the “iceberg” shows the hidden costs of economising on feed and other rearing inputs.

Whether the figures are exactly accurate for every herd is not important; the point is that by cutting costs to save cents now, the enterprise will be losing dollars both now and in the longer term. Long term productivity will certainly be compromised, possibly at a time when milk prices have risen and dairies are looking for maximum productivity to make up ground lost to poor prices.

The above illustration calculates the costs associated with sick calves and associated lost productivity to be a startling $53,340. Read more »

I am just writing the report required of me by RIRDC at the conclusion of my year as Tasmanian Rural Woman of the Year. In writing this report, I gave some thought to the question of what I perceived to be the greatest challenge facing the calf rearing industry.

I have seen 100’s of 1000’s of calves being reared for a variety of reasons, both within Australia and in other parts of the world. My belief is that the greatest problem facing the calf rearing industry is a common thread uniting these different enterprises and different countries.

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