Genetic indexes have been part of the cattle breeding industry for over 25 years yet
even today, many of us feel unfamiliar with their use and unsure of their exact
meaning. In this guide, we go back to basics on some of the most regularly used
indexes, and explain some of the common dairy cattle breeding jargon.
What is a genetic index?
A genetic index is a measure of an animal's ability to transmit its genes on to
the next generation. These could be genes for production, for conformation, for
a combination of the two, or for any other heritable trait that can be
measured. It is important to understand that a genetic index is not a measure
of a physical trait in an animal, which is technically known as it’s phenotype
(e.g. coat colour, stature, milk production etc), but is a reflection of its
genotype (i.e. its genetic make-up). This distinction is important to
understand, since many cows could produce high quantities of milk, but their
high production may be more a function of their management regime than their
genotype. A genetic index therefore makes the best estimate possible of an
animal's ability to transmit a quality, by discounting the effects of its
management regime and other environmental factors. This makes it possible to
How is a genetic index calculated?
For
male dairy animals, genetic indexes are calculated largely from the performance
of daughters, with adjustment made according to the production levels and
management of the herds in which they are milking. However, other factors also
come into play, including the genetic indices of other relatives of the bull
(particularly his sire).
For a female dairy cow the same principles apply but naturally, the animal's
own performance is also an important component of the genetic index.
A summary of the main information used to
calculate an index is as follows:
· The animal's own performance (if available)
· The performance of progeny
· The genetic index of the sire
· The genetic index of the dam
· The genetic indices of all other known relatives
As more information becomes available for the animal itself, so the importance
of the ancestor information declines.
Genetic index for production
PTA - The Predicted Transmitting Ability (PTA) is probably the most commonly used index
and forms the basis of many other indices in the UK. It is primarily a genetic index
for production - specifically the components of milk (weight), fat (weight and per cent)
and protein (weight and per cent). PTAs are calculated for males and females. An example
PTA for a bull could be expressed as follows:
384
kg milk
24.1 kg fat
+0.13% fat
17.4 kg protein
+0.07% protein
Reliability 78%.
This
bull's daughters are expected to give, on average and in their first
lactation, 384 kg more milk, 24.1 kg more fat and 17.4 kg more
protein than cows whose sire has a PTA of 0 kg milk, 0 kg fat and 0 kg protein.
The reliability figure is a reflection of the number of daughters that have
contributed to the bull's proof and their distribution across herds. When
published for bulls, reliability ranges from 50% to 99%. The higher the
reliability, the more likely is the proof to be an accurate reflection of the
bull's true transmitting ability. It is therefore recommended that bulls of low
reliability are not used on a widespread basis within any one herd.
As with all genetic indexes, it is important not to directly compare PTAs from
more than one country. These should be expressed on a UK scale when they can be
easily compared.
PTA 2005 - Currently the PTA is termed PTA2005, the 2005 denoting the year in which the
base, or average cow production was measured. It is less important to
understand the principles of base change, than it is to be aware that PTA2005's
must never be directly compared with PTA2000's (their predecessor). Base
changes generally take place every five years and the next change is due in the
UK and Ireland in 2010.
Genetic index with economic weightings
PIN - Profit Index (PIN) is an economic index derived from the production PTA. It
is expressed as a financial value in £s and reflects the expected increase in
revenue per lactation for each daughter, relative to an animal with a PIN of
zero. The formula for PIN has been calculated to reflect expected future milk
prices; the amount of feed energy required to produce each component of the
milk; the cost of quota and the cost of cooling and transporting milk.
Although pricing systems for milk can now vary widely (ranging from 'white
water' contracts to high protein milk for cheese manufacture), any economic
index has to reflect the economic situation predicted to exist five years ahead
when the offspring of matings currently being planned are having their maximum
effect on a herd. All indications are that the current PIN formula reflects
these predictions.
PLI - Profitable Life Index (PLI) is a very similar economic index to PIN,
but it recognises that milk production is not the only factor to influence profitability,
but that herd life also has an important part to play. The core of its formula is the
same as PIN, but added to this is a weighting for Lifespan.
Lifespan - The Lifespan value itself is measured directly from actual daughter survival and from certain type traits.
The type traits with the strongest association with Lifespan (major reasons for involuntary
culling decisions) are leg and foot composite, mammary composite and somatic cell count.
Locomotion is used where available for UK tested sires to replace leg and foot composites.
Fertility is another major factor involved in culling decisions and the new Fertility Index
was incorporated into overall £PLI in August 2006.
Genetic indexes for type
TM or PTAT - Type Merit (TM) or Predicted Transmitting Ability for Type (PTAT) is a genetic index for
conformation, which indicates a sire's ability to transmit his genes for physical traits on to the
next generation. It is not a measure of the sire's own type, but is calculated from the linear scores
of daughters and how far from the breed ideal the daughters score in comparison to their dams and
contemporaries for each trait.
Type Merit is expressed on a scale of around -3 to +3, with both extremes denoting a likelihood
of movement. A few exceptional animals will fall outside this range. These figures are known as
standard deviations and indicate how far the animal is from breed average.
Pedigree indexes
The Pedigree Index has been developed by Holstein UK to provide an estimate of a young animal's genetic
potential before it has any production information of it’s own. The effective use of the index enables
breeding decisions to be made at an early age and can therefore reduce heifer rearing costs. The
Pedigree Index is based on the average of the parents' genetic merit, but in the absence of parent
information, indexes from further back in the pedigree are used. Pedigree Indexes can be calculated
for almost any index, ranging from the basic PTA to the more complex economic indexes or even type
indexes. Naturally, the Pedigree Index is less reliable than the genetic index the animal receives
after it has production information of its own. It should therefore be used with the appropriate
caution. The reliability of Pedigree Indexes generally ranges from about 20 to 40%.
Other indexes
Confusion often arises over the term Production Index, indicated on milk records, which is in
fact not a genetic index, but an indicator of a female's actual performance (based on weight
of fat and protein) in any lactation within a herd. This is adjusted to a heifer equivalent and
corrected for age and season of calving. The average Production Index of any herd is always 100.
This is often a useful method to determine prospective replacement breeders within any system.
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