There are different ways of assessing
genetic progress depending on the focus
of the study. An individual breeder with
a limited number of mares, and a
studbook which is responsible for an
entire population, although on a
different scale, are both trying to
achieve the same goal: that is the
improvement of the next generation of
horses.
Figure 1. Each year the best stallions are
selected to go into a Performance Test during
the ‘Stallion Selection’ event at Den Bosch,
Netherlands. The Performance Test is the last
stage before approval. (Photo: Dirk Caremans,
courtesy KWPN)
Most European studbooks have a
method of scoring horses for their
individual genetic potential in the
different disciplines (dressage, and show
jumping, and some also score for
hunting). For now it is sufficient to
know that the method of identifying
the suitability of individual horses for
their sport is based on scientific
principles, and that it is currently the
best way of adjusting for biasing factors
like nutrition, training method, rider
and other factors that affect the actual
performance, but which are nonhereditary.
The end result is a number
assigned to each individual animal,
which tells how good the horse is
relative to the rest of the population. As
an example, the KWPN (Royal Dutch
Warmblood Studbook) has set the
average for jumping ability and for
dressage ability of all registered dressage horses
and show jumpers to 100. Any horse that is genetically
better than the average has a value
higher than 100; any horse that is
below average gets value less than 100.
This number is called the breeding value
(sometimes also a ‘breeding index’) and
the value tells us how far the horse
deviates genetically from the population
average. So if a horse in this studbook
has a value below 80 for dressage
ability, it means that this horse is not a
naturally talented dressage horse, and it is
unlikely to produce foals that are. But
this does not necessarily make the horse
undesirable. The same horse may have
a breeding value for show jumping of
130, meaning that it is far above
average for show-jumping ability and is
more likely to produce foals that have
similar ability. As a breeder we have a
goal: we want to breed an excellent
dressage horse, or an excellent show jumper,
or maybe both. Therefore, we
want to pick a stallion that has a high
probability of passing that ability on to
the next generation.
Figure 2. Champion of three-year old show
jumpers 2008 at the Stallion Selection at Den
Bosch, Netherlands: A.Adermie, sired by
Namelus R (Concorde x Joost), a potential
candidate to improve jumping ability. (Photo:
Dirk Caremans, courtesy KWPN)
This implies that we should use a
stallion that has a high breeding value
in the discipline we are interested in. At
the same time, we may be trying to
correct for conformational and/or
movement traits in the mare as well.
But if you follow these principles it is
more likely that genetic progress will be
made on the trait that is important to
you, and your foal will have a better
ability to perform in your discipline of
choice than its parent, your mare. The
studbook is trying to achieve the same
thing but on a much larger scale. It
strives for a generation of horses that
are better at what they are bred for,
than their ancestors; so the studbook is
also seeking genetic progress.
Let’s have a closer look at the methods
that the studbooks use to achieve
genetic progress. We consider the
‘population’ as all horses that are
registered with a studbook- the
principle is generic so this could be any
studbook, like KWPN, Holsteiner
Verband, Hanoveraner Verband,
Oldenburg etc. All horses within the
population have one breeding value for
jumping ability and one for dressage
ability. For each breeding value, the
population can be subdivided into
different classes of ability, let’s say
‘average’, ‘very good’, ‘superb’, ‘poor’
and ‘very poor’. For the KWPN we could
create classes for the range below 60
(very poor), 60 to 80 (poor), 80 to 120
(average), 120 to 140 (very good), 140
and higher (superb). As you may expect,
most horses will fall in the category of
‘average’ (68.3% of the population are
in this category). A much smaller
number falls in the category of poor
and very good (13.6% each) and an
even smaller number in the categories
of very poor and superb (2.3% each).
This distribution pattern is identical for
the male and female horses in the
population.
If we graph this, we see a pattern like in
figure 3.
Figure 3. Sizes of the groups with a low to high genetic merit within the population.
In order to be able to improve the next
generation, the studbook has to follow
a number of guiding principles. The
most important one is to select within
the population of stallions, those that
are above average, and preferably as far
above average as possible. The system of
stallion-approval into the studbook
represents their policy to select stallions
that have a breeding value that is
higher than the population average or
that have a high probability of being so.
Depending on the number of stallions
required to maintain the desired
population size (there being a maximum
number of mares that can be bred by
one stallion), and to avoid problems
associated with inbreeding, the
studbook may have a policy of only
approving stallions that have a breeding
value of higher than 110, or higher
than 120. In the case of the KWPN, the
average breeding value for dressage and
jumping of approved stallions is 110,
with extremes up to 170 and higher. By
using the best stallions in the
population, breeders can make genetic
progress.
Let’s assume breeders choose stallions
with a breeding value of 120. This
would mean that the average breeding
value of the next generation will be 110
(the average of the parents) and the
genetic progress in one generation is 10
points. In figure 4 you can see how the
whole population has moved to a higher
average. The difference between the old
peak and the new one shows genetic
progress.
Figure 4. Breeding values of the offspring (coloured) compared to the original mare population (blue
line), when stallions of a high genetic merit are used.
The studbooks work continuously on
selecting the best stallions within their
respective populations, so each
generation of horses improves over the
previous one. They keep track of
individual performances of the stallions,
and equally important, of their progeny.
By following the principle of selecting
the best parents to produce the
following generation, the studbooks
have achieved enormous results for
their members.