Specialists or All-rounders?
Degrees of heritability and genetic correlations in the Hanoverian breed.
by Dr. Ludwig Christmann


Reprinted with permission from Dr. Christmann

"How safe is it to assume that the shape of a horse's head will be passed on from one generation to the next?  What happens to the basic gaits and the typiness if we select for jumping ability only? Ludwig Christmann researched these kinds of statements in the course of his study of the development of the breed value estimation based on the results from mare inspections and mare performance tests at the Institute  of Animal Breeding at the University in Goettingen. These results are not only necessary for the estimation of the breed values, they, in themselves, also bear important information with respect to breeding."

The scores of mare inspections and mare performance tests of a total of 5,347 mares tested from 1987 to 1993 were taken in to consideration for this project.

The degree of heritability is a way to measure the genetic strength of a characteristic, up to which degree a characteristic will be passed on to the next generation. The investigation is based on the  fact that related animals are more alike than unrelated animals. The heritability can be expressed in percentage. A heritability of less than 20% is considered insignificant, a heritability of 20-40% is considered average, and above 40% strong. Obviously it makes only sense to evaluate heritable characteristics and use these as selection criteria. The stronger the heritability, the faster we can achieve progress in our breeding program. An average heritability degree is a very useful tool in animal breeding.

Chart 1 portrays the degrees of heritability of those characteristics scored at mare inspections. Clearly the type determining traits - head, neck, saddle position and frame - are located in the average to strong area with 23 % (frame) and 41 % (head). The degrees of heritability for the foundation characteristics like front legs (16%), hind legs (18%), correctness (14%) are significantly lower. This statement conforms to the results of other authors, even with the results regarding other animal species like cattle.

We have to presume that the environment greatly influences the development of the limbs. I define the word environment as the area where a horse grows up starting at its position in the womb to hoof care feeding  and keeping. The heritability of the size (%36) is fairly high, which coincides with observations in real life.

Chart 1: Degrees of heritability of characteristics scored at mare inspections
(computed by means of a BLUP animal model)
Breed &
sex type
***** ***** ***** ***** ***** ***** 34%
Head ***** ***** ***** ***** ***** ***** ***** ***** 41%
Neck ***** ***** ***** ***** ***** **28%
Saddle 
position
***** ***** ***** ***** ***** ***** ***** 38%
Frame ***** ***** ***** ***** **23%
Forelegs ***** ***** ***** *16%
Hindlegs ***** ***** ***** **18%
Correctness
of gaits
***** ***** **14%
Size ***** ***** ***** ***** ***** ***** ***** 36%
%   5  10  15      20     25      30     35      40      45     50

Chart 2 reflects the degrees of heritability for traits evaluated at mare performance tests. The heritability of the basic gaits lies between 26% (walk) and 37% (trot), an area which is very useful in breed issues. The trot is the movement with the highest degree of heritability. This fact coincides with statements from other research programs. The heritability degree for rideability lies with 29 % in the same area as the heritability degree for basic gaits.

Chart 2: Degrees of heritability of characteristics scored at mare performance test
(Computed by means of a BLUP animal model)
Trot ***** ***** ***** ***** ***** ***** ***** *37%
Canter ***** ***** ***** ***** ***** ***** 30%
Walk ***** ***** ***** ***** ***** *26%
Rideability ***** ***** ***** ***** ***** ***** 29%
Free jump ***** ***** ***** ***** ***** ***** ***** ***** 40%
   %       5     10     15     20     25     30     35    40     45    50

Very important: Free Jumping
With 40% it was established that the predisposition to be able to free jump has the highest heritability degree of all mare performance test criteria. This fact also proved to be true in other research programs, i.e. the stallion performance test. The high degree of heritability underlines the importance of free jumping: in a young horse free jumping gives the strongest indication for an aptitude to jump. Even though we know that other factors come into place when converting the aptitude for jumping into actually jumping courses. It is safe to say that a predisposition for being able to jump or not will most likely be passed on to the next generation. This means that progress in breeding jumpers can be achieved fairly easily  and quickly. The development in breeding jumpers in Hannover as well as in other breeding areas validates the statement above. Last year, a delegation from Sweden visited our breeding area including Professor Ingvar Fredricson, the Director of the National Stud Farm Flyinge. From what he says, Sweden used to be famous for it's dressage horse breeding program. During the last years, Sweden rapidly progressed in its breed program for jumpers, while they noticed a backlog in their dressage horse breeding program.

Genetic Correlations
I now address the correlation between the various characteristics known as genetic correlation. It gauges if and to what extent the development of a characteristic depends on another characteristic.

Genetic correlations are portrayed in units from negative one to positive one. A genetic correlation of positive one between two characteristics means the following: a selection for one characteristic causes the same breed progress for another characteristic without specifically selecting for it. A genetic correlation of negative one means: while achieving a certain breed progress for one characteristic, at the same time you achieve the same amount of setback for another characteristic. A correlation of zero means that two characteristics are independent from one another. When selecting characteristic one, neither progress nor setback is to be expected with characteristic two.

In table 1 genetic correlations between mare performance test criteria can be determined. It is clearly visible that all three basic gaits are positively connected with values of 0.52 to 0.82. Also, the correlation  between the basic gaits and the assessment of the rideability is positive and high 90.68 to 0.83). This means that the assessment of the rideability depends greatly on the quality of the basic gaits. Of the three basic gaits, the trot has the largest influence on the rideability. Considering the strong heritability, as shown above, this fact underlines the importance of this movement in a breeding program.

Table 1: Estimated values of genetic correlations between criteria of the mare performance test.
Walk Trot Canter Rideability Free Jumping
Walk   + (0.52) + (0.60) + (0.68)   -  (-0.10)
Trot     ++ (0.82) ++ (0.83)  +/- (0.02)
Canter        + (0.73)  + (0.11)
Rideability          +/- (0.06)

Table 2.: Estimated values of genetic correlations between criteria of the mare inspection and criteria of the mare performance test (computed by means of a BLUP animal model)
Walk Trot Canter Rideability Free jumping
Head  +  (0.10)  +  (0.20)  +  (0.19)  +  (0.30)  +/-  (0.00)
Neck  +  (0.27)  +  (0.27)  +  (0.40)   +  (0.29)  +/-  (0.09)
Saddle position  +  (0.26)  +  (0.28)  +  (0.33)  +  (0.32)  +/-  (-0.27)
Frame  +  (0.48)  +  (0.60)  +  (0.60)   +  (0.67)  -     (-0.20)
Breed/sex type  +  (0.35)  +  (0.54)  +  (0.52)  +  (0.60)  -     (-0.11)

The Importance of Elasticity
The interplay between rideability and basic gaits indicates the importance of elasticity. A horse allows a rider to sit comfortably when he vigorously pushes off the ground and the movements swing through his entire body with a powerful but elastic and swinging back. This horse causes less problems with the contact, the activity in the mouth and with the reactions on the rider's aids, than a horse whose appearing stiffness influences not only the quality of the basic gaits but also the suppleness.

The relation between the assessment of the free jumping and the evaluation of the basic gaits/rideability is of a different nature. The genetic correlation between the ability to jump and the trot and rideability is almost zero. This indicates their independence from one another. A slightly positive relation lies between the ability to jump and the canter with 0.11, a slightly negative relation between the jumping ability and the walk with negative 0.10. Therefore, if we select only for jumping ability, we shall achieve a slight improvement of the canter and a slight deterioration of the walk in the entire horse population.

Table 2 describes the genetic correlation between various exterior characteristics and performance criteria. The exterior evaluation results for the head, the neck, the saddle position and the frame, hold a positive relation to the dressage results, trot, canter, walk, rideability. The score for the frame has the highest positive relation to the basic gaits/rideability. This is not surprising since the score for the frame includes to a large extent the development of the top line. Weaknesses in this area like too short, too long, straight croup, tight or hollow loins, have a negative impact on the quality of the basic gaits and the rideability.

In summary, this research confirms that a rideable horse with good basic gaits can be bred when applying the traditional standards of evaluating the exterior.

They All Jump With a Different Style
This, however, does not apply to free jumping. We have a saying in the racing community: "They all run in very different styles." It is similar with the jumpers. A negative correlation can be found for almost all exterior criteria with exception of the head. The lowest correlation lies in the relation between the saddle position and the free jumping with negative 0.27. This does indicate that a poorly developed saddle position does not hinder a horse to become an outstanding jumper. In fact, there are a number of first class jumper producers like Grannus, Wanderer, or Calypso II who – as we all know – produce barely sufficient saddle positions. In general this means that the type will suffer if we select exclusively for jumping ability without considering other criteria. However, the relations between the above mentioned criteria lie nowhere close to the extreme negative. A large enough number of above average jumpers exist who carry the preferred type characteristics. If we increase the use of these sires, it is absolutely possible to improve the type while breeding jumpers. Numerous examples for a positive development can be found, I.e. Espri, Hanoverian Stallion of the Year, or his son Escudo who represents the almost ideal type. A couple of weeks ago the first free jumping event for horses who belong to the project Hanoverian Jumper Breeding took place. This event also exhibited Hannover’s significant progress in this field. The genetic correlations will ultimately change with a corresponding selection.

The portrayed genetic relations between the three areas - type, dressage ability and jumping ability clearly demonstrate: the predisposition for jumping and the predisposition for dressage are almost independent from one another. A slightly negative relation exists between the different criteria for type and the jumping ability. However, this relation is not negative enough not to allow a type improvement in jumpers. The results substantiate the versatility of horses within type and talents. Compromises always deem necessary. It appears unrealistic to strive for breeding only horses in ideal type and with the equal predisposition to be a top dressage horse as well as a top jumper. It is, however, realistic to breed an appealing horse with a significant predisposition for either dressage or jumping and an, at least average talent for the other area. ~