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Genetic parameters for traits at performance test of stallions

and correlations with traits at progeny tests in Swedish

warmblood horses

a ,

_*

_´

b a a

¨

Elisabeth Gerber Olsson

, Thorvaldur Arnason , Anna Nasholm , Jan Philipsson

Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden

IHBC AB, Knubbo, S-744 94 Morgongava, Sweden

Received 12 July 1999; received in revised form 12 November 1999; accepted 12 November 1999

Abstract

Genetic parameters for traits evaluated in the Swedish stallion performance test (SPT) and correlations between stallion performance test traits and 4-year-old offspring results from field tests were estimated. Heritability coefficients were on average 0.40 for individual gaits under rider and 0.23–0.47 for jumping traits at SPTs. Repeatabilities between tests at 6-month intervals were 0.75–0.77 for gaits and 0.38–0.58 for jumping traits. The genetic correlation between the gaits ranged from 0.30 to 0.71. Positive genetic correlations were found between gaits under rider and jumping traits (0.14–0.54), thus breeding for both characteristics is facilitated. Free jumping results were highly correlated (0.93) to results in jumping under rider, and because of their higher heritability the former are very suitable for selection purposes. Genetic correlation estimates between a trait in SPT and the same trait at 4-year-old offspring field test were unity for gaits under rider and jumping. The genetic correlations between gaits and jumping at the two different tests were 0.26–0.35. It was concluded that the field tests are well suited for early progeny testing of the stallions, and will improve the accuracy in selection of stallions

Keywords: Riding horses; Performance; Animal model; Heritabilities; Correlations

1. Introduction based on experiences and results of European

100-day tests (Bade, 1974; Bruns et al., 1985; Huizinga The Swedish system for performance testing of et al., 1991b) and genetic analyses of Swedish field warmblood stallions (SPT) was developed in the late tests of 4-year-old riding horses (Thafvelin et al., 1970s and the early 1980s. The design and gradual 1980; Darenius et al., 1982). The evaluations at the development of the Swedish performance tests were SPT were done by experienced national and interna-tional judges and trainers. The stallions could partici-pate in three tests, located at Flyinge in the southern

*Corresponding author. Tel.: 146-18-672-789; fax: 1

46-18-part of Sweden, with an interval of 6 months from

672-648.

3.5 to 5.5 years of age. The stallions were at each

E-mail address: elisabeth.olsson@hgen.slu.se (E. Gerber

Ol-sson) occasion tested during 4 days, comprising evaluation

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for soundness, conformation, gaits under rider, free genetic trend in the population based on BLUP-jumping and BLUP-jumping under rider. The stallions were Animal model evaluations is estimated with infor-also scored for temperament and general appearance mation from the RHQT and the trend has been

´ in the tests for gaits under rider, free jumping and increasing rapidly during the last 10 years (Arnason jumping under rider. The stallions usually had to be et al., 1997). When considering horses born from approved at two out of three tests to be licensed for 1988 to 1992, the annual genetic progress for gaits breeding. Annually about 200 stallions were pre- and jumping, respectively, have been about 0.05 and inspected for conformation and soundness. About 0.03 genetic standard deviations per year.

40–60 were selected to take part in the stallion The purpose of the SPT is to select sires of the performance tests and on average 8–12 of these coming generation of successful sport horses. As stallions were finally approved for breeding. Ohlsson and Philipsson (1992) have concluded, In 1997 some changes were introduced in the performance at RHQT is highly correlated to later stallion performance test. The test is now done as a competition performance, it is of great interest to 9-day-long station test at 4 years of age at Flyinge. find out to what extent the SPT and RHQT results The stallions are preselected in the autumn at 3.5 are genetically related.

years of age, for conformation, soundness and free The objectives of this study were to estimate jumping. The latter trait is therefore excluded from heritabilities, genetic and phenotypic correlations and the regular stallion performance test. The stallions repeatabilities for traits scored in the stallion per-are also judged by foreign test riders in the new test. formance tests as well as correlations between The SPT is used for phenotypic selection among stallion performance test results and 4-year-old field prospective breeding stallions, but until now it has performance records of their offspring. These param-not been investigated if heritabilities and variances eters are necessary for construction of integrated for the traits tested are high enough for an efficient indexes to be used for performance evaluations and selection. Because the SPT was done as repeated continued evaluations of stallions based on early tests it was important to investigate the re- progeny results. The parameters are also useful for peatabilities of the results, and thereby determine the determining the opportunities of improving or sim-effects of reducing the number of tests. The correla- plifying the stallion performance tests.

tions between traits in the Swedish stallion per-formance test have not been estimated before and it

would be interesting to know their interrelationships 2. Materials

in order to make optimal use of the information for

selection purposes, which may indicate if the tests 2.1. Stallion performance test data can be further simplified and less costly.

Riding horse quality tests (RHQT), a 1-day field A dataset comprising 378 stallions evaluated at the test for 4-year-old horses, have taken place in SPT during 1979–1993 was analysed. The stallions Sweden since 1973. Annually about one third, or took part in one to three performance tests. The 500–1000, of all the 4-year-old riding horses in dataset contained observations from a total of 683 Sweden participate at the tests which take place at tests and was used to estimate heritabilities, genetic about 20 different places every autumn. The traits and phenotypic correlations and repeatabilities be-scored are health status, conformation, gaits under tween tests. The number of observations in the rider and jumping. A temperament and general dataset is shown in Table 1. All traits were given appearance score is given at the latter two parts of scores between 1 and 10. The temperament and the tests. All traits are given scores between 1 and general appearance scores for the two jumping traits 10. The tests are more precisely described by Gerber have only been recorded since autumn 1985 and the et al. (1997). The RHQT system aims at genetic corresponding score for gaits under rider since evaluations of stallions and mares and of providing autumn 1988. Also within a test some stallions were information about the quality of the individual young not shown in jumping under rider if the free jumping horses as a guideline for choice of sport horses. The result was poor. The scores for conformation and

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Table 1

Number of observations per trait and combining of traits in the data from the stallion performance test

Trait 1 2 3 4 5 6

1 Walk, trot, gallop 536

2 Temp. gaits 217 217

3 Free jumping 458 596

4 Temp. free jumping 189 478 478

5 Jumping under rider 315 278 235 315

6 Temp. jumping under rider 174 239 236 264 266

Temp., temperament and general appearance.

soundness were not included in the material as the tions from the SPT contained 683 records for gaits stallions were pre-selected mainly on these criteria. and jumping. There were 234 stallions with offspring Means and standard deviations for the different traits in the RHQT. Of these, 136 had a minimum of five

scored in SPT are given in Table 2. offspring in the test and were included in the

The evaluated stallions had 160 sires and 167 correlation analysis with RHQT results. There were maternal grandsires, 60 of them appeared as both 2964 mares tested in the RHQT with offspring, that sires and maternal grandsires. Among the sires and also had participated in the RHQT.

maternal grandsires 56 had themselves participated For estimation of the genetic correlations between

in the SPT. SPT and RHQT traits new combined traits were

created from the original ones. The trait ‘gaits’ was 2.2. Stallion performance test and riding horse created as the average of the four scores for walk,

quality test data trot, gallop and temperament and general appearance,

for the data from the RHQT and SPT. The score for To estimate the genetic correlations between traits ‘jumping’ was in the RHQT defined as the average tested at SPT and RHQT, respectively, the material of either two or four scores for free jumping and from the SPT, described above, and results from the jumping under rider (both were not always re-RHQT during the period 1983–1993 were merged. corded), and temperament and general appearance Results for mares with foal at foot when tested were for the two disciplines whenever recorded. Approxi-excluded from the analysis. The final dataset for this mately 50% of the horses were tested for free analysis comprised 6674 horses tested for gaits under jumping only, 40% for jumping under rider only, and rider and for jumping in the RHQT. In the SPT, 327 the remaining had scores for both traits. In the SPT and 359 stallions were tested for gaits under rider data the average of all four scores for the jumping and jumping, respectively, and another 8559 horses traits were used. However, at the age of 4.5 years occurred in the pedigrees. The dataset with observa- and above, the free jumping test was exchanged for

Table 2

Means, standard deviations (S.D.), minimum (Min) and maximum values (Max) and coefficients of variation (CV) of the different traits in the data from the stallion performance test

Trait Means S.D. Min Max CV (%)

Walk 6.91 1.18 3 10 17.1

Trot 6.72 1.29 4 10 19.2

Gallop 7.09 1.09 5 10 15.4

Temp. gaits 6.52 1.21 1 10 18.6

Free jumping 6.49 1.72 1 10 26.5

Temp. free jumping 6.56 1.87 1 10 28.5

Jumping under rider 6.70 1.56 2 10 23.3

Temp. jumping under rider 6.61 1.68 1 10 25.4

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Table 3

Number of horses (N ), means, standard deviations (S.D.), minimum (Min) and maximum (Max) values and coefficients of variation (CV) for the average scores of different traits in the combined data set from the stallion performance test (SPT) and riding horse quality test (RHQT)

Trait (average score) N Mean S.D. Min Max CV %

Gaits (RHQT) 6674 6.52 0.89 3.00 9.33 13.7

Jumping (RHQT) 6674 6.79 1.50 0.50 10.00 22.1

Conformation (RHQT) 6674 7.47 0.49 5.00 9.60 6.6

Gaits (SPT) 136 6.83 0.92 4.33 9.67 13.5

Jumping (SPT) 136 6.24 1.63 1.00 10.00 26.2

another jumping test under rider. All tests at the the random effect of the kth tested stallion, mean50

different ages were included and means were calcu- and variance5Asa; pekl is the random effect of the lated for each created trait. The trait ‘conformation’ permanent environment, mean50 and variance5

in the RHQT data was defined as the average of five spe; eijkl is the random residual effect, mean50 and

scores comprising type, head-neck-body, legs, walk variance5se.

and trot at hand. Table 3 contains means and Repeatability for the traits scored in the SPT was

2 2 2 2 2

standard deviations for the traits scored. estimated as (s 1 sa pe)) /sP where s 5 s 1P a

2 2 2

spe1se, and sa is the additive genetic variance. Standard errors of the variance and covariance components were provided by the DMU programme

3. Methods

and standard errors for heritabilities, repeatabilities and genetic correlations were approximated by using A multivariate animal model was used to estimate

Taylor series expansions. repeatabilities and (co)variance components for the

Bivariate analyses with an animal model were traits in the SPT. For estimation of (co)variance

performed to estimate (co)variances for the average components the traits were analysed five by five

scores of the traits in the combined data from the and / or four by four, due to restrictions in computer

RHQT and SPT. For the analysis the derivative free capacity. The model included the additive

relation-REML programme by Meyer (1993) was used. The ship matrix (A) with information about sires and

animal model used for traits in RHQT contained the maternal grandsires. Only pedigree information from

fixed effects of year and place of scoring and the males was available. Parameters were estimated by

random effects of the breeding values of the in-use of restricted maximum likelihood (REML)

pro-dividual horses tested as follows: cedures using the DMU computer programme

pack-age (Jensen and Madsen, 1994).

Yij5m 1year2placei1horseij1e (Model 2)ij

To estimate repeatabilities and (co)variance com-ponents for the traits in the data from the stallion

where Y is the average score of each trait for the

performance test the following animal model was ij

ijth horse; m is the population mean; year2place is

used: i

the fixed effect of ith year and place of testing

Yijkl5m 1agei1yearj1stallionk1pekl (i51, . . . , 211); horseij is the random effect of the breeding value of the ijth tested horse, mean50 and

1eijkl(Model 1) ₂

variance5Asa; eij is the random residual effect,

where, Yijkl is the score of each trait for the ijlth test mean50 and variance5se.

of the kth stallion; m is the population mean; age isi For SPT traits in the data the animal model used the fixed effect of the ith age (i53.5, 4.0, 4.5, 5.0, included the fixed effects of year of birth of the 5.5) of the stallion at test; year is the fixed effect ofj stallion and the random effect of the individual

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Table 4

Yij5m 1year of birthi1stallionij1e (Model 3)ij 2

Heritabilities (h ), standard errors (S.E.) and repeatabilities (t) of traits scored in the stallion performance test

where Yij is the score of each trait for the ijth Trait h (S.E.) t (S.E.)

stallion; m is the population mean; year of birth isj _Walk _{0.46 (0.13)} _{0.77 (0.03)}

the fixed effect of ith year of birth (i5 #82, 83 . . . , _Trot _{0.37 (0.11)} _{0.77 (0.03)}

89); stallionij is the random effect of the breeding Gallop 0.39 (0.11) 0.75 (0.03)

value of the ijth tested stallion, mean50 and addi- Free jumping 0.47 (0.13) 0.58 (0.04)

2 _{Temp. free jumping} _{0.23 (0.14)} _{0.38 (0.07)}

tive genetic variance5Asa; eij is the random

re-2 _{Jumping under rider} _{0.32 (0.14)} _{0.47 (0.06)}

sidual effect, mean50 and variance5se. _{Temp. jumping under rider} _{0.33 (0.23)} _{0.43 (0.09)} In model 1 an age effect was considered, but not

Temp., temperament and general appearance.

in model 3 as the traits in the latter case consisted of average values of several observations measured at different ages. It was assumed that the environmental correlation between the SPT and RHQT traits was

the heritability estimates for gaits and jumping zero, as the traits were recorded on different

in-estimated by Bruns et al. (1985), Huizinga et al. dividuals and at different occasions. Estimates of the

(1991a,b), von Velsen-Zerweck (1998) and Friemel sampling variance was not obtainable by this version

et al. (1998). of the derivative free REML programme.

The heritability estimates for temperament and general appearance for free jumping were lower than the corresponding estimate for free jumping ability. The explanation for this may be that the

tempera-4. Results and discussion

ment and general appearance traits are more difficult to assess, but also that they were scored on a smaller 4.1. Stallion performance test data

number of horses, and thus subjected to larger errors. It was not possible to estimate heritabilities, re-4.1.1. Means and standard deviations

peatabilities and correlations for the trait tempera-Table 2 shows that the scores for individual gaits

ment for gaits, because of too few observations. generally had a relatively lower variation than the

The highest repeatabilities were found between the scores for jumping. Among these, free jumping

gaits, 0.75–0.77. Free jumping was the trait within scores showed the largest variation. For gaits the

jumping that showed the highest repeatability, 0.58. scale was not fully used. However, a score of 5

The temperament traits for free jumping and jumping means that the stallion has an acceptable ‘clean’ gait,

under rider had a lower repeatability than the corre-while lower values indicate lameness or some

ir-sponding jumping trait, but none of the traits showed regularities. For jumping the whole scale was better

a low repeatability. The high repeatabilities for gaits used and the scores showed a normal distribution.

show that the number of tests for gaits can be reduced without affecting the accuracy in the judge-4.1.2. Heritabilities and repeatability ment. The lower repeatability for the temperament The gaits generally had high heritabilities as well traits indicates that these are more difficult to assess as repeatabilities (Table 4). Jumping under rider had in a correct way at a single occasion. These traits had a lower heritability than free jumping. The reason for less observations which may affect the accuracy of this may be that the riders influence the horses so the results, though, the estimates of repeatability that the natural variation among horses is not ex- cannot be compared to repeatabilities estimated in pressed as much. Furthermore, some of the poorly other countries because the repeatabilities in this free jumping horses did not participate in jumping study are repeatabilities between independent tests at under rider. Thus, a certain amount of pre-selection different testing periods and not between repeated took place which also may cause the lower value. tests within the same testing period, as has been The heritabilities in this study are within the range of common in other studies.

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4.1.3. Genetic and phenotypic correlations variation. The average conformation score had the Positive genetic correlations were found between highest mean and the lowest coefficient of variation. all gaits, the highest between trot and gallop, 0.71

(Table 5). These results are in agreement with Bruns 4.2.2. Heritabilities

et al. (1985), Huizinga et al. (1991a) and Christman Table 6 contains the heritabilities for traits in the and Bruns (1997), who got genetic correlations in SPT and RHQT data. The highest heritabilities were the range of 0.5 to 0.9 between the individual gaits. estimated for the traits scored in the SPT. The main Gallop was quite naturally also the gait most associ- reasons for this are that in the SPT generally the ated with jumping results and more highly correlated same very experienced judges are used in this test for to jumping under rider than to free jumping. Huiz- several years, and the stallions are judged at the inga et al. (1991a) got similar results, although same place. Furthermore the judges, as regards the somewhat lower correlations for gallop and show gaits, have the opportunity to judge the stallions jumping and free jumping, respectively. Jumping twice during consecutive days before the scores are under rider was highly correlated to free jumping set. The heritabilities calculated for traits in the SPT (0.93), which indicates that they to a great extent are in this analysis were in the same range (0.43) (Table governed by the same genes. The genetic correla- 6) as when the traits were analysed as individual tions between the four jumping scores were in traits (Table 4). The explanation for this could be general high (0.44–0.99), but somewhat lower with that the model is not accounting for the permanent temperament and general appearance for jumping environmental effect that affects the results of the under rider. This may partly be explained by larger stallion in the SPT, which may contribute to a standard errors as the number of observations for this greater heritability. On the other hand the trait was smaller. The phenotypic correlations among heritabilities of means of repeated measures should the jumping traits (0.43–0.89) were slightly lower

than the genetic ones.

Table 6

4.2. Stallion performance test and riding horse _{Heritabilities (diagonal), genetic (above the diagonal) and}

pheno-quality test data typic correlations (under the diagonal) estimated for the mean of

traits scored in the stallion performance test (SPT) and in the riding horse quality test (RHQT)

4.2.1. Means and standard deviations

Table 3 shows the means and standard deviations Trait (average score) 1 2 3 4 5

for the traits in data from SPT and RHQT. For the _{1 Gaits (RHQT)} _0.31 _1.00 _0.35

stallions, jumping had a lower mean and a greater 2 Jumping (RHQT) 0.17 0.26 1.00

3 Conformation (RHQT) 0.27 0.75 0.07

coefficient of variation than the gaits. In the RHQT,

4 Gaits (SPT) 0.29 0.16 0.30 0.43

jumping had a higher mean than the gaits and, as in

5 Jumping (SPT) 0.13 0.20 0.08 0.43

the SPT-data, also showed a higher coefficient of

Table 5

Genetic correlations with standard errors (above the diagonal) and phenotypic correlations (under the diagonal) for stallion performance test data. Standard errors within brackets

Trait 1 2 3 4 5 6 7

1 Walk 0.40 (0.08) 0.30 (0.06) 0.27 (0.07) 0.21 (0.06)

2 Trot 0.40 0.71 (0.16) 0.24 (0.06) 0.14 (0.05)

3 Gallop 0.36 0.67 0.40 (0.10) 0.54 (0.18)

4 Free jumping 0.14 0.22 0.31 0.99(.03) 0.93 (0.23) 0.44 (0.14)

5 Temp. free jumping 0.89 0.79 (0.19) 0.50 (0.26)

6 Jumping under rider 0.11 0.29 0.37 0.57 0.51 0.90 (0.06)

7 Temp. jumping u.r. 0.43 0.44 0.84

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be higher, though the trait was not always the same, competition for show jumping, 0.90. The corre-e.g. gaits recorded as individual gaits or as means for sponding genetic correlation for dressage was 0.68.

the three scored traits. Also, between gaits in the SPT and conformation

The heritability for the traits in the RHQT are in the RHQT the genetic correlation was high (0.75). ´

similar to the heritabilities estimated by Arnason This can partly be explained by the fact that walk (1993). The author estimated the heritabilities for and trot at hand are two of the five traits scored in gaits and temperament at the gait test to 0.35 and the conformation evaluation. It is also clear that the 0.27, respectively, and in this study the average genetic correlation between conformation in the value of gaits and temperament and general appear- RHQT and jumping in the SPT is low (0.07). Gaits ance for gaits was 0.31. The genetic correlation are positively associated with jumping traits, though between the two former traits were 0.84 according to at a modest level, 0.26–0.35.

Arnason (1993), which may explain the high In this analysis it was assumed that the environ-heritability for the average trait gaits in this analysis. mental correlation between SPT and RHQT is zero. For conformation and jumping the heritability was This assumption may not be completely correct as also very similar between the results in this study performance test results of the stallions may affect

and the one by Arnason (1993) which were 0.29 for how their offspring are trained and the judges in the conformation and for jumping and temperament for RHQT may unconsciously consider this when judg-jumping, 0.20 and 0.14, respectively. The heritability ing. This fact may cause the correlations in Table 6 for gaits in the RHQT are similar to the ones to be somewhat overestimated. The judges in the

estimated by Kuhl (1991) and von Velsen-Zerweck RHQT are all participating in a regular training (1998), but for jumping the authors got a higher programme where the judges of the SPT are the heritability, 0.37 and 0.35, respectively. The explana- instructors, which also can be a part of an explana-tion for this could be that the results are based on tion for the high correlations.

15–50 days stationary tests and 1-day field tests.

4.3. Aspects on testing and selection 4.2.3. Genetic and phenotypic correlations

The genetic correlations were estimated to unity Analysis of data from the Swedish stallion per-between gaits scored in the SPT and in RHQT and formance testing scheme show moderately high between jumping scored in the SPT and in the heritabilities for all recorded traits, and it can be RHQT (Table 6). Therefore, it can be assumed that concluded that the heritabilities of the traits are high the same gene complex affects the traits judged in enough for an effective, selection. Then having the two different tests, although they are scored with estimated the heritabilities it is possible to estimate different heritabilities. Correlations between sire and breeding values for the stallions in the SPT and to progeny tests were also analysed in Germany by use all available pedigree information in a BLUP-Schade (1996) and von Velsen-Zerweck (1998). The evaluation. To make the breeding values more tests are primarily made at the age of 3.5 years. The accurate, the goal should be to include the results of former author found high genetic correlations (0.83– both SPT and RHQT data in the breeding value 0.96) between traits scored at stallion performance estimations of stallions at the performance tests as tests and field tests for mares, while the latter came the genetic correlations between the results of these to the conclusion that traits at performance tests for tests are very high. A complicating factor is that a stallions and mares cannot be seen as exactly the number of the stallions prospects at the tests are same. However, the genetic correlations were high foreign bred and thus largely lack pedigree infor-and ranged from 0.74 to 0.90. Brockmann infor-and Bruns mation from the two types of Swedish tests. (1997) got a genetic correlation between results at Jumping traits showed the largest variation, while stallion performance test and mare performance tests individual gait scores showed the highest re-which was 0.68. van Veldhuizen (1997) found a high peatabilities. For a given accuracy in assessing genetic correlation between results for jumping in expected breeding values fewer tests at the age of stallion performance tests and offspring results in four are required for evaluation of the gaits under

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rider than for jumping traits. The stallion perform- selection criterion. Positive genetic correlations were ance tests could therefore be shortened and then also found between gaits and jumping traits, thus breed-less costly. Free jumping test results, which can be ing for both characteristics is facilitated. The genetic done at a lower age than jumping under rider, are correlations between the stallion performance test highly correlated to results under rider, and because and the riding horse quality test are very high and of their higher heritability very suitable for selection therefore similarly defined traits in the two different purposes. However, an argument of ‘the riders and tests can be regarded as virtually the same genetic the owners of the stallions’ is that the stallions must traits.

be jumped under rider because that is how they are going to be used later, and coincides with the

breeding objective. It is therefore difficult to get References

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¨ ˚ ¨

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