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Relative importance of performance tests and progeny tests in horse breeding
Livestock Production Science, 5 (1978) 303--312 303 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
RELATIVE IMPORTANCE OF PERFORMANCE TESTS AND PROGENY TESTS IN HORSE B R E E D I N G
H. S T R O M
and J. P H I L I P S S O N
Department of Animal Breeding and Genetics, The Swedish Universityof Agricultural Sciences, S-750 07 Uppsala 7 (Sweden)
(Received 6 October 1977)
ABSTRACT StrSm, H. and Philipsson, J., 1978. Relative importance of performance tests and progeny tests in horse breeding. Livest. Prod. Sc~, 5: 303--312. Calculations of genetic gain have been made in order to compare mass selection with two-stage selection (mass selection and progeny testing) for trotters and riding horses. The main conclusions drawn are as follows. (1) In horse breeding, mass selection and suitably designed performance tests are of great importance, mainly because of the long generation intervals and also because most of
the traits can be measured in every animal in both sexes. (2) For traits such as conformation and trotting speed, which can be measured early in life and have a high or medium heritability, selection should be based mainly on the individual's own phenotype. When the heritability is low, however, progeny testing will be more important. Progeny testing also imparts increased accuracy of selection, which is in itself of great importance for the individual breeds. (3) For some traits that are measured later in life, such as competitive ability and other riding horse traits, the choice of selection scheme depends on the accuracy of the performance test, on the breeding objective and on the magnitude of the genetic relationship between the breeding objectives. As a general conclusion, it could be said that progeny testing is quite valuable as a complement to performance testing for these traits.
INTRODUCTION In m o s t b r e e d i n g w o r k , p r o g e n y testing is a v e r y valuable t o o l in achieving genetic progress. This is especially t h e case in species w i t h a high rate o f r e p r o d u c t i o n , and f o r traits t h a t either have a low heritability or are measurable in o n l y o n e sex. O n e d r a w b a c k , h o w e v e r , is t h a t p r o g e n y testing is followed b y l o n g e r g e n e r a t i o n intervals. This is especially so in h o r s e breeding, w h e r e the g e n e r a t i o n interval generally ranges b e t w e e n 8 and 12 years, d e p e n d i n g m a i n l y o n h o w early in t h e a n i m a l ' s life the trait can be measured. C o n s e q u e n t ly, it is o f p a r t i c u l a r interest t o c o m p a r e mass selection with p r o g e n y testing in selection p r o g r a m m e s f o r stallions. I n a d d i t i o n t o t h e g e n e r a t i o n interval, t h e level o f h e r i t a b i l i t y m u s t be c o n s i d e r e d . This m a y v a r y b e t w e e n d i f f e r e n t p o p u l a t i o n s a n d b e t w e e n characteristics. Its level is n a t u r a l l y also d e p e n d e n t
304
on how accurately the measuring of various characteristics can be accomplished. The purpose of this study was to estimate the genetic improvement for certain characteristics with varying levels of heritability when using mass selection or progeny testing in the selection programmes for stallions. In particular, the paper discusses the problems as they apply to riding homes and trotters. METHODS Genetic gain has been calculated by comparing mass selection vis-a-vis a two-stage selection procedure including mass selection and a second stage based on progeny testing o f stallions. For trotters, calculations were made for a single characteristic expressing competition results. For a population of riding homes, genetic gain was estimated for a breeding objective including two groups of characteristics -- one that is measurable early in life and one at a later age. According to Rendel and Robertson (1950) the genetic improvement may be calculated as: A G=Iss
+ IsD + IDs + IDD
Lss + LSD + LDS + LDD
where AG = the genetic improvement per year I = genetic improvement per generation = i . r v i , ov where i = selection differential ral = correlation between index and true breeding value o a = genetic standard deviation L = generation interval SS = path sire to son S D = path sire to daughter D S = path dam to son D D = path dam to daughter. In a two-stage selection scheme the genetic variation in the second stage diminishes, thus affecting the genetic gain o f t h a t stage. This effect has been incorporated in a m e t h o d described by RSnningen (1969). Genetic parameters
The assumptions on which the calculations were based are summarized in Table I. For trotters, two heritabflity levels were chosen -- 0.2 and 0.3 -- in line with most of the estimates in the literature (Linnet and Osterkorn, 1974; Minkema, 1975; RSnningen, 1975). If the horses are selected at 5 years and progeny tested at 10, the generation interval will be 9 and 12.5 years, respectively, provided average ages of 12 and 14 years, respectively, are reached by the two groups of selected stallions, and t h a t 50% of the foals are
305
TABLE I Assumptions for calculations on genetic gain for trotters and riding horses Selection intensity (%)
Heritability
Phenotypic standard deviation (%)
Generation interval (years)
2 4 50 100
0.2, 0.3 0.2, 0.3 0.2, 0.3
20 20 20
9 9 9 9
251 502
0.2, 0.3 0.2, 0.3
20 20
12.5 11.5
10 20 50 90
0.4 0.4 0.3 0.2
15 15 17 21
8 8 8 8
203 404
0.2 0.2
21 21
11.5 8.9
0.5, 0.3 0.5, 0.3 0.2 --
20, 26 20, 26 32
8.5 8.5 8
--
8
0.3 0.3
26 26
Trotters, mass selection sire--son sire--daughter dam--son dam--daughter Trotters, progeny test sire--son sire--daughter Type and conformation, mass selection sire--son sire--daughter dam--son dam--daughter Type and conformation, progeny testing sire--son sire--daughter Performance test sire--son sire--daughter dam--son dam--daughter
20 33 50 100
Progeny test for competitive ability sire--son sire--daughter In the two-stage selection i 8% in the first stage 2 8% in the first stage 350% in the first stage 450% in the first stage s50% in the first stage 650% in the first stage
40 s 676
the selection intensity is 25% in the second 50% in the second 20% in the second 40% in the second 40% in the second 67% in the second
13.5 10.5
306
sired by progeny tested stallions. For mares, only mass selection was assumed. For riding horses, one character that can be measured early in life is type and conformation, which was used in the present calculation. Characters that must be measured later in life are competitive ability in jumping, dressage and so on, which can be measured either b y performance tests or b y progeny testing through competition. The heritability for t y p e and conformation was assumed to be of moderate degree (Varo, 1965; Dusek, 1970; Stamp, 1973) and was considered to be greater in the paths sire--son and sire--daughter, as the judging is usually more exact for stallions. The heritability for performance testing of stallions was assumed to be 0.5 when they are trained together and carefully judged, and 0.3 if they are tested in a field trial and n o t trained together. For the path dam--son the heritabilities 0.2 (performance) and 0.3 (conformation) were chosen, as the judging is done in the field and therefore cannot be so precise. The heritability for the path dam--son is assumed to be higher than that for dam--daughter, which is true if there is a special selection of dams of stallions. In all alternatives, only mass selection was assumed for mares. The heritabilities for performance tests and results from competitions agree closely with those reported by Langlois (1974), Bade et al. (1975) and Philipsson (1975). A phenotypic standard deviation o f 20% for trotting was chosen, a value very similar to that set by RSnningen (1975). For type and conformation and the other riding horse traits, the phenotypic standard deviation has been chosen in such a way that the genetic variation is the same for a certain trait irrespective of the heritability level, which reflects the quality of the judging. The standard deviation chosen for type and conformation agrees well with figures given by Stamp (1973) and Varo (1965) and the values chosen for riding traits agree with those o f Bade et al. (1975) and Philipsson (1975). The generation interval for the characteristic conformation can be quite short -- 8 years -- assuming that the animals can be judged at an age of 2--3 years. The longest generation interval is for characters measured in competitions, when the stallions must have at least 5--6-year-old progeny. The size of the progeny group was assumed to be 20, for both trotters and riding horses. The genetic gain was calculated for different magnitudes of the genetic relationship between performance test and competition ability as measured by progeny testing. The selection intensity in both stages of the two-stage selection procedure was so chosen that the total selection intensity is the same as when only mass selection is used. RESULTS AND DISCUSSION Trotters
The estimates of genetic improvement in competition results for trotters are given in Table II. At the lower heritability level, 0.2, the two-stage selection procedure gave a b o u t 17% higher genetic gain than selection based only on
307
the individual's o w n results. When the heritability was 0.3, mass selection gave almost as high a genetic gain as two-stage selection. The greater accuracy o f the breeding values through progeny testing was in this case almost cancelled o u t by the negative effects of increased generation interval. As regards trotters, the most important instrument of selection must be individual testing, especially as nearly every horse is tested on the race course. However, the results of these calculations depend very much on the relation between the selection intensity in the first and the second stage. In the present calculations, a strong selection intensity was assumed in the first stage (8%) and a weaker intensity in the second (25--50%) among the already selected animals. Nevertheless, in some populations the reverse is the case, with stronger selection intensity in the second stage, especially in the path sire--son, which means that two-stage selection will be more effective than mass selection alone. Furthermore, it is quite possible that the breeders will wait before using a stallion on a larger scale until his progeny have appeared on the race courses, as the accuracy of selection improves after completion of the progeny test (Table III). This means in practice that the selection intensity at progeny testing may be increased, which should justify the use of this m e t h o d as a selection criterion. A very important trait for trotters is the ability to endure the strain of competitions over several years, in other words their longevity. It may be questioned whether, ordinarily, this complex of traits has a heritability large enough to achieve an acceptable genetic gain without progeny testing. In the practical breeding of trotters the conclusion may be drawn that the strongest selection intensity should be based on individual performance, b u t that the genetic evaluation should later be supplemented with progeny testing, T A B L E II A n n u a l genetic i m p r o v e m e n t for t r o t t e r s w h e n using mass s e l e c t i o n versus two-stage selection h2
0.2 0.3
Mass s e l e c t i o n
Two-stage selection
%
Rel.
%
Rel.
0.60 0.98
100 163
0.70 1.00
117 167
T A B L E III A c c u r a c y o f b r e e d i n g values for t r o t t e r s
(rGl) w h e n
mass s e l e c t i o n a n d two-stage s e l e c t i o n are used
h2
Mass s e l e c t i o n
T w o - s t a g e selection
0.2 0.3
0.45 0.55
0.70 0.74
308
especially if some of the desired traits have a lower heritability than in the calculations above.
Riding horses For type and conformation of riding horses, our calculations gave a genetic improvement o f 0.75% per year when using mass selection only, compared with 0.66% per year when using two-stage selection, which gives a relative difference of 14%. This means that, due to the relatively high heritability and the longer generation intervals that follow progeny testing, the two-stage selection is inferior to mass selection. However, since conformation can be measured rather early in life, progeny testing may be used as a complement if it can be done w i t h o u t lengthening the generation interval t o o much. Often, sufficient information is at hand to carry o u t progeny testing and of course this information should then be used. The important point is that the selection intensity should be high in the first stage of the two-stage selection scheme. For competitive ability, the'breeding objective, the level of correlation between performance test and competition results, and the subsequent generation intervals, all determine whether mass selection or two-stage selection is the more efficient method. If the breeding objective is defined as competitive ability alone, then, as shown in Table IV, the two-stage selection m e t h o d is superior to performance testing when the correlation between performance tests and competitive results is below the 0.8 level. This is quite conceivable, since a performance test should measure not only pure competitive ability but also other riding traits such as willingness to work, ease of handling, etc. However, it is reasonable to assume that the breeding objective for most populations of riding horses is n o t competitive ability alone, b u t also those characteristics that are recorded in a performance test, as mentioned above. Ascribing equal importance to these two groups of characteristics and assuming that the level of heritability for the performance test is 0.5, it is shown in Table IV that performance testing is superior to two-stage selection -- at least when the correlation is 0.4 or higher. If, however, the heritability of the performance test is lower, e.g., 0.3, then the two-stage selection m e t h o d will almost match performance testing when the correlations are 0.4 or higher. From the practical breeder's point of view the genetic gain of the whole population may n o t be as important as the fact that the accuracy of the breeding values increases considerably when using progeny testing as a complement to performance testing (Table V). One point of interest is that the accuracy of the breeding values -- in the cases of two-stage selection -- decreases at higher levels of the genetic correlations between the breeding objectives. This is a result of the reduced genetic variance among stallions remaining in the last stage of the two-stage selection procedure. When conclusions from these calculations are drawn, it will be obvious
(equal weights)
Two-stage selection
0.3 0.5
0.3 0.5
Two-stage selection (performance + progeny test)
Performance test only
0.5
Performance test only
Competitive ability
Performance test characteristics+ competitive ability
0.5
Selection criteria
Breeding objective
0.52 73 0.57 80
0.50 70 0.62 87
0.51 96
0.35 66
0.58 0.64
0.58 0.71
0.60
0.53
82 90
82 100
113
100
134
0.64 0.70
90 99
0.65 92 0.79 111
0.68 128
0.71
145
166
0.70 0.77
99 108
0.72 101 0.89 125
0.77
0.88
Heritability Genetic correlation between performance test and of performan- competitive ability ce test for 0.4 0.6 0.8 1.0 stallions % Rel. % Rel. % Rel. % Rel.
Comparison of annual genetic improvement of riding horses when selection is based on performance test and a two-stage selection procedure. The relative values are comparable only within the same breeding objective
TABLE IV
O
(equal weights)
Two-stage selection
Performance test only
0.3 0.5
0.3 0.5
0.5
Performance test characteristics + competitive ability
0.5
Performance test only
Two-stage selection (performance progeny test)
Competitive ability
Heritability
Selection criteria
Breeding objective
0.70 0.73
0.46 0.59
0.78
0.28
0.4
0.75 0.80
0.49 0.63
0.78
0.42
0.6
0.81 0.87
0.52 0.67
0.78
0.57
0.8
0.77 0.77
0.55 0.71
0.77
0.71
1.0
Genetic correlation between performance test and competitive ability
Comparison of accuracy of breeding values for riding-horse traits when selection is based on performance testing and the two-stage selection procedure
TABLE V
O
311
that the results depend very much on which parameters (length of generation interval, percentage selected in the different stages, heritability, genetic correlations, etc.) are chosen. Nonetheless, it is reasonable to assert that a second selection stage after the performance test is defensible, since it is likely that the heritability for a performance test is often somewhat lower than 0.5. However, as the performance test increases in confidence and precision, its heritability rises and a second selection stage based on progeny testing loses its importance, from the aspect o f annual genetic gain. The main conclusion to be drawn here is to emphasize the importance of accurate performance testing, rather than considering what can be gained by progeny testing as a complement to the performance test. Progeny tests may in any case be easily computed on the basis of competition statistics and will, of course, constitute valuable information for practical breeders. REFERENCES Bade, B., Glodek, P. and Shormann, H., 1975. Die Entwicklung von Selektionskriterien fdr die Reitpferdezucht. I. Genetische Parameter fiir Kriterien der Eigenleistungspriifung yon Junghengsten auf Station. Ziichtungskunde, 47: 68--77. Dusek, J., 1970. Zur Heritabilit~it des K~rperbaues und des Ganges bei Pferden. Z. Tierz. ZuechtungsbioL, 87: 14--19. Langlois, D., 1974. Resultats de recherches en mati6re g~n~tique chez le chevai. I. Etude des gains des chevaux dans les Concours de Saut d'ostaclea Journ. et Comm. Cheval, Fed. Europ. Zootech. Copenhague, 17--21, August 1974, 14 pp. Linner, M.-T. and Ostarkorn, K., 1974. Ziichterische Auswertung der Rennleistung yon Traberpferden der Jahrgiinge 1963 und 1964 in der BRD. Ziichtungskunde, 46: 168--176. Minkema, D., 1975. Studies on the genetics of trotting performance in Dutch trotters. Ann. G~n~t. S~I. Anita., 7: 99--121. Philipsson, J., 1975. Studies on population structure in Swedish horses. Proc. Int. Syrup. on Genetics and Horse-Breeding, Dublin, Ireland, 17--18 September 1975, pp. 59--62. Rendel, J. and Robertson, A., 1950. Estimation of genetic gain in milk yield by selection in a closed herd o f cattle. J. Genet., 50: 1--8. RSnningen, K., 1969. Studies on selection in aminai breeding. I. The efficiency of two-stage selection compared with single-stage selection with respect to progeny testing in animal breeding. Acta Agric. Scand., 19: 149--174. RSnningen, K., 1975. Genetic and environmental factors for traits in the North-Swedish trotter. Z. Tierz. Zuechtungsbiol., 92: 164--175. Stamp, H., 1973. Populationsgenetische Untersuchung an Holsteiner Warmblutpferden. Diss. Institut fiir Tierzucht und Tierhaitung der Christian-Albrecht-Universitiit, Kiel, 59 pp. Varo, M., 1965. Some coefficients o f heritability in horses. Ann. Agric. Fenn., 4: 223--237. RESUME StrSm, H. et Philipsson, J., 1978. Importance respective des tests de performance et des tests de descendance dans la selection du cbevai. Livest. Prod. Sc~, 5 : 3 0 3 - - 3 1 2 (en anglais). On a calcul6 le gain g~n~tique afin de comparer la s~lection massale ~ la s~lection ~ deux
312
6tages (s61ection massale + testage) pour les chevaux de trot et de loisir. Les prlncipales conclusions obtenues sont les suivantes: (1) Dans la s61ection du cheval, la s61ection massale et un testage sur descendance bien adapt6 sont d ' u n grand int6r~t, principalement ~ cause du long intervalle entre g6n6rations et aussi parce que les caract~res peuvent ~tre mesur6s sur chaque animal des deux sexes. (2) Pour des caract~res tels qui la conformation et la vitesse de trot, qui peuvent ~tre mesur6s pr~cocement dans la vie et qui ont une h6ritabilit6 61ev6e ou moyenne, la s61ection doit ~tre bas6e principalement sur le ph6notype et l'individu. Cependant quand l'h6ritabilit6 est faible, le testage sur descendance sera plus important. Le restage sur descendance entrafne en m~me temps une exactitude am61ior6e de la s61ection, cequi en soi a une importance consid6rable pour les races individuelles. (3) Pour quelques caract6res qui sont mesur6s plus tard darts la vie, tels qui l'aptitude la comp6tition et d'autres caract~res des chevaux de loisir, le choix du sch6ma de s61ection d6pend de la pr6cision du contrble individuel de performance, des objectifs de s~lection et de la magnitude de la relation g6n6tique entre les objectifs de s61ection. Mais en conclusion g6n6rale, on peut dire que le testage sur descendance est extr~mement pr6cieux en compl6ment du contr5le individuel de performance. KURZFASSUNG StrSm, H. und Philipsson, J., 1978. Die relative Bedeutung der Eigenleistungs- und Nachkommenprtifungen in der Pferdezucht. Livest. Prod. 8ci., 5 : 3 0 3 - - 3 1 2 (in Englisch). In der vorliegenden Arbeit wurde der genetische Fortschrit berechnet, um die Einzelselektion mit der Zweistufenselektion zu vergleichen (Einzelselektion + Nachkommenpriifung) fiir Traber und Reitpferde. Die wichtigsten Schlussfolgerungen sind: (1) In der Pferdezucht ist die Einzelselektion und eine zweckm~ssig gestaltete Eigenleistungspriifung yon grSsster Bedeutung, weft verhiiltnism~ssig lange Generationsinterval!e vorliegen und weil die meisten Eigenschaften fast an allen Tieren und bei beiden Geschlechtern gemessen werden kSnnen. (2) Fiir Eigenschaften, z.B. ExteriSr und Rennleistung, die eine hohe oder mittlere • Heritabilitiit aufweisen, sollte hauptsiichlich die Selektion nach dem Phiinotyp erfolgen. Mit sinkender Heritabilitiit w~ichst die Bedeutung der Nachkommenpriifung. Die Nachkommenpriifung beschafft auch eine h~here Genauigkeit der Selektion, welche an sich fiir die einzelnen Rassen yon grosser Bedeutung ist. (3) Fiir gewisse Eigenschaften, z.B. Eignung fiir den Turniersport und andere Reitpferdeeigenschaften, h~'lgt die Wahl des Selektionsplanes teils vom Zuchtziel und teils yon der Genauigkeit der Eigenleistungspriifung und der Genetischen Korrelation zwischen den Zuchtzielen ab. Es kann jedoch die Schlussfolgerung gezogen werden, dass die Nachkommenpriifung eine sehr wertvolle Erg~nzung zur Eigenleistungspriifung fdr diese Eigenschaften ist.
RELATIVE IMPORTANCE OF PERFORMANCE TESTS AND PROGENY TESTS IN HORSE B R E E D I N G
H. S T R O M
and J. P H I L I P S S O N
Department of Animal Breeding and Genetics, The Swedish Universityof Agricultural Sciences, S-750 07 Uppsala 7 (Sweden)
(Received 6 October 1977)
ABSTRACT StrSm, H. and Philipsson, J., 1978. Relative importance of performance tests and progeny tests in horse breeding. Livest. Prod. Sc~, 5: 303--312. Calculations of genetic gain have been made in order to compare mass selection with two-stage selection (mass selection and progeny testing) for trotters and riding horses. The main conclusions drawn are as follows. (1) In horse breeding, mass selection and suitably designed performance tests are of great importance, mainly because of the long generation intervals and also because most of
the traits can be measured in every animal in both sexes. (2) For traits such as conformation and trotting speed, which can be measured early in life and have a high or medium heritability, selection should be based mainly on the individual's own phenotype. When the heritability is low, however, progeny testing will be more important. Progeny testing also imparts increased accuracy of selection, which is in itself of great importance for the individual breeds. (3) For some traits that are measured later in life, such as competitive ability and other riding horse traits, the choice of selection scheme depends on the accuracy of the performance test, on the breeding objective and on the magnitude of the genetic relationship between the breeding objectives. As a general conclusion, it could be said that progeny testing is quite valuable as a complement to performance testing for these traits.
INTRODUCTION In m o s t b r e e d i n g w o r k , p r o g e n y testing is a v e r y valuable t o o l in achieving genetic progress. This is especially t h e case in species w i t h a high rate o f r e p r o d u c t i o n , and f o r traits t h a t either have a low heritability or are measurable in o n l y o n e sex. O n e d r a w b a c k , h o w e v e r , is t h a t p r o g e n y testing is followed b y l o n g e r g e n e r a t i o n intervals. This is especially so in h o r s e breeding, w h e r e the g e n e r a t i o n interval generally ranges b e t w e e n 8 and 12 years, d e p e n d i n g m a i n l y o n h o w early in t h e a n i m a l ' s life the trait can be measured. C o n s e q u e n t ly, it is o f p a r t i c u l a r interest t o c o m p a r e mass selection with p r o g e n y testing in selection p r o g r a m m e s f o r stallions. I n a d d i t i o n t o t h e g e n e r a t i o n interval, t h e level o f h e r i t a b i l i t y m u s t be c o n s i d e r e d . This m a y v a r y b e t w e e n d i f f e r e n t p o p u l a t i o n s a n d b e t w e e n characteristics. Its level is n a t u r a l l y also d e p e n d e n t
304
on how accurately the measuring of various characteristics can be accomplished. The purpose of this study was to estimate the genetic improvement for certain characteristics with varying levels of heritability when using mass selection or progeny testing in the selection programmes for stallions. In particular, the paper discusses the problems as they apply to riding homes and trotters. METHODS Genetic gain has been calculated by comparing mass selection vis-a-vis a two-stage selection procedure including mass selection and a second stage based on progeny testing o f stallions. For trotters, calculations were made for a single characteristic expressing competition results. For a population of riding homes, genetic gain was estimated for a breeding objective including two groups of characteristics -- one that is measurable early in life and one at a later age. According to Rendel and Robertson (1950) the genetic improvement may be calculated as: A G=Iss
+ IsD + IDs + IDD
Lss + LSD + LDS + LDD
where AG = the genetic improvement per year I = genetic improvement per generation = i . r v i , ov where i = selection differential ral = correlation between index and true breeding value o a = genetic standard deviation L = generation interval SS = path sire to son S D = path sire to daughter D S = path dam to son D D = path dam to daughter. In a two-stage selection scheme the genetic variation in the second stage diminishes, thus affecting the genetic gain o f t h a t stage. This effect has been incorporated in a m e t h o d described by RSnningen (1969). Genetic parameters
The assumptions on which the calculations were based are summarized in Table I. For trotters, two heritabflity levels were chosen -- 0.2 and 0.3 -- in line with most of the estimates in the literature (Linnet and Osterkorn, 1974; Minkema, 1975; RSnningen, 1975). If the horses are selected at 5 years and progeny tested at 10, the generation interval will be 9 and 12.5 years, respectively, provided average ages of 12 and 14 years, respectively, are reached by the two groups of selected stallions, and t h a t 50% of the foals are
305
TABLE I Assumptions for calculations on genetic gain for trotters and riding horses Selection intensity (%)
Heritability
Phenotypic standard deviation (%)
Generation interval (years)
2 4 50 100
0.2, 0.3 0.2, 0.3 0.2, 0.3
20 20 20
9 9 9 9
251 502
0.2, 0.3 0.2, 0.3
20 20
12.5 11.5
10 20 50 90
0.4 0.4 0.3 0.2
15 15 17 21
8 8 8 8
203 404
0.2 0.2
21 21
11.5 8.9
0.5, 0.3 0.5, 0.3 0.2 --
20, 26 20, 26 32
8.5 8.5 8
--
8
0.3 0.3
26 26
Trotters, mass selection sire--son sire--daughter dam--son dam--daughter Trotters, progeny test sire--son sire--daughter Type and conformation, mass selection sire--son sire--daughter dam--son dam--daughter Type and conformation, progeny testing sire--son sire--daughter Performance test sire--son sire--daughter dam--son dam--daughter
20 33 50 100
Progeny test for competitive ability sire--son sire--daughter In the two-stage selection i 8% in the first stage 2 8% in the first stage 350% in the first stage 450% in the first stage s50% in the first stage 650% in the first stage
40 s 676
the selection intensity is 25% in the second 50% in the second 20% in the second 40% in the second 40% in the second 67% in the second
13.5 10.5
306
sired by progeny tested stallions. For mares, only mass selection was assumed. For riding horses, one character that can be measured early in life is type and conformation, which was used in the present calculation. Characters that must be measured later in life are competitive ability in jumping, dressage and so on, which can be measured either b y performance tests or b y progeny testing through competition. The heritability for t y p e and conformation was assumed to be of moderate degree (Varo, 1965; Dusek, 1970; Stamp, 1973) and was considered to be greater in the paths sire--son and sire--daughter, as the judging is usually more exact for stallions. The heritability for performance testing of stallions was assumed to be 0.5 when they are trained together and carefully judged, and 0.3 if they are tested in a field trial and n o t trained together. For the path dam--son the heritabilities 0.2 (performance) and 0.3 (conformation) were chosen, as the judging is done in the field and therefore cannot be so precise. The heritability for the path dam--son is assumed to be higher than that for dam--daughter, which is true if there is a special selection of dams of stallions. In all alternatives, only mass selection was assumed for mares. The heritabilities for performance tests and results from competitions agree closely with those reported by Langlois (1974), Bade et al. (1975) and Philipsson (1975). A phenotypic standard deviation o f 20% for trotting was chosen, a value very similar to that set by RSnningen (1975). For type and conformation and the other riding horse traits, the phenotypic standard deviation has been chosen in such a way that the genetic variation is the same for a certain trait irrespective of the heritability level, which reflects the quality of the judging. The standard deviation chosen for type and conformation agrees well with figures given by Stamp (1973) and Varo (1965) and the values chosen for riding traits agree with those o f Bade et al. (1975) and Philipsson (1975). The generation interval for the characteristic conformation can be quite short -- 8 years -- assuming that the animals can be judged at an age of 2--3 years. The longest generation interval is for characters measured in competitions, when the stallions must have at least 5--6-year-old progeny. The size of the progeny group was assumed to be 20, for both trotters and riding horses. The genetic gain was calculated for different magnitudes of the genetic relationship between performance test and competition ability as measured by progeny testing. The selection intensity in both stages of the two-stage selection procedure was so chosen that the total selection intensity is the same as when only mass selection is used. RESULTS AND DISCUSSION Trotters
The estimates of genetic improvement in competition results for trotters are given in Table II. At the lower heritability level, 0.2, the two-stage selection procedure gave a b o u t 17% higher genetic gain than selection based only on
307
the individual's o w n results. When the heritability was 0.3, mass selection gave almost as high a genetic gain as two-stage selection. The greater accuracy o f the breeding values through progeny testing was in this case almost cancelled o u t by the negative effects of increased generation interval. As regards trotters, the most important instrument of selection must be individual testing, especially as nearly every horse is tested on the race course. However, the results of these calculations depend very much on the relation between the selection intensity in the first and the second stage. In the present calculations, a strong selection intensity was assumed in the first stage (8%) and a weaker intensity in the second (25--50%) among the already selected animals. Nevertheless, in some populations the reverse is the case, with stronger selection intensity in the second stage, especially in the path sire--son, which means that two-stage selection will be more effective than mass selection alone. Furthermore, it is quite possible that the breeders will wait before using a stallion on a larger scale until his progeny have appeared on the race courses, as the accuracy of selection improves after completion of the progeny test (Table III). This means in practice that the selection intensity at progeny testing may be increased, which should justify the use of this m e t h o d as a selection criterion. A very important trait for trotters is the ability to endure the strain of competitions over several years, in other words their longevity. It may be questioned whether, ordinarily, this complex of traits has a heritability large enough to achieve an acceptable genetic gain without progeny testing. In the practical breeding of trotters the conclusion may be drawn that the strongest selection intensity should be based on individual performance, b u t that the genetic evaluation should later be supplemented with progeny testing, T A B L E II A n n u a l genetic i m p r o v e m e n t for t r o t t e r s w h e n using mass s e l e c t i o n versus two-stage selection h2
0.2 0.3
Mass s e l e c t i o n
Two-stage selection
%
Rel.
%
Rel.
0.60 0.98
100 163
0.70 1.00
117 167
T A B L E III A c c u r a c y o f b r e e d i n g values for t r o t t e r s
(rGl) w h e n
mass s e l e c t i o n a n d two-stage s e l e c t i o n are used
h2
Mass s e l e c t i o n
T w o - s t a g e selection
0.2 0.3
0.45 0.55
0.70 0.74
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especially if some of the desired traits have a lower heritability than in the calculations above.
Riding horses For type and conformation of riding horses, our calculations gave a genetic improvement o f 0.75% per year when using mass selection only, compared with 0.66% per year when using two-stage selection, which gives a relative difference of 14%. This means that, due to the relatively high heritability and the longer generation intervals that follow progeny testing, the two-stage selection is inferior to mass selection. However, since conformation can be measured rather early in life, progeny testing may be used as a complement if it can be done w i t h o u t lengthening the generation interval t o o much. Often, sufficient information is at hand to carry o u t progeny testing and of course this information should then be used. The important point is that the selection intensity should be high in the first stage of the two-stage selection scheme. For competitive ability, the'breeding objective, the level of correlation between performance test and competition results, and the subsequent generation intervals, all determine whether mass selection or two-stage selection is the more efficient method. If the breeding objective is defined as competitive ability alone, then, as shown in Table IV, the two-stage selection m e t h o d is superior to performance testing when the correlation between performance tests and competitive results is below the 0.8 level. This is quite conceivable, since a performance test should measure not only pure competitive ability but also other riding traits such as willingness to work, ease of handling, etc. However, it is reasonable to assume that the breeding objective for most populations of riding horses is n o t competitive ability alone, b u t also those characteristics that are recorded in a performance test, as mentioned above. Ascribing equal importance to these two groups of characteristics and assuming that the level of heritability for the performance test is 0.5, it is shown in Table IV that performance testing is superior to two-stage selection -- at least when the correlation is 0.4 or higher. If, however, the heritability of the performance test is lower, e.g., 0.3, then the two-stage selection m e t h o d will almost match performance testing when the correlations are 0.4 or higher. From the practical breeder's point of view the genetic gain of the whole population may n o t be as important as the fact that the accuracy of the breeding values increases considerably when using progeny testing as a complement to performance testing (Table V). One point of interest is that the accuracy of the breeding values -- in the cases of two-stage selection -- decreases at higher levels of the genetic correlations between the breeding objectives. This is a result of the reduced genetic variance among stallions remaining in the last stage of the two-stage selection procedure. When conclusions from these calculations are drawn, it will be obvious
(equal weights)
Two-stage selection
0.3 0.5
0.3 0.5
Two-stage selection (performance + progeny test)
Performance test only
0.5
Performance test only
Competitive ability
Performance test characteristics+ competitive ability
0.5
Selection criteria
Breeding objective
0.52 73 0.57 80
0.50 70 0.62 87
0.51 96
0.35 66
0.58 0.64
0.58 0.71
0.60
0.53
82 90
82 100
113
100
134
0.64 0.70
90 99
0.65 92 0.79 111
0.68 128
0.71
145
166
0.70 0.77
99 108
0.72 101 0.89 125
0.77
0.88
Heritability Genetic correlation between performance test and of performan- competitive ability ce test for 0.4 0.6 0.8 1.0 stallions % Rel. % Rel. % Rel. % Rel.
Comparison of annual genetic improvement of riding horses when selection is based on performance test and a two-stage selection procedure. The relative values are comparable only within the same breeding objective
TABLE IV
O
(equal weights)
Two-stage selection
Performance test only
0.3 0.5
0.3 0.5
0.5
Performance test characteristics + competitive ability
0.5
Performance test only
Two-stage selection (performance progeny test)
Competitive ability
Heritability
Selection criteria
Breeding objective
0.70 0.73
0.46 0.59
0.78
0.28
0.4
0.75 0.80
0.49 0.63
0.78
0.42
0.6
0.81 0.87
0.52 0.67
0.78
0.57
0.8
0.77 0.77
0.55 0.71
0.77
0.71
1.0
Genetic correlation between performance test and competitive ability
Comparison of accuracy of breeding values for riding-horse traits when selection is based on performance testing and the two-stage selection procedure
TABLE V
O
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that the results depend very much on which parameters (length of generation interval, percentage selected in the different stages, heritability, genetic correlations, etc.) are chosen. Nonetheless, it is reasonable to assert that a second selection stage after the performance test is defensible, since it is likely that the heritability for a performance test is often somewhat lower than 0.5. However, as the performance test increases in confidence and precision, its heritability rises and a second selection stage based on progeny testing loses its importance, from the aspect o f annual genetic gain. The main conclusion to be drawn here is to emphasize the importance of accurate performance testing, rather than considering what can be gained by progeny testing as a complement to the performance test. Progeny tests may in any case be easily computed on the basis of competition statistics and will, of course, constitute valuable information for practical breeders. REFERENCES Bade, B., Glodek, P. and Shormann, H., 1975. Die Entwicklung von Selektionskriterien fdr die Reitpferdezucht. I. Genetische Parameter fiir Kriterien der Eigenleistungspriifung yon Junghengsten auf Station. Ziichtungskunde, 47: 68--77. Dusek, J., 1970. Zur Heritabilit~it des K~rperbaues und des Ganges bei Pferden. Z. Tierz. ZuechtungsbioL, 87: 14--19. Langlois, D., 1974. Resultats de recherches en mati6re g~n~tique chez le chevai. I. Etude des gains des chevaux dans les Concours de Saut d'ostaclea Journ. et Comm. Cheval, Fed. Europ. Zootech. Copenhague, 17--21, August 1974, 14 pp. Linner, M.-T. and Ostarkorn, K., 1974. Ziichterische Auswertung der Rennleistung yon Traberpferden der Jahrgiinge 1963 und 1964 in der BRD. Ziichtungskunde, 46: 168--176. Minkema, D., 1975. Studies on the genetics of trotting performance in Dutch trotters. Ann. G~n~t. S~I. Anita., 7: 99--121. Philipsson, J., 1975. Studies on population structure in Swedish horses. Proc. Int. Syrup. on Genetics and Horse-Breeding, Dublin, Ireland, 17--18 September 1975, pp. 59--62. Rendel, J. and Robertson, A., 1950. Estimation of genetic gain in milk yield by selection in a closed herd o f cattle. J. Genet., 50: 1--8. RSnningen, K., 1969. Studies on selection in aminai breeding. I. The efficiency of two-stage selection compared with single-stage selection with respect to progeny testing in animal breeding. Acta Agric. Scand., 19: 149--174. RSnningen, K., 1975. Genetic and environmental factors for traits in the North-Swedish trotter. Z. Tierz. Zuechtungsbiol., 92: 164--175. Stamp, H., 1973. Populationsgenetische Untersuchung an Holsteiner Warmblutpferden. Diss. Institut fiir Tierzucht und Tierhaitung der Christian-Albrecht-Universitiit, Kiel, 59 pp. Varo, M., 1965. Some coefficients o f heritability in horses. Ann. Agric. Fenn., 4: 223--237. RESUME StrSm, H. et Philipsson, J., 1978. Importance respective des tests de performance et des tests de descendance dans la selection du cbevai. Livest. Prod. Sc~, 5 : 3 0 3 - - 3 1 2 (en anglais). On a calcul6 le gain g~n~tique afin de comparer la s~lection massale ~ la s~lection ~ deux
312
6tages (s61ection massale + testage) pour les chevaux de trot et de loisir. Les prlncipales conclusions obtenues sont les suivantes: (1) Dans la s61ection du cheval, la s61ection massale et un testage sur descendance bien adapt6 sont d ' u n grand int6r~t, principalement ~ cause du long intervalle entre g6n6rations et aussi parce que les caract~res peuvent ~tre mesur6s sur chaque animal des deux sexes. (2) Pour des caract~res tels qui la conformation et la vitesse de trot, qui peuvent ~tre mesur6s pr~cocement dans la vie et qui ont une h6ritabilit6 61ev6e ou moyenne, la s61ection doit ~tre bas6e principalement sur le ph6notype et l'individu. Cependant quand l'h6ritabilit6 est faible, le testage sur descendance sera plus important. Le restage sur descendance entrafne en m~me temps une exactitude am61ior6e de la s61ection, cequi en soi a une importance consid6rable pour les races individuelles. (3) Pour quelques caract6res qui sont mesur6s plus tard darts la vie, tels qui l'aptitude la comp6tition et d'autres caract~res des chevaux de loisir, le choix du sch6ma de s61ection d6pend de la pr6cision du contrble individuel de performance, des objectifs de s~lection et de la magnitude de la relation g6n6tique entre les objectifs de s61ection. Mais en conclusion g6n6rale, on peut dire que le testage sur descendance est extr~mement pr6cieux en compl6ment du contr5le individuel de performance. KURZFASSUNG StrSm, H. und Philipsson, J., 1978. Die relative Bedeutung der Eigenleistungs- und Nachkommenprtifungen in der Pferdezucht. Livest. Prod. 8ci., 5 : 3 0 3 - - 3 1 2 (in Englisch). In der vorliegenden Arbeit wurde der genetische Fortschrit berechnet, um die Einzelselektion mit der Zweistufenselektion zu vergleichen (Einzelselektion + Nachkommenpriifung) fiir Traber und Reitpferde. Die wichtigsten Schlussfolgerungen sind: (1) In der Pferdezucht ist die Einzelselektion und eine zweckm~ssig gestaltete Eigenleistungspriifung yon grSsster Bedeutung, weft verhiiltnism~ssig lange Generationsinterval!e vorliegen und weil die meisten Eigenschaften fast an allen Tieren und bei beiden Geschlechtern gemessen werden kSnnen. (2) Fiir Eigenschaften, z.B. ExteriSr und Rennleistung, die eine hohe oder mittlere • Heritabilitiit aufweisen, sollte hauptsiichlich die Selektion nach dem Phiinotyp erfolgen. Mit sinkender Heritabilitiit w~ichst die Bedeutung der Nachkommenpriifung. Die Nachkommenpriifung beschafft auch eine h~here Genauigkeit der Selektion, welche an sich fiir die einzelnen Rassen yon grosser Bedeutung ist. (3) Fiir gewisse Eigenschaften, z.B. Eignung fiir den Turniersport und andere Reitpferdeeigenschaften, h~'lgt die Wahl des Selektionsplanes teils vom Zuchtziel und teils yon der Genauigkeit der Eigenleistungspriifung und der Genetischen Korrelation zwischen den Zuchtzielen ab. Es kann jedoch die Schlussfolgerung gezogen werden, dass die Nachkommenpriifung eine sehr wertvolle Erg~nzung zur Eigenleistungspriifung fdr diese Eigenschaften ist.