Thorvaldur Árnason

Genetic Studies on Conformation and Performance of Icelandic Toelter Horses

Abstract The breeding values of 3975 horses, included in the dataset, were assessed simultaneously for the ten traits by a Best Linear Unbiased Prediction (BLUP) procedure. The correlations among the traits were considered and the additive genetic relationship among the horses were accounted for. A description is given of the computational procedure. Canonical transformation of the variates substantially reduced the computing effort entailed and made it possible to solve the BLUP equations separately for each trait. The method is currently being applied to routine evaluation of breeding horses in the Icelandic toelter horse population.

Genetic parameters for traits at performance test of stallions and correlations with traits at progeny tests in Swedish warmblood horses.

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 for performance traits.

Contribution of various factors to genetic evaluations of stallions

Abstract Genetic evaluations (BLUP) of 262 stallions were obtained for the following subjectively scored traits of the Swedish Riding Horse Quality Events: conformation, gaits, temperament in the gaiting test, jumping ability and temperament in the jumping test. The genetic evaluations were partitioned into the various factors included in the statistical model applied to the data. The partitioned sire solutions were compared with genetic evaluations obtained from application of an animal model. The differences between the evaluations indicated the influence of the genetic merit of mates. The following correlations were found between the estimated breeding values (from the animal model) of sires and their mates for the five traits in the same order as above: 0.101, 0.046, 0.096, 0.004, 0.020. Evaluations of genetic values within birth years (1973–1979) indicated a slightly negative genetic trend in conformation score and slightly positive genetic trend in the other four traits.

A note on the fertility and some demographical parameters of Icelandic toelter horses

Mating results of 5649 pairings taking place in the years 1978 and 1979 were analysed. The average fertility rate for 316 stallions was found to be 85.6%. On average, 17.9 mares were mated to each stallion during the breeding season. The overall fertility (conception) rate was estimated to 82.1%, the apparent fertility rate 81.1% and the numerical productivity rate 78.0%. The sex ratio for the newborn foals was found to be 52.5% males vs 47.5% females, which is significantly different from an equal ratio of the two sexes. The generation interval from parents birth to birth of offspring, LPO, was estimated to be 9.7 years. The generation intervals for the four different paths were: LSS=8.6, LSD=8.7, LDS=11.4 and LDD=11.6.

Genetic selection indices for Swedish trotter broodmares

Abstract Studbook performance records for 2734 Swedish standardbred trotter broodmares have been analysed. Transformation of most variables was necessary in order to approach normality of the records. The heritability for the traits varied between 0.10 and 0.68, the lowest value for transformed “number of starts” and highest for transformed “earnings per start”. Genetic correlations were very high or high between all traits except for transformed “number of starts”. Transformed “earnings” was found to be genetically almost identical with transformed “best life-time racing time”, but more suitable for selection purposes. Several selection indices were constructed and their use in mass-selection and for adjusting for non-random distribution of dam genotypes when progeny-testing stallions is discussed.

The heritability of degenerative joint disease in the distal tarsal joints in Icelandic horses

The heritability of radiographic signs of degenerative joint disease (DJD) in the distal tarsal joints and hind limb lameness after flexion test of the tarsus was estimated in a population of Icelandic horses being used for riding. Furthermore, the genetic and phenotypic correlation between the radiographic signs and lameness was estimated. Of the 614 examined horses, aged 6–12 years, 420 belonged to progeny groups from 17 selected stallions and 194 were sired by unselected stallions. The heritability was analyzed by two statistical models based on the threshold liability concept, a non-linear sire model and a linear animal model. The heritability of radiographic signs of DJD in the distal tarsal joints was not found to be significantly different from zero. The h2 estimate for lameness was higher and in the order of 0.4. A positive genetic and phenotypic correlation was identified between radiographic signs of DJD in the distal tarsal joints and lameness. The results indicate that the prevalence of DJD in the distal tarsal joints could be reduced in Icelandic riding horses by breeding selection based on flexion test of the tarsus followed by radiographic examination.

Predicting Genetic Trend by Uni- and Multitrait Models

Abstract The effects of evaluating milk yield in the first three lactations by a single-trait animal model or by a repeatability animal model instead of by the true multitrait animal model were investigated using stochastic simulation. The models were compared both in terms of how accurately they predicted genetic trend when applied to the same dataset and regarding difference in true genetic progress when selecting on breeding values predicted by these models. A breeding structure resembling the Icelandic dairy cattle population was used for the simulation. The single-trait model underpredicted the true genetic trend in second and third lactations quite heavily but only by 2.3% in the first lactation, while the repeatability model overpredicted the true genetic trend in the first three lactations by 9.4% on average. The multitrait animal model, however, estimated genetic trends with a high degree of accuracy. When selection was made on all three lactations with equal economic weights, the multitrait mode...

Genetic studies on carcass traits in iceland twin ram lambs II. Analysis of principal components and construction of selection indices

Abstract The relationship between 20 carcass measures in twin ram lambs at the Experimental Sheep Farm, Hestur, Iceland, was studied by means of principal components. The genetic and phenotypic correlation matrices were modified by the recently developed “bending” procedure in order to reduce certain undesirable effects of sampling errors. The results of the principal component analysis indicated that a reduced number of representative variates can be extracted to form a selection criterion without incurring any great loss in efficiency. The breeding objective is assumed to be the production of compact carcasses having a large muscle to fat ratio. The selection criterion should include measures of fatness, amount of various valuable muscles, and carcass conformation. The ratio of cannonbone circumference to cannonbone length (MC/ML) was found to be a fair measure of the desired conformation. Seven traits were selected for the construction of substitution selection indices for progeny testing of rams. A carcass quality index is proposed, formed by weighting three of the substitution indices with economic values.

Multistage index selection in finite populations.

Multistage selection with fixed proportions and selection indices based on covariates of the target variable is studied. Assuming a multivariate normal distribution before the selection, expressions are presented for the expectation and the variance of the target variable in the retained subpopulation. As the numerical evaluation for finite populations requires lengthy computations, some approximations using methods for infinite populations are proposed. Numerical illustrations are given for selections in up to three stages.

Genetic Studies on Conformation and Performance of Icelandic Toelter Horses: III. Study on Covariance Matrices and Breeding Objectives by Principal Component Analysis

The relationship among ten correlated traits was studied by means of principal component analysis. The first principal component contrasted horses with generally high versus low scores for all traits. The four largest principal components accounted for 91%, 77%, 84% and 66% of the variation in the G, E, r G and r E covariance and correlation matrices, respectively. Principal component analyses were also used to examine the effects of variously defined vectors of relative weights, upon resulting selection indices.