Browsing by Author "Jordaan, G. F."

Now showing 1 - 7 of 7
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    The effect of Charolais and Hereford sires and straightbred and crossbred dams on pre-weaning growth of calves
    (South African Society for Animal Science, 2002) Dadi, H.; Jordaan, G. F.; Schoeman, S. J.; Van Der Westhuizen, J.
    Weaning performance of calves sired by Charolais and Hereford on dams of Angus, Bonsmara and Hereford and various crossbred dam combinations was compared. Charolais sired calves were on average 5 kg heavier at birth and 20 kg heavier at weaning than Hereford sired calves. Among the three straightbred dam breeds, the Bonsmara weaned calves that were approximately 6% heavier than both the Angus and the Hereford calves. The use of crossbred dams having intermediate levels of Charolais composition is suggested in crossbreeding programmes in intensive production systems based on cultivated pastures under irrigation.
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    A genetic analysis of epistaxis as associated with EIPH in the Southern African Thoroughbred
    (South African Society for Animal Science, 2004) Weideman, H.; Schoeman, S. J.; Jordaan, G. F.
    Pedigree and race run data from Thoroughbreds racing in Southern Africa, covering the period 1986-2002 (63 146 horses in pedigree data-set and 778 532 race runs), were analysed in order to study genetic and environmental factors affecting the incidence of epistaxis as associated with "exercise-induced pulmonary haemorrhage" (EIPH). Variables that were tested as significant in preliminary data analyses were included as fixed effects for the model. Various combinations of such variables namely age, weight, altitude, sex, month and going were tested. Fixed effects that were included in the final model were gender, going and altitude. The heritability estimates from a logit transformed analysis for epistaxis fitting both the animal and sire models were 0.23 and 0.40, respectively, which indicated that epistaxis as associated with EIPH in Southern African Thoroughbred sires has a strong genetic basis. Genetic trends indicating an increase in epistaxis were also found. Affected stallions and those racing whilst being treated with furosemide should be barred from breeding and not be considered as future sires. Estimated breeding values for epistaxis should be used as a tool for selecting against it and be considered in breeding programmes to decrease the incidence thereof.
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    Genetic parameter estimates of early growth traits in the tygerhoek Merino flock
    (South African Society for Animal Science, 2002) Duguma, G.; Schoeman, S. J.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Jordaan, G. F.
    Genetic parameters were estimated for birth weight (BWT), weaning weight (WWT) and pre-weaning average daily gain (ADG) using Restricted Maximum Likelihood (REML) procedures. Four different animal models were fitted, differentiated by including or excluding maternal effects. The direct heritability estimates (h2) ranged from 0.19 to 0.38, 0.25 to 0.36 and 0.26 to 0.34 for BWT, WWT and ADG, respectively. The estimates were substantially higher when maternal effects, either genetic or environmental, were ignored from the model. The maternal heritability (m2) for BWT was 0.38 when only maternal genetic effects were fitted in the model but decreased to 0.25 when the maternal permanent environmental effect (c2) was fitted. The m2 for WWT and ADG ranged from 0.05 to 0.11 and from 0.04 to 0.10, respectively. The respective c2 estimates ranged from 0.06 to 0.10 and from 0.06 to 0.09. Moderate negative genetic correlations (ram)between direct and maternal genetic effects were observed in BWT, while close to zero estimates were obtained for WWT and ADG. The direct genetic correlation estimates between BWT and WWT and BWT and ADG were 0.16 and 0.04, respectively. The corresponding maternal genetic correlation estimates were 0.93 and 0.60. The direct and maternal genetic correlation estimates between WWT and ADG were 0.99 and 0.85, respectively. It is suggested that selection should be applied to WWT.
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    Genetic parameters of testicular measurements in Merino rams and the influence of scrotal circumference on total flock fertility
    (South African Society for Animal Science, 2002) Duguma, G.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Schoeman, S. J.; Jordaan, G. F.
    Genetic parameter estimates for scrotal circumference (SC), testis diameter (TD) and two-tooth liveweight (LW) were obtained for 1380 two-tooth Merino rams born from 1986 to 1998 on the Tygerhoek Experimental Farm. The effect of SC of service sires (n = 263) on ewe fertility was also investigated. Year of birth, selection group and LW were significant sources of variation for both SC and TD. SC had a significant effect on ewe fertility. Heritability estimates ranged from 0.29 to 0.40, 0.25 to 0.38 and from 0.49 to 0.52 for SC, TD and LW, respectively. Adjustment for LW decreased heritability estimates of SC and TD and the genetic correlations between the latter traits. Rams with an unadjusted SC of less than 30 cm should not be used.
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    The influence of multicollinearity on crossbreeding parameter estimates for weaning weight in beef cattle
    (South African Society for Animal Science, 2002) Schoeman, S. J.; Aziz, M. A.; Jordaan, G. F.
    Data on 17258 weaning weight records of calves from a crossbreeding project were utilized to investigate the problem of collinearity and its effect on the estimation of direct, maternal and the non-additive genetic effects. Several criteria were used to detect the near-dependency among the independent variables. The results indicated that there was a near-dependency among both the direct and the maternal genetic effects causing unstable estimates. It was attempted to solve the collinearity problem using ridge regression. An improved model fit was evident at a ridge value of k = 0.8 with large reductions in standard errors and estimates with more meaningful biological interpretation. Ridge regression is recommended for the estimation of crossbreeding effects where inevitable collinearity amongst the independent variables is evident.
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    The inheritance of liability to epistaxis in the Southern African Thoroughbred
    (AOSIS, 2004) Weideman, H.; Schoeman, S. J.; Jordaan, G. F.
    ENGLISH ABSTRACT: This study was carried out to estimate the heritability of liability to epistaxis in the southern African Thoroughbred population. Data of all horses that suffered epistaxis while racing in southern Africa and Mauritius from 1986 to 2002 and involving 1252 bleeders were analysed. Pedigree data covering the period 1960-1986 was used as required to calculate the incidence of bleeding amongst ancestors of the post-1986 era. Only pedigrees of horses that raced were included in this study as it was not possible to predict whether non-runners would have bled had they raced. Consequently all non-runners and also those that raced overseas in countries where bleeding occurrence was not recorded, were excluded. The heritability of liability method as described by Falconer (1989) was used to estimate the relative importance of heredity and environment. For the period investigated, the population incidence for epistaxis in southern African horses was 2.1 %. The estimation of heritability of liability showed that 1st-degree relatives had a figure of 55.4 %. Second-degree relatives had a heritability of 41.3 % and 3rd-degree relatives came in at 30.4 %. The heritability of liability shown in this study could be regarded as being at the lower end of the range but could be appreciably higher. The data depict horses that bled almost exclusively on race days, as only a small percentage (~5 %) was reported as having bled during exercise. Accordingly, the full extent of epistaxis amongst racing Thoroughbreds in southern Africa is difficult to gauge. It is clear that epistaxis in the racing Thoroughbred has a strong genetic basis. It is suggested that horses showing frank bleeding from the nostrils after racing or exercise be suspended, and not used for breeding purposes. This should result in relatively fast progress being made towards eradicating this costly scourge of the modern Thoroughbred racehorse.
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    Multitrait estimation of direct and maternal (co)variances for growth and efficiency traits in a multibreed beef cattle herd
    (South African Society for Animal Science, 1999) Schoeman, S. J.; Jordaan, G. F.
    Estimates of (co)variance components were obtained for growth and efficiency traits in a multibreed synthetic beef cattle herd. Components were estimated simultaneously by fitting four alternative seven-trait models. Direct heritabilities varied according to the model fitted but were higher for preweaning than for postweaning traits. Applying a model which also included maternal heritabilities, the direct-maternal covariance and the permanent environmental effect, direct heritabilities of weaning weight, weaning index, preweaning Kleiber ratio, preweaning relative growth rate, cow efficiency, postweaning Kleiber ratio and postweaning relative growth rate were 0.57, 0.40, 0.35, 0.71, 0.54, 0.16 and 0.13, respectively. In general, maternal heritabilities varied from 0.03 for postweaning Kleiber ratio to 0.45 for cow efficiency. Direct-maternal correlations were negative and varied from -0.31 to -0.58 for weaning weight related traits, but was -0.77 for cow efficiency.