SUNScholar - Browsing by Author "Van Wyk, J. B."

Browsing by Author "Van Wyk, J. B."

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Across flock genetic parameter estimation for yearling body weight and fleece traits in the South African Dohne Merino population
(South African Society for Animal Science, 2008) Van Wyk, J. B.; Swanepoel, J. W.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Olivier, J. J.; Delport, G. J.
Accurate genetic parameter estimates are needed upon which to perform multiple-trait across flock breed analyses. Genetic parameters for yearling body weight (BW), clean fleece weight (CFW) and mean fibre diameter (MFD) were estimated using records of 107 389 individuals (the progeny of 1 530 sires and 45 178 dams) collected between 1992 and 2004 in the South African Dohne Merino population. Fixed effects included in the model were flock-year-season-sex- management group (1 594 classes), type of birth (singles, multiples), age of dam (2 - 7+ years) and age at measurement, fitted as a linear covariate (385 ± 12 days). Six different single-trait animal models were fitted, where different combinations of the following random effects were fitted: direct additive, the sire-flock interaction, the sire-flock-year-season interaction (SFYS), the dam genetic effect, the direct-maternal correlation and the dam permanent environmental effect. These analyses were followed by a three-trait analysis structured according to the log likelihood ratios obtained for the single-trait analysis. This analysis allowed the calculation of relevant correlations among traits together with their respective standard errors. Direct heritability estimates from the three-trait analysis were 0.17 for BW, 0.19 for CFW and 0.45 for MFD. Maternal heritability estimates were 0.01 for BW and 0.006 for CFW, with corresponding dam permanent environmental ratios of respectively 0.03 and 0.02. The genetic correlation between animal effects amounted to 0.48 for BW. Derived proportions of the total phenotypic variance due to SFYS were 0.02, 0.02 and 0.02, respectively. Genetic correlations of BW with CFW and MFD were 0.11 and 0.13, respectively, and of CFW with MFD 0.16. It was concluded that the inclusion of some form of a genotype by environmental interaction as part of the national evaluation is essential, although it controlled only a modest portion of the overall phenotypic variation.
Crossbreeding Holstein-Friesian with Ethiopian Boran cattle in a tropical highland environment : preliminary estimates of additive and heterotic effects on milk production traits
(South African Society of Animal Science, 2000) Demeke, S.; Neser, F. W. C.; Schoeman, S. J.; Erasmus, G. J.; Van Wyk, J. B.; Gebrewolde, A.
Genetic parameters and relationships of faecal worm egg count with objectively measured wool traits in the Tygerhoek Merino flock
(South African Society for Animal Science, 2014-07-09) Matebesi-Ranthimo, P. A. M.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Van Wyk, J. B.; Olivier, J. J.
The costs of internal parasite control and treatment are potentially very high in grazing sheep. Faecal worm egg count (FEC) has been suggested as a suitable criterion for selection for resistance to nematode infestation in livestock. Genetic parameter estimates for FEC and its relationship with wool traits were assessed in the current study, using data from Merino sheep from a selection experiment maintained at Tygerhoek Research Farm. Data consisted of ~7 100 animals born between 1989 and 2010. Rectal faecal samples were taken from individual sheep at 13 to 16 months of age, after drenching had been withheld for at least 10 weeks, generally in July to September. Nematode eggs were counted with the McMaster technique, with a sensitivity of 100 eggs per gram of wet faeces. The fixed effects of birth type, sex, birth year and sex x birth year interaction were included in the operational model for FEC. Only the effect of additive animal affected the data for FEC. Heritability estimates of FEC ranged from 0.10 for untransformed FEC to 0.16 for Log (FEC + 100). The genetic relationships of Log (FEC + 100) with staple strength (SS), staple length (SL), coefficient of variation of fibre diameter (CVFD) and standard deviation of fibre diameter (SDFD) were favourable. The genetic correlations of FEC with wool weight traits were unfavourable in absolute terms, but not significantly different from zero. Selection for FEC is unlikely to result in unfavourable correlated responses to wool traits in South African Merinos, with the exception of wool weights.
Genetic parameters for subjectively assessed wool and conformation traits in the Tygerhoek Merino flock
(South African Journal of Animal Sciences, 2009) Matebesi, P. A.; Van Wyk, J. B.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe)
Records of the Tygerhoek Merino resource flock were used to estimate genetic, phenotypic and environmental parameters for subjectively assessed wool and conformation traits. The database consisted of records of 4 495 animals, the progeny of 449 sires and 1 831 dams born in the period 1989 to 2004. The pedigree records have been collected between 1969 and 2004. Direct heritability estimates (h2a) for subjective wool traits ranged from 0.15 for face cover score to 0.50 for woolly face score. Corresponding h2a for subjective conformation traits ranged from 0.13 for topline (TOPL) to 0.39 for total fold score (TOT). Maternal heritability estimates were all below 10% where applicable. The proportion of the total phenotypic variance due to the maternal permanent environment variance (c2pe) amounted to 5% for general head conformation (GEN). The genetic correlation between animal effects ranged from -0.70 to 0.21 where applicable. Among the subjective wool traits favourable genetic correlations (rg) were estimated between regularity of crimp (ROC) and wool colour (COL) (0.31), for wool quality (QUAL) with ROC (0.49) and COL (0.26) and between staple formation (STAPL) and belly and points (BANDP) (0.58). The relationships between ROC and STAPL (-0.49) and for QUAL with STAPL (-0.45) and BANDP (-0.20) were unfavourable. The noteworthy relationships among subjective conformation traits were those between the conformation of the hind legs and the conformation of the front legs (0.71) and of GEN and TOPL with TOT (-0.31 and -0.47 respectively). The rg of significance between subjective wool and conformation traits were variable in sign and magnitude. These results indicated the possibility to achieve sustained genetic improvement by selection for subjective wool and conformation traits in South African Merino sheep.
Genetic prediction models and heritability estimates for functional longevity in dairy cattle
(South African Society for Animal Science, 2015) Imbayarwo-Chikosi, V. E.; Dzama, K.; Halimani, T. E.; Van Wyk, J. B.; Maiwashe, A.; Banga, C. B.
Longevity is a major component of the breeding objective for dairy cattle in many countries because of its high economic value. The trait has been recommended for inclusion in the breeding objective for dairy cattle in South Africa. Linear models, random regression (RR) models, threshold models (TMs) and proportional hazard models (PH) have been used to evaluate longevity. This paper discusses these methodologies and their advantages and disadvantages. Heritability estimates obtained from these models are also reviewed. Linear methodologies can model binary and actual longevity, while RR and TM methodologies model binary survival. PH procedures model the hazard function of a cow at time t derived from survival from first calving to culling, death or censoring. It is difficult to compare methodologies for sire evaluation and ranking across countries because of the variation in the definition of longevity and the choice of model. Sire estimated breeding values (EBVs) are derived differently for the models. Sire EBVs from PH models are expressed as deviations of the culling risk from the mean of the base sires, expected percentage of daughters still alive after a given number of lactations, expected length of productive life in absolute terms or as standard deviation units. In linear, TM and RR modelling, sire EBVs for longevity have been expressed as deviations of survival from the mean estimated with Best Linear Unbiased Prediction (BLUP). Appropriate models should thus be developed to evaluate functional longevity for possible inclusion in the overall breeding objective for South African dairy cattle.
Inbreeding in the Dohne Merino breed in South Africa
(South African Society for Animal Science, 2007) Swanepoel, J. W.; Van Wyk, J. B.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Delport, G. J.
The actual level of inbreeding and the effect of inbreeding depression on yearling body weight and fleece traits in the South African Dohne Merino population were investigated. All available pedigree data, which comprised of 266 268 records (302 169 animals including base parents) for the period 1975 to 2003 were used for calculating individual inbreeding coefficients. Inbreeding depression was estimated as the regression of performance (corrected for fixed effects) on the individual and dam inbreeding coefficients, fitting an animal model. The rate of inbreeding (ΔF) was estimated as the difference between the individual inbreeding (Ft) and the inbreeding of the parents (Ft-1) divided by (1-Ft-1). The level of inbreeding (F) in the SA Dohne Merino sheep population is very low. The proportion of animals that was inbred to some extent increased from 0.00 (average F = 0) in 1980 to 0.38 in 2003 (average F = 0.012). No significant inbreeding depression on body weight and fleece traits could be found. In general the results suggest that inbreeding at present is not a serious problem in the South African Dohne Merino breed.
Non-linear model analysis of categorical traits related to female reproduction efficiency in beef cattle
(South African Society for Animal Science, 2009) Rust, T.; Schoeman, S. J.; Van Der Westhuizen, J.; Van Wyk, J. B.
The utilization of non-linear threshold models and linear animal models to estimate variance components for categorical reproductive traits in beef cattle was investigated. Three traits, retention (RET), stayability (STAY) and calf tempo (CT) were defined. The data set consisted of 36 880 Afrikaner beef cattle measurements recorded over a period of 10 years. After editing, the records of 7 746 females from 473 sires were available for the genetic parameter estimation for RET. The corresponding number of records for STAY and CT were 3 018 (243 sires) and 7 653 (465 sires), respectively. Using GFCAT, sire variances for RET, STAY and CT were estimated as 0.202, 0.072 and 0.114, respectively. With error variances pre-set to one, the heritability estimates on the underlying scale were calculated as 0.67, 0.27 and 0.41 for RET, STAY and CT, respectively. From the three traits analyzed in this study CT probably reflects the true fertility of the bull's female progeny best.
Parameter estimates for reproductive output and product quality traits of ostrich females within breeding seasons
(South African Society for Animal Science, 2011) Fair, M. D.; Van Wyk, J. B.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe)
Data involving monthly records of egg production (EP), chick production (CP), hatchability (H), mean egg weight (MEW) and mean day-old chick weight (MCW) were analysed using REML procedures. All traits were treated as hen traits. Egg production of young birds increased to reach a peak of approximately 4 to 5 eggs per month relatively late in the breeding season (September to December). Older hens reached a higher peak of 6 to 9 eggs per month earlier in the breeding season (August to September). There was a secondary peak in older birds from November to December. All birds except 2-year olds exhibited evidence of a slump of production during October. Chick production, MEW, MCW and H followed similar trends. The random effects of direct additive genetic, permanent environment (PE), temporary environment (TE - unique hen-year combinations) and service sire (SS) were estimated from the data. Heritability estimates of the direct additive genetic effect (h2a) of the hen were 0.04 for EP, 0.05 for CP, 0.44 for MEW, 0.67 for MCW and 0.02 for H. The estimates of permanent environment (c2pe) as a ratio of phenotypic variance for the four traits (MCW did not exhibit a significant c2pe effect) were 0.08, 0.08, 0.09 and 0.11, respectively. The effect of service sire as a ratio (c2ss) was significant but relatively low for all traits, ranging from 0.04 for MEW and MCW to 0.06 for CP. Monthly EP and CP were highly correlated at all levels, ranging from 0.74 for the TE correlation to unity (1.00) for the genetic correlation. Egg production was favourably correlated with H at a genetic level (0.98). The genetic correlations of EP and CP with MEW and MCW were variable and in some cases antagonistic as is often found in poultry. The genetic correlations of H with MEW and MCW were positive (0.52 and 0.47, respectively). As expected, the genetic correlation of MEW and MCW was very high at 0.94. The results indicate that selection for improved reproduction (reproductive output and product quality traits) in ostriches is possible. Selection is unlikely to be complicated by unfavourable correlations with H, MEW and MCW.
Pedigree analysis of an ostrich breeding flock
(South African Society for Animal Science, 2012) Fair, M. D.; Van Wyk, J. B.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe)
Pedigree records, maintained from 1978 to 2005 at the Oudtshoorn Research Farm, South Africa, of 40 074 birds of a pair-breeding ostrich flock were used to estimate the effective number of founders (fe), the effective number of ancestors (fa), the effective population size and the effective genome equivalents (fg) under random mating, to assess the genetic variability present in the population. The average level of completeness of the pedigrees was high (99.3%) in the first generation, and the average level of inbreeding (F), calculated from the pedigrees, was 0.51%. The reference population was defined as the 39 784 birds hatched from 1990 to 2005. The estimated measures of variability were fg = 47.3, fe = 59 and fa = 58, with an fe/fa ratio of 1.02. The numbers of ancestors responsible for 100%, 50% and 20% of the genes in the reference population were 254, 21 and 6, respectively. The largest individual contribution to the population hatched from 1990 to 2005 was from a male that was responsible for 4.85% of the genetic variability. The generation interval for the four selection pathways – calculated as the average age of parents when offspring that were kept for reproduction were born – were sire to son (7.74 ± 4.92), sire to daughter (7.77 ± 5.13), dam to son (7.50 ± 4.29) and dam to daughter (7.90 ± 4.92). The average generation interval of the reference population was 7.72 ± 4.79 years. The linear regressions of mean annual individual rate of inbreeding on year of birth for the two distinct periods 1995–2002 and 2003–2005 were 0.08% and -0.07% per year, respectively. The estimate of effective population size (Ne), computed via the increase in the individual rate of inbreeding, was 112.7. Estimates of Neusing the alternative methods of tracing the numbers of generations were 73.6, 177.4 and 95.3 for complete, maximum and equivalent complete generations. The results of this study indicated that the population under study was at an acceptable level of genetic variability.
Relationship of ewe reproduction with subjectively assessed wool and conformation traits in the Elsenburg Merino flock
(South African Society for Animal Science, 2017-11-06) Matebesi-Ranthimo, P. A. M.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Van Wyk, J. B.; Olivier, J. J.
Subjectively assessed wool and conformation traits are widely used to select breeding ewes and rams in the sheep industry. Data from a Merino flock that is maintained at Elsenburg Research Farm were used to investigate animal model (co)variance components for ewe reproduction traits with subjectively assessed wool and conformation traits. Ewe reproduction traits were assessed at their first lambing opportunity at two years, or over a three-year period from their lambing opportunities at two to four years old. Relationships of ewe reproduction traits with subjectively measured wool and conformation traits were also investigated. All these traits were heritable, with a range from 0.16 ± 0.03 for topline (TOPL) to 0.64 ± 0.04 for woolly face score (WFS). Genetic correlations of number of lambs born (NLB1) with colour (COL), number of lambs weaned (NLW1) with COL and belly and points (BANDP) and total weight of lamb weaned (TWW1) with COL were negative and significant. Significant genetic correlations of ewe reproduction traits over three lambing opportunities were found between number of lambs born (NLB3) and WFS (0.23 ± 0.11) and between total weight weaned (TWW3) and face cover score (FCS) (-0.33 ± 0.16). Among these traits, the noteworthy favourable genetic correlation between total fold score (TOT) and NLB1 suggested that plainer ewes were more reproductive. This is important for the South African Merino industry as plainer sheep are more desirable because of their faster growth and higher lambing percentages and reduced chances of fly strike. Selection for improved ewe reproduction in Merino sheep thus would not result in marked unfavourable correlated responses in most of these subjective wool and conformation traits.
The relationships between faecal worm egg count and subjectively assessed wool and conformation traits in the Tygerhoek Merino flock
(South African Society for Animal Science, 2014-08-02) Matebesi-Ranthimo, P. A. M.; Cloete, S. W. P. (Schalk Willem Petrus van der Merwe); Van Wyk, J. B.; Olivier, J. J.
Subjectively assessed wool and conformation traits form part of the selection objective in wool sheep enterprises. The present study investigated the genetic, phenotypic and environmental correlations for nematode resistance (using faecal worm egg count (FEC)) with subjectively assessed wool and conformation traits. The Merino sheep flock (consisting of four lines) maintained on Tygerhoek Research Farm was used. Fixed effects of selection line, birth type, sex, age of dam in years, year of birth, and sex*birth year interaction had a significant effect on all subjective traits. At genetic level, log transformed FEC was significantly related to wool oil only at 0.18 ± 0.09, staple formation at 0.29 ± 0.10, and topline at −0.33 ± 0.11. These correlations suggested that sheep with high FEC are likely to have excessive wool oil, thicker and bulkier staples, and lower scores for topline. Selection for resistance to and resilience against nematodes in Merino sheep thus will not result in marked unfavourable correlated responses in the vast majority of these subjective wool and body conformation traits.
A survey of the prevalence of blowfly strike and the control measures used in the Ruens area of the Western Cape Province of South Africa
(AOSIS, 2011) Scholtz, A. J.; Cloete, S. W. P.; Du Toit, E.; Van Wyk, J. B.; Van der Linde, T. C. de K.
Blowfly strike and the methods used to combat blowfly strike were recorded on 33 properties in the Rûens area of South Africa during 2003/2004. Data were recorded on Merino and Dohne Merino hoggets (n = 4951) with at least 3 months' wool growth. The following data were captured: presence or absence of strike, site of the strike (body or breech), presence or absence of dermatophilosis as well as subjective scores for wool quality and wool colour. Control measures recorded include: chemical treatment (preventative and spot treatment), crutching, mulesing and the use of the Lucitrap® system. Blowfly strike was not significantly influenced by gender or breed. Hoggets suffering from dermatophilosis were more likely to be struck, compared with contemporaries not suffering from the skin disorder (0.057 vs 0.027; P < 0.05). Merino hoggets generally had higher scores than their Dohne Merino contemporaries for wool quality (32.6 vs 27.4; P<0.05) and wool colour (29.0 vs 27.2; P<0.05). There was an indication that the Lucitrap® system may have reduced flystrike, but the effect was not statistically significant (P = 0.19 for overall strikes and P = 0.12 for body strike). The Mules operation benefited overall flystrike (0.013 vs 0.110; P < 0.05); mainly through an effect on breech strike (0.010 vs 0.109; P < 0.05). The proportion of fly strikes increased with wool length, and declined with an increase in farm size in wool colour score. None of the ethically acceptable control measures assessed could substantially reduce blowfly strike on their own, and an integrated pest management programme was proposed.

Browsing by Author "Van Wyk, J. B."

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