A genetic evaluation of production traits in Nguni cattle

[摘要] English: Genetic parameters may well aid the prediction of selection and correlated response when selectionis based on production traits. The animal model, fitting an additional random factor such as thepermanent maternal environmental effect is common practice to increase the accuracy of estimates.As selection in a beef breed such as the Nguni takes place in various environmental conditions, theinclusion of an additional random factor i.e. herd-year-season x sire interaction seems justified.Production traits analyzed included records of birth weight (BW), weaning weight (WW), yearlingweight (YW) and eighteen-month weight (EW). Herd-year-season, sex, age at recording (exceptBW) and age of dam had a significant effect. Herd-year-season x sire interaction (HYSxS) wasincluded as an additional random factor. Estimates obtained by univariate analyses were used asstarting values in the multivariate analyses. In multivariate analyses another data set was used whichincluded only herds linked with at least two other herds through sires used. This data set was alsoused in two bivariate analyses being the analyses between the maternally influenced traits (BWandWW) and WW and YW to gain knowledge as to a possible carry-over effect of the maternal geneticeffect. Genetic trends were calculated using the regression of the average breeding value estimates. (EBV) on year of birth. Direct heritability estimates for BW, WW, YW and EW obtained bymultivariate analyses were 0.31, 0.32, 0.23 and 0.27 respectively. Genetic correlations betweendirect and maternal effects for BW and WW were mostly low to moderate and negative (-0.35 to -0.59 and -0.17 to -0.50 for BW and WW respectively), while direct genetic correlations among thefour growth traits were moderate to high (0.51 to 0.95). The maternal genetic correlation betweenBW and WW was moderate (0.49). It was concluded that selection on WW alone would beundesirable as all other weights would increase. In review, breeding values for production traitsshould be seen in conjunction with one another so as to take into account undesirable effects onother traits. Genetic trends derived showed no major change.Nguni cattle, like most other beef producing breeds, are kept under a wide variety of extensiveenvironments throughout South Africa. Based on this, a genotype x environment interaction (G xE) in this breed may be expected. Three most important aspects while considering the nature ofinteractions are the ranking order of the genotypes, the relative magnitudes of genotypic, environmental and interaction effects and the statistical significance of interaction effects.Because of limitations in the data set, only weaning weight (WW) records were used. Records fromherds were allocated to one of three veld types that described the specific region in which each berdwas situated, i.e. sweetveld (Region 1), sourveld (Region 2) and mixed veld (Region 3). Bivariateanalyses of WW in the three different regions were carried out to determine the (co)variancecomponents and weighted direct heritability estimates obtained ranged from 0.23 to 0.45. Directgenetic correlations of Regions 2 and 3 with Region I were significant and were 1.00 and 0.71respect ivcly. This indicates that no Cl x E occurred between Regions I and 2, but indeed betweenRegions I and 3. The genetic correlation between Regions 2 and 3 (0.60) were non-sign ificant.Correlation coefficients between breeding values of sires, obtained by multivariate analyses and thedifferent bivariate analyses, was also calculated (range 0.77 to 0.8]) indicating no major re-rankingof sires when selection is based on breeding values obtained by multivariate analyses which includea HYSxS interaction. Top performing sires showed more re-ranking.

[发布机构] University of the Free State