Estimation of genetic parameters for growth traits in South African Brahman cattle

[摘要] English: The present study was carried out to estimate genetic, phenotypic and environmentalparameters for growth traits, genetic trends as well as the level of inbreeding in the SouthAfrican Brahman breed. The data used were obtained from the South African BrahmanBreeders' Society and consisted of pedigree information of 181 508 animals and 221 015performance records, ranging from birth to 18-months weight /final weight collectedbetween 1955 and 2002. Performance records available before editing as classifiedaccording to age ranges were birth weight (BWT) = 67 336, weaning weight (WWT)(80�?00 days) = 62 159, yearling weight (YWT) (301-500 days) = 41 313 and finalweight (FWT) (501-900 days) = 32 602. The linear animal model used included the fixedeffects of contemporary group (herd-year-season), sex, management group, age of thecalf, age of the dam. Direct genetic, maternal genetic and permanent maternalenvironmental as random effects as well as sire x herd-year-season interaction as anadditional random effect. Preliminary analyses showed that all fixed effects had asignificant (P<0.0001) effect on all traits studied.Genetic parameters for growth traits were estimated using ASREML software by fittingunivariate and bivariate animal models. Production traits considered after editing wereBWT = 41 509, WWT = 37 705, YWT = 22 682 and FWT = 13 055 records collectedbetween 1985 and 2002. The reason for using the data only from 1985 is that only a smallnumber of records were recorded up to 1984. The direct heritability estimates of BWT,WWT, YWT and FWT were 0.28, 0.14, 0.14 and 0.18 respectively. The correspondingmaternal heritability estimates were 0.11, 0.06, 0.05 and 0.03 respectively. The maternal permanent environmental component due to the dam contributed 3 - 7 % of the totalphenotypic variance of the traits under consideration. The corresponding contribution ofsire x herd-year-season interaction ranges from 5 - 6%. The genetic correlation betweenanimal effects was -0.36 for BWT, other traits studied did not show an antagonismbetween animal effects. The prospects of improvements of these traits by selection seempossible and the contributions of permanent maternal environmental effect are notequally important as maternal effects though the effects are small.Estimates of phenotypic correlations were low to moderate (0.22 to 0.64), whereasgenetic correlations were moderate to high (0.47 to 0.91). Genetic correlations betweenBWT and WWT, YWT and FWT were 0.62, 0.47, and 0.52 respectively. Thecorresponding genetic correlations between direct and maternal effects of the same trait(ram, rma) were (-0.4204, -0.014), (-0.4241, 0.0567) and (0.05170, 0.0879) respectively.Genetic correlations between WWT and postweaning weights were 0.88 and 0.91 forYWT and FWT respectively. The corresponding maternal correlation was at unity.Lastly, the genetic and maternal correlation between YWT and FWT were 0.83 and unity,respectively.Positive genetic trends were observed for all traits except for the maternal genetic trendfor BWT, which had a slightly negative slope. Direct genetic progress for BWT was0.0207 kg/year (1987-2002), whereas progress for WWT was 0.1kg/year (1985-1995)and 0.4 kg /year (1996-2001). Progression in direct genetic effect for YWT was0.1kg/year (1985- 1995) and 0.3 kg/year (1996-2001) whereas progress for FWT was0.13 kg/year (1985-2001). The maternal genetic trends were -0.003, 0.04, 0.008 and 0.003 kg/year for BWT, WWT, YWT and FWT, respectively. Genetic progress was ingeneral low over the years for all traits. Therefore, estimation based on BLUP of breedingvalues may be more important in the future and should be considered to maximiseopportunities for genetic change.Inbreeding level and its possible influence on growth traits of the South African Brahmancattle were investigated. Inbreeding coefficients were calculated by inverting the diagonalof the inverse relationship matrix using the MTDFNRM program of the MTDFREMLpackage. Inbreeding depression was estimated as the regression of performance onindividual and dam inbreeding coefficients using an animal model. The mean inbreedingof the population was very low (0.01) with an average of (0.03) for inbred animals.Although regression coefficients of BWT, WWT, YWT and FWT on inbreeding ofanimal and dam were significant for all traits, the results showed that inbreeding atpresent is not a serious problem in the South African Brahman breed.

[发布机构] University of the Free State