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

English: The present study was carried out to estimate genetic, phenotypic and environmental parameters for growth traits, genetic trends as well as the level of inbreeding in the South African Brahman breed. The data used were obtained from the South African Brahman Breeders’ Society and consisted of pedigree information of 181 508 animals and 221 015 performance records, ranging from birth to 18-months weight /final weight collected between 1955 and 2002. Performance records available before editing as classified according to age ranges were birth weight (BWT) = 67 336, weaning weight (WWT) (80–300 days) = 62 159, yearling weight (YWT) (301-500 days) = 41 313 and final weight (FWT) (501-900 days) = 32 602. The linear animal model used included the fixed effects of contemporary group (herd-year-season), sex, management group, age of the calf, age of the dam. Direct genetic, maternal genetic and permanent maternal environmental as random effects as well as sire x herd-year-season interaction as an additional random effect. Preliminary analyses showed that all fixed effects had a significant (P<0.0001) effect on all traits studied. Genetic parameters for growth traits were estimated using ASREML software by fitting univariate and bivariate animal models. Production traits considered after editing were BWT = 41 509, WWT = 37 705, YWT = 22 682 and FWT = 13 055 records collected between 1985 and 2002. The reason for using the data only from 1985 is that only a small number 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 corresponding maternal 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 total phenotypic variance of the traits under consideration. The corresponding contribution of sire x herd-year-season interaction ranges from 5 - 6%. The genetic correlation between animal effects was -0.36 for BWT, other traits studied did not show an antagonism between animal effects. The prospects of improvements of these traits by selection seem possible and the contributions of permanent maternal environmental effect are not equally important as maternal effects though the effects are small. Estimates of phenotypic correlations were low to moderate (0.22 to 0.64), whereas genetic correlations were moderate to high (0.47 to 0.91). Genetic correlations between BWT and WWT, YWT and FWT were 0.62, 0.47, and 0.52 respectively. The corresponding 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 for YWT 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 trend for BWT, which had a slightly negative slope. Direct genetic progress for BWT was 0.0207 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 was 0.1kg/year (1985- 1995) and 0.3 kg/year (1996-2001) whereas progress for FWT was 0.13 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 in general low over the years for all traits. Therefore, estimation based on BLUP of breeding values may be more important in the future and should be considered to maximise opportunities for genetic change. Inbreeding level and its possible influence on growth traits of the South African Brahman cattle were investigated. Inbreeding coefficients were calculated by inverting the diagonal of the inverse relationship matrix using the MTDFNRM program of the MTDFREML package. Inbreeding depression was estimated as the regression of performance on individual and dam inbreeding coefficients using an animal model. The mean inbreeding of 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 of animal and dam were significant for all traits, the results showed that inbreeding at present is not a serious problem in the South African Brahman breed.