The genetic basis of gestation length in Bonsmara cattle

English: The purpose of this study was to investigate the genetic and phenotypic parameters for gestation length and birth weight in Bonsmara cattle in South Africa. The genetic and phenotypic parameters were determined when the traits were firstly treated as traits of the calf and secondly as traits of the dam. A total of 26962 gestation length and birth weight records were used for this investigation. The data was collected from 1989 to 2000. The phenotypic correlation between gestation length and birth weight was moderate and positive (0.22). The sources of non - genetic variation for gestation length and birth weight were sex, herd x year x season (HYS) and age of dam (linear and quadratic). When the traits were treated as traits of the calf all the sources of non-genetic variance were highly significant (P<0.001). Age of dam quadratic was not significant for birth weight when the trait was treated as a trait of the dam. The traits were analysed through univariate and bivariate analyses. Five basic models were used for the univariate analysis. The five models took the direct genetic, maternal genetic, and/or permanent maternal environmental genetic effects into account. The appropriate random effects were also included in, or excluded from the models. The model that fitted the data best was then used for a bivariate analysis. The direct heritability through the bivariate analysis was 0.38 for gestation length and 0.23 for birth weight. Maternal heritability was lower, 0.09 for gestation length and 0.11 for birth weight. The direct genetic correlation (0.40) and the maternal genetic correlation (0.57) between gestation length and birth weight were positive. In the above results the traits were treated as traits of the calf. The direct heritabilities for gestation length and birth weight, when the traits were treated as traits of the dam, were 0.15 and 0.16 respectively through bivariate analysis. The maternal heritabilities for gestation length and birth weight were very low to negligible (0.02 and 0.003). The direct genetic correlation (0.41) between gestation length and birth weight was positive, but the maternal genetic correlation (-0.83) was negative. Estimated breeding values (EBVs) for gestation length and birth weight were calculated for each animal in the data. The genetic trends for gestation length and birth weight were also determined. The trend for gestation length showed that there was a correlated response for shorter gestation length through the possible direct selection for lower birth weight. The trend for birth weight showed that there was selection for lower birth weight.