Abstract

The objective of this study was to determine the environmental and genetic factors affecting reproductive traits and calf survival from birth to weaning in Tswana breed of cattle. Analyses of environmental and genetic effects for calf survival traits were done using 7223 records of animals which were born between 1996 and 2013 from 1659 dams and 188 sires in 54 contemporaries. Analyses of environmental and genetic effects for age at first calving were done using 818 records of animals born between 1998 and 2013 from 611 dams and 136 sires in 49 contemporaries, while calving interval analyses were done using 1804 records of cows born between 1999 and 2013 from 496 dams and 121 sires in 45 contemporaries. Reproductive traits analysed were age at first calving (AFC) and calving interval (CI). AFC was analysed using univariate animal model while CI was analysed using repeatability model. Calf survival to weaning (CS) was analysed as a binomial trait using generalised mixed linear logistic model with logit as link function in the ASREML program. Significant environmental effects for reproductive traits were selection line, calving year and season. CS was significantly influenced by calf sex, selection line, calf-birth weight and dam age. The estimated heritability values for reproductive traits were 0.07±0.02 for CI and 0.10±0.07 for AFC. Heritability estimate obtained for CS was 0.07±0.05. Low genetic variability obtained in reproductive traits and calf survival to weaning trait indicates that improvement of these traits through genetic selection may prove to be slow.

Key words: heritability estimates, binomial trait, logistic model, repeatability model.