Estimates of genetic, environmental and phenotypic trends in indigenous Nguni cattle of Zimbabwe: implication for management and breeding program

Growth traits in indigenous cattle are desirable economic parameters which should be improved and their trends monitored over time to investigate direction of change. Traits considered were birth weight (BWT) and weaning weight (WWT). BLUP breeding values were regressed on year to obtain annual direct and maternal genetic trends, while fixed effects solutions and mean performances were regressed on year to obtain environmental and phenotypic trends, respectively. Linear regression representing genetic trends for direct effects indicated a decrease of -0.02 kg per year for BWT, and an increase of 0.12 kg per year for weaning weight. The environmental trend was positive for birth weight and negative for weaning weight. A cubic regression produced a relative better fit for environmental values for WWT (R-square= 34%), as compared to a linear regression of (R-square=18%). Environmental values for WWT decreased from approximately 1983, suggesting that the environmental conditions had a negative effects on WWT. There was a net phenotypic gain for birth weight of 0.48 kg per year, and a net phenotypic loss for WWT of -01.57 kg per year. The trend of maternal ability for BWT increased at the first quarter of the study (1983 to 1991), and then declined thereafter (1992 to 1997), and was insignificant for WWT. The results obtained in the present study on various trends demonstrate that very small genetic gains were realized signaling need for change in selection and/or management procedure in the scheme.