Genetic aspects of control of anaemia development in trypanotolerant N'Dama cattle

148 one-year-old N'Dama cattle, progeny of 29 sires, were exposed for 92 days to a medium natural tsetse-trypanosome challenge in Gabon, Central Africa. Matching health and performance data were recorded on 11 occasions. Average packed red cell volume percent (PCV) and lowest PCV reached during the period were evaluated as measures of ability to control the development of anaemia. Attempts were made to systematically control other possible causes of anaemia. In animals detected as parasitaemic, those with above average average PCV values or above average lowest PCV reached had 34% and 35% respectively higher daily weight gains than those with below average. Even when not detected as parasitaemic, those with above average average PCV values or above average lowest PCV reached had 14% and 12% respectively higher gain indicating that a proportion of these animals actually were parasitaemic. When all environmental and parasitaemia information was taken into account, the heritability of growth, average PCV and lowest PCV reached was 0.39±0.31, 0.64±0.33 and 0.50±0.32 respectively. The genetic correlation between average PCV and growth was 0.70±0.42 and between lowest PCV reached and growth was 0.28±0.55. While the standard errors are large, the higher heritabilities of PCV measures compared to animal growth and the positive genetic correlations between PCV and growth do indicate an opportunity for selection on PCV when animals can be detected as parasitaemic. All heritabilities and genetic correlations increased in size when parasitaemia information was utilized in the analysis. Thus more sensitive field tests for trypanosome detection, and early onset of parasitaemia through very high natural infection or an acceptable experimental alternative, could speed up selection for ability to control the development of anaemia.