Abstract
BACKGROUND: Crossbreeding is successfully implemented worldwide to improve animal productivity and adaptability. However, recurrent failures of crossbreeding programmes in African countries imply the need to design effective strategies for the predominant smallholder production systems. METHODS: A comprehensive simulation procedure mimicked body weight (BW(L)) and tick count (TC(L)) incidence in a local taurine cattle breed and in an exotic indicine beef cattle breed (BW(E) and TC(E), respectively). The two breeds were crossed to produce F1 and rotational animals. Additionally, synthetic breeds were created by applying four schemes defined as farm bull (FB), intra-village bull (IVB), exchanged-village bull (EVB), and population-wide bull (PWB) scheme. These schemes reflect different strategies to select and allocate bulls to smallholder farms. The different crosses were compared with the local breed over 20 generations by varying the genetic correlation between the traits ( rg = - 0.4, 0, 0.4), genotype-by-environment effects (GxE) between local and exotic environment ( rg×e = 0.4, 0.6, 0.8), and the relative emphasis of TC(L) compared to BW(L) in a selection index (SI_TCL10%, SI_TCL30%, SI_TCL50%). RESULTS: Regardless of rg and rg×e , EVB achieved the highest phenotypic and genetic gains for BW(L) and TC(L) over the 20 generations with SI_TCL50%. However, EVB displayed lower phenotypic means than F1 crosses in the first seven generations due to the loss of heterosis. Additive genetic variances were generally larger in synthetic crosses than in F1 and local animals, explaining the larger responses to selection. In addition, the EVB was the most effective strategy to stabilize inbreeding and retain heterosis in the advanced generations of synthetic animals. Low emphasis on TC(L) (SI_TCL30%, SI_TCL10%) resulted in negative phenotypic gain for TC(L) in synthetic animals when r(g) = - 0.4. In contrast to F1 and rotational crosses, GxE effects did not affect phenotypic gain in synthetic crosses. CONCLUSIONS: The study demonstrates opportunities for long-term genetic improvement of adaptive and productive performances in smallholder cattle farms using synthetic breeding. Extensive exchange of semen between villages or regions controls inbreeding and additionally contributes to increasing genetic gain. Furthermore, the definition of a suitable selection index prevents antagonistic selection responses caused by negative correlations between traits and GxE effects.