Crop genetic resources are the building blocks of sustainable agricultural development due to their relevance not only as inputs for variety development but also as indigenous crop insurance mechanisms through traditional variety portfolio management. Their continuous survival is, however, threatened by natural and human driven factors. This threat has induced the need for designing conservation measures. Among the in situ and ex situ conservation options available to conserve crop genetic resources, on-farm conservation has recently attracted enormous attention. To make this option operational, placing incentives (that link conservation with utilization) and removal of perverse incentives are believed to be crucial so that landraces of no immediate interest to farmers can be conserved. However, before designing sound incentives and/or removing perverse incentives, we have to understand farmers' motives for managing a portfolio of traditional varieties. To address our objective, we have adopted a utility based model that considers on-farm diversity as a positive externality of farmers' livelihood decisions. Accordingly, on-farm diversity is considered as the derived outcome of farmers' revealed preferences subject to their concerns and constraints. To empirically test the relationships, a Poisson regression model is estimated using rural household survey data collected from 198 sorghum growing farmers in East Ethiopia. The results have shown the most important diversity promoting factors and those factors detaching the link between farmers' 'survival first' motives and their spillover effects on sorghum diversity. Based on the results, the paper concludes outlining the policy implications of the findings.