This study analyzes the dynamics of farmer-to-farmer distribution, the primary means by which new bean varieties spread in Rwanda. Two processes determine the speed of diffusion: the creation of new users through seed distribution and the disappearance of a variety from individual farms (mortality). Small bean plots, modest seed quantities received, environmental stresses and poverty put farmers at risk of losing varieties, even ones they may value. Overall, distribution tends to be both numerically limited and socially restricted. Analysis shows the probability of survival to be correlated with the rate at which farmers distribute seed. The paper presents a demographic model for measuring varietal expansion which takes account of loss as well as gain. Farmer-to-farmer distribution is differentially effective in diffusing varieties; salient factors include the growers' appreciation of the variety, its productivity and harshness of environment. Finally, we outline a diffusion strategy for improving small farmers' access to new varieties, recognizing that adoption is not a one-time affair and that the poorer have particular difficulty in receiving and keeping new cultivars. © 1993.
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