ObjectivesCurrent models of transgene dispersal focus on gene flow via pollen while neglecting seed, a vital vehicle for gene flow in centers of crop origin and diversity. We analyze the dispersal of maize transgenes via seeds in Mexico, the crop's cradle.MethodsWe use immunoassays (ELISA) to screen for the activity of recombinant proteins in a nationwide sample of farmer seed stocks. We estimate critical parameters of seed population dynamics using household survey data and combine these estimates with analytical results to examine presumed sources and mechanisms of dispersal.ResultsRecombinant proteins Cry1Ab/Ac and CP4/EPSPS were found in 3.1% and 1.8% of samples, respectively. They are most abundant in southeast Mexico but also present in the west-central region. Diffusion of seed and grain imported from the United States might explain the frequency and distribution of transgenes in west-central Mexico but not in the southeast.ConclusionsUnderstanding the potential for transgene survival and dispersal should help design methods to regulate the diffusion of germplasm into local seed stocks. Further research is needed on the interactions between formal and informal seed systems and grain markets in centers of crop origin and diversification.
Controlling for the endogeneity of activity choice and considering both household-head and family schooling, we findevid ence of high returns from schooling in both crop andnoncrop activities. As schooling levels increase, the returns from schooling shift away from crop production. Failure to control for activity selection andfamily schooling is likely to result in biasedestimates of the returns from schooling in individual activities. Copyright 2000, Oxford University Press.
Crop genetic diversity is an indispensable resource for farmers and professional breeders responding to changing climate, pests, and diseases. Anecdotal appraisals in centers of crop origin have suggested serious threats to this diversity for over half a century. However, a nationwide inventory recently found all maize races previously described for Mexico, including some formerly considered nearly extinct. A flurry of social studies seems to confirm that farmers maintain considerable diversity. Here, we compare estimates of maize diversity from case studies over the past 15 y with nationally and regionally representative matched longitudinal data from farmers across rural Mexico. Our findings reveal an increasing bias in inferences based on case study results and widespread loss of diversity. Cross-sectional, case study data suggest that farm-level richness has increased by 0.04 y −1 nationwide; however, direct estimates using matched longitudinal data reveal that richness dropped −0.04 y −1 between 2002 and 2007, from 1.43 to 1.22 varieties per farm. Varietal losses occurred across regions and altitudinal zones, and regardless of farm turnover within the sector. Extinction of local maize populations may not have resulted in an immediate loss of alleles, but low varietal richness and changes in maize's metapopulation dynamics may prevent farmers from accessing germplasm suitable to a rapidly changing climate. Declining yields could then lead farmers to leave the sector and result in a further loss of diversity. Similarities in research approaches across crops suggest that methodological biases could conceal a loss of diversity at other centers of crop origin.in situ conservation | crop evolution | climate change | corn
Using a unique panel data set from rural Mexico, we find strong evidence of a negative relationship between farm size and both productivity and technical efficiency: large farms not only have a lower value of output per hectare than small farms, they also produce further from the efficiency frontier. Our findings suggest that, in spite of the ongoing transformation of agricultural supply chains and economists' recommendations for small farmers to exit crop production, there may be sustained advantages for smallholder farms. Our analysis offers new insights into inverse-farm size relationship, the productivity-efficiency relationship, and the use of stochastic frontier techniques.
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