A Review of the Evidence - Search CIMMYT repository

Table 22. Maize production, variety, and seed use by environment, Swat District, Pakistan, 1989Irrigated Rainfed Swat Districtvalleys mountain zones (total)Maize area per farm (ha) 1.76 1.36 1.56% area planted to improved maize 23 9 17% area planted to Azam variety 16 4 13% farmers growing local variety only 76 85 81% farmers growing improved OPV only 18 8 13% farmers growing local and improved 6 7 6Age of local varieties (years) 16.7 14.1 15.5Age of improved varieties (years) 3.4 2.1 3.2Source: Longmire et al. (unpublished).along with samples of commercial Azam seed. Data were recorded on grain yield, plantand ear height, and maturity (measured as days to 50% silking).In the on-farm trials, plants grown from commercial seed yielded an average of 14% morethan plants grown from recycled seed. Plant and ear height differed with a high degree ofsignificance; plants grown from recycled seed were 10% taller on average, and ears on thoseplants were 12% higher on average. Small but significant differences were also observed inmaturity, with plants grown from recycled seed showing shorter duration. In the on-stationtrials, the only significant differences detected involved plant and ear height, with plantsgrown from recycled seed again slightly taller, and ears on those plants again placedslightly higher. No difference was detected between plants grown from commercial versusrecycled seed in terms of grain yield or maturity. The authors speculated that the lack ofsignificance of these two characteristics in the on-station trials may have resulted from therelatively high levels of management, which might have reduced seed-related performancedifferences.Contrary to expectations, no direct correlation was detected between the number of yearsthe farmers had recycled Azam and the characteristics of interest (grain yield, plant and earheight, maturity). In other words, the observed genetic changes did not occur graduallythrough time. The authors concluded that this may have been due to the fact thatsuccessive generations of Azam grown in farmers’ fields may have been exposed todifferent rates of contamination.This example from Pakistan shows that it is not always possible to distinguish between theeffects of multiple causes of genetic change that may be operating simultaneously. In thecase of Azam, it is difficult to identify precisely the factor or factors responsible for geneticchanges occurring through time. The increase in plant height resulted at least in part fromfarmer selection practices, but if these practices were consistent, the observed rate ofgenetic change should have been gradual. Since it was not gradual, unintentionalcontamination presumably was occurring as well.47