A Review of the Evidence - Search CIMMYT repository

to 41% for the single cross (Figures 7 and 8). In percentage terms, the yield declines weresimilar under two different fertilizer application rates, a result that fails to support thefindings of Zambezi et al. (1997) that fertilization reduces inbreeding depression inadvanced-generation hybrids.This trial produced unexpected results concerning the relative yield performance ofrecycled hybrids versus recycled OPVs. At the F1 generation, the hybrids tested byWaddington et al. outyielded the improved OPVs, but only under favorable conditions;under poor conditions, the yield advantage of the hybrids was negligible. At the F2generation, the yield rankings were reversed: the improved OPVs outyielded the hybrids,with the yield advantage particularly pronounced under poor conditions. These findingssuggest that in Zimbabwe, planting recycled hybrid seed under conditions of low rainfalland low soil fertility can be risky.Combined effectsMost of the studies cited thus far involved trials that were specifically designed to isolatethe effect of a particular source of genetic change. It is important to remember, however,that more than one source may be operating at the same time. Our search of the literatureturned up several additional studies reporting on genetic changes in maize in which it wasnot possible to pinpoint the exact source of the change.Perhaps the best example of how genetic changes can result from a combination of causescomes from a study carried out in northern Pakistan to assess the extent and rate of geneticchange in Azam, an improved OPV (Longmire et al., unpublished). Following its release in1985, Azam was widely adopted throughout the Swat Valley (Table 22). In an effort to assessthe rate of genetic change in farmer-recycled Azam, seed samples of varying ages werecollected from 50 farmers. These samples were grown out in on-station and on-farm trials,Yield (kg/ha)Commercial seed (F1),,,y Second-generation seed (F2), y,,yy,y,,yy, y,,yy,y,,yy,y,y,,,, y,,yy,,,y,,,, yy,, y,y,, ,, ,,,, y,,yyy,,,, yy,, y,y,, ,, ,,,, y,,yy ,,,y,,,, yy , , y,y,, ,, ,,,, y,,yyy,,,, yy , , y,y6543210-41%-41%+0%-26%SR52 SC601 R215 R201 KEP ZM607(single cross) (three-way (three-way (three-way (improved (improvedcross) cross) cross) OPV) OPV)-36%Yield (kg/ha)6543210, y,y , y, y,y,,, yy,y,,,, yy,y, ,,,,,y,y,yyy,,,, yy,y, ,,,,,y,y,yyy,,,, yy,y, ,,,,,yyyy,y,,yy,y-41%-40%,y,,-33%Commercial seed (F1)Second-generation seed (F2)SR52 SC601 R215 R201 KEP ZM607(single cross) (three-way (three-way (three-way (improved (improvedcross) cross) cross) OPV) OPV)-32%Figure 7. Effects of seed recycling on yields of fourmaize hybrids (high fertilizer treatment, Zimbabwe.Source: Waddington et al. (1997).Figure 8. Effects of seed recycling on yields of fourmaize hybrids (low fertilizer treatment), Zimbabwe.Source: Waddington et al. (1997)46