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accidental cross-pollination, random mutation, and gene segregation. Since both types ofselection pressure are highly variable, it is difficult to generalize about the rate of geneticchange; depending on the circumstances, the genotype of a landrace or improved OPV canchange significantly from one generation of plants to the next, or it can remain essentiallyunchanged across many generations of plants.In hybrids, by far the most important source of genetic changes is segregation — randomrecombination of alleles that occurs when seed is recycled. Key results of a simulationexercise designed to show the likely effects of inbreeding in maize hybrids appear to besupported by findings published in the empirical literature on seed recycling:♦ When maize hybrids are recycled, yield usually decreases significantly from the F1 to theF2 generation. Yield tends to stabilize in subsequent generations, however, and mayeventually begin to increase again if farmers are exerting selection pressure.♦ When maize hybrids are recycled, the size of the yield decrease observed between the F1and F2 generations depends in large part on the level of inbreeding of the originalparents. Generally speaking, the greater the degree of inbreeding in the parents, thegreater the degree of heterosis in the F1 generation, and the greater the yield declineobserved between the F1 and F2 generations. This relationship may be confounded byenvironmental factors, however.♦ The degree of inbreeding of the parents affects not only the size of the expected yielddecrease but also its variability. The greater the level of heterozygosity in F1 plants, thegreater the variability in inbreeding depression expected in F2 and F3 plants.♦ Whether or not advanced-generation hybrids outyield landraces and improved OPVsdepends on the original difference in yield and on the magnitude of the decline in yieldcaused by recycling. In some instances, recycled hybrids continue to outyield the othertypes of materials, which explains why hybrid recycling may make sense.♦ Recycling of hybrids may have little effect on qualitative traits such as kernel size andshape, grain texture, and pounding quality.The finding that seed recycling often leads to significant genetic changes in farmermaintainedcultivars suggests that there may be a need to reassess the categoriestraditionally used to classify maize varieties (e.g., landraces, improved OPVs, hybrids). Inaddition, the rapid rate of genetic change observed to take place in farmers’ fields hasimportant implications for research impacts assessment studies. Practical guidelines for usein estimating the returns to maize breeding research are presented in the appendix.v
AcknowledgmentsAny report that is based on a comprehensive review of the literature owes a great deal topreviously published work, and this one is no exception. We would like to recognize thecontributions of the many researchers whose findings we have liberally cited; without theirefforts, this synthesis would not have been possible. We especially acknowledge thepioneering work on farmers’ varietal and seed management practices carried out by ourCIMMYT colleagues Dominique Louette, Melinda Smale, and Mauricio Bellon.Drafts of this paper were reviewed by Gregory Edmeades, Melinda Smale, and MiguelLópez-Pereira, all of whom provided useful comments. In addition, many CIMMYTcolleagues and national program collaborators took the time to go through their files insearch of studies of potential relevance. The manuscript was copyedited by Kelly Cassadayof the CIMMYT Publications Unit.vi
Page 1 and 2: E C O N O M I C SWorking Paper 99-0
Page 3 and 4: CIMMYT (www.cimmyt.mx or www.cimmyt
Page 5: Executive SummaryThis paper summari
Page 9 and 10: followed a very different path comp
Page 11 and 12: Farmers’ Management of Maize Vari
Page 13 and 14: In recent years, new evidence has e
Page 15 and 16: state of Guanajuato and discusses t
Page 17 and 18: Based on the results of a survey co
Page 19 and 20: elied with greater frequency on the
Page 21 and 22: hybrid seed. Initially, it was gene
Page 23 and 24: In Veracruz State, Mexico, where mo
Page 25 and 26: Recent work in the highlands of Mex
Page 27 and 28: producers) tend to rely on family,
Page 29 and 30: Unintentional seed mixingUnintentio
Page 31 and 32: Table 11. Yield depression resultin
Page 33 and 34: Genetic driftWhen farmers select ea
Page 35 and 36: (a) Production of a single-cross hy
Page 37 and 38: Breeding hybrid maize begins with t
Page 39 and 40: According to this theory, genes tha
Page 41 and 42: Wright’s finding, which is based
Page 43 and 44: Similarly, the mean of F3 generatio
Page 45 and 46: 2. Between the F1 and F2 generation
Page 47 and 48: In a series of on-station trials co
Page 49 and 50: As part of the same trial, Ramírez
Page 51 and 52: Table 19. Inbreeding depression obs
Page 53 and 54: to 41% for the single cross (Figure
Page 55 and 56: DiscussionEvery time a farmer recyc
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The finding that genetic change in
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Brennan, J.P., and D. Byerlee. 1991
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Murillo Navarrete, P. 1978. Estimac
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AppendixGuidelines for Estimating t
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exactly how different the plants wo
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As a general rule, we propose that
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