A Review of the Evidence - Search CIMMYT repository

(2) Productivity gains attributable to adoption of improved germplasmIn theory, the productivity gains attributable to the adoption of improved germplasm areexpressed as the difference between the yields obtained with the farmer’s current variety(which depending on the circumstances may be a landrace, a local variety, or an older MV)and the MV developed by the breeding program, holding all other inputs constant. Inpractice, this difference is difficult to measure for at least two reasons. First, because thegenetic potential of any cultivar interacts with environmental factors (GxE interaction), theyield difference tends to vary considerably across locations and between cropping seasons.Second, when farmers recycle seed, the genetic composition changes from one generation tothe next, which may also affect the yield difference.Practical approaches for estimating research impactsTheoretical and empirical evidence reviewed in this paper make clear that because of seedrecycling practices and other factors, genetic change in farmer-maintained maize cultivars isubiquitous and constant. This finding has important implications for the practical businessof estimating research impacts. Once we concede that landraces, improved OPVs, andhybrids may be genetically diverse, and once we accept that materials in all three categoriestend to change constantly, it becomes very difficult to define precisely what constitutes“improved germplasm” and to quantify the productivity gains associated with adoption.In order to make any progress in measuring the two key parameters described above, it isnecessary to formulate workable criteria for distinguishing between improved andunimproved germplasm. Once such criteria have been defined, it may be possible to drawuseful conclusions about the size of the productivity gains associated with the use ofimproved germplasm. The fact that the criteria to some extent must be arbitrary does notnecessarily undermine their usefulness; what is important is that they be workable,defensible, and consistent.Estimating the area planted to MVsIn estimating the area planted to MVs, what criteria can be used to distinguish betweenimproved and unimproved germplasm?Genotype-based criteria. From a technical point of view, the most rigorous way to defineimproved germplasm would be on the basis of genotypic information. For example, animproved cultivar might be defined as one whose genotype is identical to the genotype ofan improved OPV or hybrid produced by a scientific plant breeding program. With theadvent of genetic fingerprinting techniques, it should soon be possible to sequenceindividual plants rapidly and inexpensively, which would facilitate such comparisons.While appealing in theory, genotype-based criteria may turn out to be tricky to define inpractice. To begin with, exactly how similar would two (or more) plants have to be in orderto be considered genetically identical? Because of their propensity to cross-pollinate, maizeplants are by nature genetically heterogeneous, and even within a field of hybrids,individual plants will tend to differ at the genetic level. Thus, we would have to decide57