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esults showed that the annual progress in grain yieldpotential achieved through resistance breeding,averaged over six trials, was 0.48% for fungicideprotected plots and 2.21% for plots not protected byfungicide. Thus, although the grain yield potential ofCIMMYT-related cultivars has improved significantlyover the past 30 years, the progress in protecting thisyield potential through rust resistance breeding wasestimated to be at least four times greater. The trialdata imply that leaf rust resistance has accounted for82% of the average annual progress in grain yieldpotential between 1966 and 1988 in northwesternMexico. This estimate was used to adjust the averageannual yield series for ME 1. The following loglinearmodel was used for this purpose:ln (y t) = α + βX + ε (4)The parameters are: ln (y t), the natural logarithm of y t,the average annual farm-level yield of CIMMYTrelatedspring bread wheat in ME 1; α, a constant; β,the average annual yield growth rate; X, time in yearsfrom 1973 to 1998; and ε, the error term.The loglinear ln (y t) of the original ME 1 yield series y twas regressed to estimate the coefficient on time β,representing the average annual percent growth inyield from 1973 to 1998. The coefficient was adjustedby 82% to include only the proportion of growthattributable to leaf rust resistance breeding, asestimated from the CIMMYT trial data by Sayre et al.(1998). This resulted in a new coefficient ˆβ, which wasused to generate a new loglinear yield series ln( ŷ t).The antilog resulted in a yield series ( ŷ t) includingonly the growth attributable to leaf rust resistance,net of yield enhancement and other research effects,and thus corresponding to around 82% of the originalseries y t. As before, we regressed the data to projectyields to 2007, and repeated the analysis bysubstituting y twith ŷ tin equations (1) to (3).Area to which yield savings applyParameter a tin equations (1) to (3) represents theaverage annual area to which yield savings apply, bygenetic resistance category and ME, from 1973 to2007. This is calculated as the product of: (1) thepercent area grown to CIMMYT-related spring breadwheat by ME since 1973; (2) the average annualpercent area potentially affected by leaf rust by ME;(3) the percent distribution of area by geneticresistance category and ME; and (4) the averageannual area sown to CIMMYT-related spring breadwheat by ME, from 1973 to 2007.Percent area grown to CIMMYT-related springbread wheat. The proportion of area sown toCIMMYT-related spring bread wheat varieties since1973 was estimated by diffusion curves with alogistic function (Griliches 1957; CIMMYT 1993).The logistic function produces an S-shaped curverepresenting the cumulative proportion of adoptionover time. This assumes slow initial growth in theuse of the new technology, followed by a more rapidincrease and then a slow rate of increase as adoptionapproaches a ceiling asymptotically. Since Griliches’study of hybrid maize adoption in 1957, the S-shaped logistic curve has often been used in studiesof seed technology adoption. The function isexpressed as:P = –––––––K(5)1+e -(a+bt)Parameters are: P, the cumulative percent arearepresenting the cumulative path of adoption; K, theceiling or upper bound of adoption; t, time; b, aconstant related to the slope or rate of adoption; anda, a constant related to the time when adoptionbegins.Historical CIMMYT Global Wheat Impacts Surveydata from 1977, 1990, and 1997 on adoption levelsand adoption lags were used to solve for the logisticfunction parameters algebraically (Table 1). Thisenabled the estimation of cumulative adoption ratesin intervening years. Estimates of the cumulativepercent area planted to CIMMYT-related springbread wheat by ME in 1997 (Heisey et al. 2002) wereassumed as the adoption ceiling in eachenvironment. The 1997 estimates were combinedwith 1977 (CIMMYT 1989) and 1990 (Byerlee andMoya 1993) 7 data to calibrate the diffusion curves atthree points in time, and subsequently to estimatethe total time period of diffusion in each ME. Thesame sources were used to estimate the adoptionlag, or the period from varietal release until its initialadoption by farmers, in each ME. We assumed that7Adoption reported in ME 3 for 1997 (Heisey et al. 2002) is lower than for 1990 (Byerlee and Moya 1993). This is explainedby the relatively high number of improved tall varieties that continued to be released and sown in Brazil. Since they aretall, they were not accounted for in the adoption estimates for semidwarf varieties by Heisey et al. (2002). However, theyprobably often contain improved and/or CIMMYT germplasm, and could still be considered as CIMMYT-relatedmaterial. For all wheat production environments other than MEs 2 and 4a, there is fairly strong evidence that adoptionceilings have been reached, unless major genetic changes are accomplished, such as for drought tolerance.19
CIMMYT-related varieties released since 1973followed similar aggregate adoption paths as thosebeginning to diffuse in 1966. The year 2007 thusproved to be the latest year predicted by the logisticcurves. Figure 3 shows the fitted diffusion curves byME from 1973 to 2007.The earliest release dates for the spring bread wheatvarieties drawn from the 1997 CIMMYT GlobalWheat Impacts Survey data and classified by geneticresistance category are consistent with ourassumptions regarding the initial years of diffusion.The most susceptible varieties in genetic resistancecategories 1 and 2 were released the earliest, in 1970.This was before the initial year (1973), in which wehave assumed the deliberate change in CIMMYT’sbreeding strategy to focus on race-nonspecificresistance. The varieties with moderate to high levelsof race-nonspecific resistance in categories 3 to 5were released thereafter, beginning in 1973, 1974, and1979, respectively. Varieties with effective racespecificresistance in category 6 were released from1983 onward, which seems to indicate that farmersare rapidly turning over the varieties. Most of thesevarieties were grown in MEs 3 (acid soils) and 4b(dry), which have the longest adoption lags (Table 1).Based on this information, it seems reasonable toassume that varieties with race-nonspecific resistancebegan to spread among farmers from 1973.Percent area potentially affected by leaf rust. Theanalysis included only the average annual percentarea potentially affected by leaf rust in each ME.Estimates were drawn from the CIMMYT WheatProgram (H.J. Dubin, personal communication; Table1) by reviewing a list of production zonescorresponding to the MEs in the countries includedin the Global Wheat Impacts Surveys. The potentiallyaffected area varied by ME, but was assumed to beconstant over the period of analysis.Percent area by genetic resistance category andmega-environment. We calculated 1997 pointestimates of the percent distribution of area to whichyield savings applied, by genetic resistance categoryand ME. Information on the resistance categoriesfrom the sample of varieties tested in trials wascombined with the areas sown to each variety, asrecorded in the 1997 CIMMYT Global Wheat Impactsdatabase. However, the 1997 database reports thearea accruing to each variety by country rather thanME. We therefore partitioned the area per varietyamong MEs in the same proportion as the country’stotal spring bread wheat area is distributed amongMEs, as indicated by the 1990 database. The samplevariety areas were then summed for each resistancecategory and ME, and expressed as the percent of thetotal area of sample varieties.Table 4 indicates that 80% of the sample area wasprotected by genes conferring race-nonspecificresistance (categories 2 to 5), while only 10% of thearea accrued to race-specific resistance (category 6). Afurther 10% of the area was sown to varietiesclassified as almost fully susceptible (category 1) inTable 2. These findings correspond with theobservations by Smale et al. (1998) that varieties withrace-specific resistance occupied a generallyArea (%)10080ME1 ME2 ME3 ME4a ME4b ME4c ME5Table 4. The percent area by genetic resistance category andmega-environment in the sample of major CIMMYT-relatedspring bread wheat varieties grown in the developing worldin 1997.Genetic resistance category †60402001973 76 79 82 85 88 91 94 97 2000 03 06YearFigure 3. Percent area in post-1972 CIMMYT-related springbread wheat releases by mega-environment from 1973 to 2007.Mega-environment 1 2 3 4 5 61 11.8 6.6 37.7 36.1 4.1 3.72 1.0 8.0 37.8 19.4 0 33.83 8.7 0 7.9 11.1 0.3 72.04a 1.1 2.9 53.6 25.2 0 17.24b 0 0 1.6 1.2 0 97.24c 8.7 5.0 36.8 41.4 4.3 3.85a 13.0 8.5 33.2 40.9 2.5 1.9Sample area (000 ha) 3,694 2,342 13,679 12,723 1,222 3,694Percentage 10 6 37 34 3 10†Genetic resistance categories are defined in Table 2.20
Page 1 and 2: E c o n o m i c s P r o g r a m P a
Page 3 and 4: CIMMYT® (www.cimmyt.org) is an int
Page 5 and 6: TablesTable 1. Summary of parameter
Page 8 and 9: The Economic Impact in DevelopingCo
Page 10 and 11: origins or pedigrees are selected f
Page 12 and 13: Most assessments of the returns on
Page 14 and 15: MethodologyIn the capital investmen
Page 16 and 17: likely behavior was predicted based
Page 18 and 19: Table 3. Estimated yield losses fro
Page 22 and 23: decreasing percentage of the bread
Page 24 and 25: arising from this assumption may be
Page 26 and 27: Table 6. Discounted gross benefits
Page 28 and 29: our base scenario was minimal (Tabl
Page 30 and 31: esistance. It is conceptually and p
Page 32 and 33: ReferencesAdusei, E.O. 1988. Evalua
Page 34 and 35: Nagarajan, S., and L.M. Joshi. 1985
Page 36 and 37: Appendix ADefinition of CIMMYT mega
Page 38: ISSN: 1405-7735International Maize

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