THE IMPACT OF AGRICULTURAL MAINTENANCE RESEARCH:

THE CASE OF LEAF RUST RESISTANCE BREEDING IN CIMMYT-RELATED SPRING BREAD WHEAT

C. N. Marasas

M. Smale

R.P. Singh

International Maize and Wheat Improvement Center, Apartado Postal 6-641,

06600 Mexico D.F., Mexico

Paper prepared for the International Conference on Impacts of Agricultural Research and Development, San José, Costa Rica, 4-7 February 2002

ABSTRACT

Leaf rust caused by Puccinia triticina is a wheat disease of major historical and economic importance worldwide. Genetic resistance, rather than the use of fungicides, remains the principal means of disease control. We estimated the impact in developing country production of efforts by the International Maize and Wheat Improvement Center (CIMMYT) to breed leaf rust resistant spring bread wheat varieties since 1973.

The challenge in estimating these benefits is in dealing with the fact that rust pathogens are able to rapidly mutate to new races, which are able to infect previously resistant varieties. Various single genes or gene complexes determine the type, level, and longevity of a variety’s resistance. Leaf rust resistance breeding is therefore an example of crop maintenance research. Whereas productivity enhancement is measured in terms of positive yield gains, productivity maintenance is estimated in terms of the yield losses that would have occurred in the absence of the research investment. Although its importance has long been argued, there are relatively few economic analyses of maintenance research, and in particular, breeding for crop disease resistance.

The benefits of CIMMYT’s investment in leaf rust resistance breeding were estimated using an economic surplus approach adjusted for maintenance research. Gross benefits were modeled as the surplus generated by avoiding a cost-increasing shift in the supply curve resulting from changes in the environment caused by evolving leaf rust pathogens. A standard capital investment analysis was applied to estimate the returns.

A sample of the major spring bread wheat varieties grown in the developing world were classified by their type and level of genetic leaf rust resistance through trials conducted at CIMMYT. The estimated yield losses of these varieties were compared with those that would have occurred had all these varieties been fully susceptible. The area to which yield savings applied were estimated by fitting historical logistic diffusion curves to the study area potentially affected by leaf rust. The analysis was separated by wheat breeding mega-environment, a classification developed by the CIMMYT Wheat Program to guide its germplasm enhancement activities. The full cost of CIMMYT’s wheat improvement effort was included in the analysis. A range of investment values was elicited by varying assumptions on several parameters.

The results of this study have two major policy implications. They firstly demonstrate that CIMMYT’s investment in wheat genetic improvement since 1973 has generated substantial economic returns from leaf rust resistance breeding in spring bread wheat only. In an era characterized by a global decline in agricultural research investments, the efficient targeting of scarce resources is becoming increasingly important. Secondly, the results emphasize the importance of maintenance research, which has often been undervalued in economic analyses. Studies at CIMMYT indicate that part of the progress in wheat yield gain through the years has been achieved by protecting this yield potential through disease resistance breeding. Failure to account for the effects of maintenance research could therefore bias rate of return estimates.

INTRODUCTION

The rate of return on investments in agricultural research has often been estimated assuming that research explains positive productivity growth, and that productivity would remain constant in the absence of research. However, this assumption ignores the losses that may result from the physical, biological and economic changes that could render existing technologies less effective. Most assessments of the returns on crop improvement programs have focused on productivity enhancement. There are comparatively fewer economic analyses of maintenance research, particularly for disease resistance breeding. Whereas productivity enhancement is often measured in terms of positive yield gains, productivity maintenance is estimated in terms of the yield losses that would have occurred in the absence of the research investment.

In this paper, we draw on the findings of our case study to underscore the importance of maintenance research in plant breeding programs. Our objective is to estimate the economic impact in developing country production of efforts by the International Maize and Wheat Improvement Center (CIMMYT) to breed genetic leaf rust resistance in spring bread wheat from 1973. We initiate our paper by outlining the background and scope of this study, before presenting the conceptual framework, methodology, results and conclusions.

BACKGROUND AND SCOPE OF THIS STUDY

Leaf rust caused by Puccinia triticina is a wheat disease of major historical and economic importance worldwide (Roelfs, Singh and Saari, 1992). It is the most widespread of three types of rusts, the other two types being stem rust caused by P. graminis and stripe rust caused by P. striiformis. Periodic rust epidemics were common in most decades of this century, and some yield losses to rusts are still suffered in many wheat-producing areas in most years. The cultivation of resistant varieties remains the principal control method in developing countries, where fungicides are not often used for this purpose. The development of leaf rust resistance has therefore been a priority of CIMMYT’s wheat breeding program since its inception.

Varieties could carry different types and levels of genetic leaf rust resistance. Genes conferring race-specific resistance produce intermediate to major reactions against specific races of the pathogen. However, the major challenge in breeding for leaf rust resistance is in dealing with the ability of the pathogen to rapidly mutate to new races, which are able to infect previously resistant varieties. The effects of race-specific resistance may thus be overcome within a relatively short time. A severe leaf rust epidemic in northwestern Mexico in 1976-77 dramatically underscored the need for more durable resistance (Dubin and Torres, 1981).

Race-nonspecific resistance is based on the interaction of a few or several genes having partial to additive effects. The genes are effective against several races of the pathogen simultaneously and result in varying levels of resistance against them. Varieties with race-nonspecific resistance may suffer comparatively larger yield losses than varieties with effective race-specific resistance. However, race-nonspecific resistance appears to endure longer. Though CIMMYT’s Wheat Program emphasizes the selection for race-nonspecific resistance (Rajaram, Singh, and van Ginkel, 1996), breeders in some national programs might place more emphasis on characteristics other than race-nonspecific leaf rust resistance. Furthermore, producers often continue to grow varieties with levels of resistance that wheat scientists may no longer consider satisfactory. CIMMYT-related varieties with both race-specific and race-nonspecific resistance can therefore be found in wheat fields today.

In their case study of the Yaqui Valley of Mexico, Smale, Singh, Sayre, Pingali, Rajaram, and Dubin (1998) estimated the yield losses that farmers would have suffered if a breeding strategy for race-specific rather than race-nonspecific resistance had been employed. The authors estimated a rate of return of 40% on the investment in race-nonspecific leaf rust resistance breeding over the period 1970-1990. Detailed information on the genes conferring for resistance and the longevity of useful resistance for each wheat variety grown in the Yaqui Valley since 1968 was employed. However, the geographical scope of that study was more limited, since it covered a mere 150,000 ha compared to the estimated 71.5 million ha of spring bread wheat in the developing world. Similar information on the genetic basis and longevity of useful leaf rust resistance was not available on a global basis to facilitate our analysis.

Our study therefore encompasses all genetic resistance mechanisms carried by CIMMYT-related spring bread wheat. We compare the yield losses suffered by varieties with different types and levels of genetic leaf rust resistance to the yield losses that would have been suffered had all these varieties been fully susceptible. Our study is limited to the developing world, because the mandate of CIMMYT’s Wheat Program is to breed advanced lines for the national agricultural research programs in these countries. We focus on spring bread wheat, because it covers about two-thirds of the wheat area in the developing world (Heisey, Lantican, and Dubin, forthcoming). Our analysis is conducted by wheat breeding mega-environment (ME), since this is a classification developed by the CIMMYT Wheat Program to guide its germplasm enhancement activities (Rajaram, van Ginkel, and Fischer, 1995). We focused on the MEs where spring bread wheat is grown at low latitudes, and thus included MEs 1, 2, 3, 4a, 4b, 4c, and 5. More information on these production environments is presented in Marasas, Smale, and Singh (forthcoming).

With the term “CIMMYT-related” we include those materials selected from advanced CIMMYT lines by wheat breeders in national agricultural research programs. These varieties generally descend from the first semidwarf varieties released during the late 1960s. Almost 80% of the spring bread wheat area in developing countries was sown to CIMMYT-related varieties in 1997 (Heisey, Lantican, and Dubin 1999). A survey of wheat breeders in these countries indicated that materials from CIMMYT International Nurseries are the most frequently crossed in pursuit of disease resistance goals (Rejesus, Smale and van Ginkel, 1997). Broad international flow of CIMMYT-related germplasm with leaf rust resistance has therefore been likely.

CONCEPTUAL FRAMEWORK

The first step in measuring the economic benefits of agricultural research, is to compare the situation with research to the one with no research, also known as the “with” and “without” scenarios. In view of the pathogen’s ability to overcome the effects of previously resistant varieties, we argued that leaf rust resistance breeding is an example of research aimed at maintaining crop productivity. We applied an economic surplus approach adjusted for maintenance research to estimate the gross benefits of CIMMYT’s investment in leaf rust resistance breeding since 1973.

The effects of productivity enhancement are often treated as a cost-reducing rightward or downward shift in the commodity supply function, as shown by S1 in Figure 1. This is assumed to result from yield increases or cost savings associated with the technology. The “without” scenario assumes constant supply in the absence of research, as represented by S0. However, the assumption of a static supply function does not remain valid in the face of evolving leaf rust pathogens. Once a variety’s resistance has been overcome, its production gains will decline and result in lower production per unit cost. If not constantly replaced by newly resistant varieties with similar productivity potential, the “without” scenario would comprise a leftward or upward shift in the supply curve, shown by S2. In an economic surplus framework, maintenance research can therefore be defined as the effort required to prevent a cost-increasing shift in the supply curve, which results from changes in the physical, economic or biological environment (Collins, 1995). The economic surplus thus generated is shown as the shaded area in Figure 1. Though full adoption and depreciation is assumed in Figure 1, these processes are in fact dynamic and proceed over a period of time.

In our case, we assume that the “with” scenario represents the actual wheat supply (S0), generated by the CIMMYT-related spring bread wheat varieties with various leaf rust resistance categories, grown in the developing world since 1973. The “without” scenario is the supply (S2) that would have prevailed had all these varieties been fully susceptible.


Figure 1. General economic surplus approach adjusted for maintenance research
Our approach is methodologically simplified, due to standard difficulties in estimating the impact of maintenance research, estimating the economic impact of agricultural research in general, and limitations imposed by the data available to us. We apply a capital investment analysis to estimate the returns instead of a fully developed equilibrium model based on a multi-market world economy. This is partly because the benefits in our analysis are aggregated over a large number of developing country wheat producers. Losses to leaf rust might have generated a shift in the short- and long-term wheat supply curve in any one of these countries. However, these changes would not have been substantial enough to affect the world wheat price in the presence of the large volumes traded by developed country wheat producers. The demand curve is therefore completely elastic at the world wheat price in our version of Figure 1. We measure the wheat supply shift avoided in units on the horizontal axis, valued at the world wheat price, for each year and wheat-producing environment included in the study. Our supply curve refers to CIMMYT-related spring bread wheat only.

METHODOLOGY

In our capital investment analysis, the research returns were estimated in terms of the net present value, internal rate of return, and benefit-cost ratio. The net present value of leaf rust resistance breeding in CIMMYT-related spring bread wheat can be most generally expressed as:

(1) Net present value = [(pt yt at) – Ct]

Essential parameters are: , the average, annual, farm-level percentage yield loss avoided by growing varieties with various leaf rust resistance categories; y, the average, annual, farm-level wheat yield, and a, the area to which yield savings apply. The product of these terms represents the production savings from leaf rust resistance breeding by genetic resistance category and wheat breeding environment. The real wheat price p is used to calculate an economic value on the wheat yield saved. The difference between the gross research benefits and the cost of research C is calculated for each year t. This begins in 1973 (t1), the year in which the first variety recognized and promoted for its race-nonspecific resistance was released (Torim 73). It ends n years later in 2007 (tn), the year the last adoption ceiling predicted in our logistic diffusion curves is reached. The research benefits are discounted using the interest rate i to obtain the net present value.

The internal rate of return is estimated by setting the net present value equal to zero in equation (1) and solving for i arithmetically:

(2) [(pt yt at) – Ct] = 0

The benefit-cost ratio is calculated by dividing the present value of the gross benefits by the present value of the research costs:

(3)

Estimation of each of the parameters in equations (1) to (3) is described next, with details related to data sources and assumptions. A summary of parameter assumptions is presented in the Annex Table 1.

Percentage yield savings

Parameter yt in equations (1) to (3) was estimated as the product of the following three terms:

1) The percentage yield savings of resistant relative to susceptible varieties by genetic resistance category. For this purpose, we used trial data for a sample of the major spring bread wheat varieties grown in the developing world, as drawn from CIMMYT’s 1997 Global Wheat Impacts Survey. Varieties with known CIMMYT origin, released since 1970, grown on more than 500 ha, and for which seed was available in the CIMMYT gene bank, were grown without fungicide protection in a field trial in El Batán, Mexico. Leaf rust epidemics were established by inoculating susceptible spreader rows planted adjacent to the trial material. The varieties were classified by type and level of genetic resistance to the current Mexican population of leaf rust, based on the modified Cobb-scale (Peterson, Campbell, and Hannah, 1948) (Table 1). Seedling evaluation tests with selected P. triticina races were conducted in the greenhouse to assess the presence of effective race-specific genes. The trial data were combined with supplementary data from previous CIMMYT trials over several years to obtain a sample of 184 varieties. The percentage infection relative to the susceptible check variety was used to calculate the associated yield savings for each resistance category.

Table 1. Definition of the leaf rust resistance categories used in this study a

CategoryPercentage leaf rust infection Type of resistance

relative to susceptible check

180 - 100% Susceptible

250 - 70% Race-nonspecific, low resistance

330 - 50% Race-nonspecific, moderate resistance

410 - 20%Race-nonspecific, high resistance

5less than 10% Race-nonspecific, high resistance

6less than 5% Effective, race-specific resistance

a Based on the modified Cobb-scale (Peterson et al., 1948).

2) The average, annual, farm-level percentage yield lost with susceptible varieties by ME. These annual yield loss data were not available over the extensive spring bread wheat producing areas of the developing world. Data on the weather conditions, management practices, and spatial distributions of pathogen and resistance types were also not available to allow prediction of the annual disease pressure or the duration of resistance. We then searched various sources of trial data and historical accounts from the literature for estimates of expected losses from secondary sources (Marasas et al., forthcoming).

However, experimental estimates were not available for all of the production areas included in our study, and the available data from small-plot evaluations tend to over-estimate disease losses. The number and significance of recorded rust epidemics vary, and estimated production losses have typically been reported anecdotally for the developing world. Even when occurrence of the disease may be recorded, it is seldom accompanied by data on yield losses, or the relationship to wheat prices, output levels, and imports. We were furthermore concerned with farm-level yield losses averaged over several years, large areas, and various production environments in developing countries. These are clearly lower than the yield losses estimated in zones of high disease pressure, or the losses reported in epidemic years. Yield loss information for developed countries, such as the comprehensive data from the Cereal Disease Laboratory ( for the United States of America, is also not appropriate. These estimates firstly do not represent all the spring bread wheat producing environments included in our study. They also do not represent the situation in all developing countries, where few farmers use fungicides to control leaf rust.