A Bio-Economic Model of Long-Run Striga Control with an Application to Subsistence Farming in Mali Jeffrey D. Mullen Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Agricultural and Applied Economics Daniel B. Taylor, Chair George W. Norton Jeffrey Alwang Bradford Mills John Caldwell February 18, 1999 Blacksburg, Virginia Keywords: Striga, Integrated Pest Management, Mali, Subsistence Farming, Dynamic Programming Copyright 1999, Jeffrey D. Mullen A Bio-Economic Model of Long-Run Striga Control with an Application to Subsistence Farming in Mali Jeffrey D. Mullen (ABSTRACT) The parasitic weeds belonging to the genus Striga are among the world’s most tenacious, prolific and destructive agricultural pests. Crop loss estimates due to Striga infestations can reach 100 percent. Furthermore, the weeds’ affinity for low-fertility soils and low rainfall means that those farming the most marginal lands are most severely affected. Nonetheless, subsistence farmer have yet to adopt seemingly beneficial control practices to any appreciable degree. This paper develops a bio-economic model capable of identifying: (1) affordable, effective Striga control practices consistent with the resource constraints of subsistence farmers; and (2) barriers to the adoption of those practices. The model is comprised of two components: a biological component modeling Striga population dynamics, and an economic component representing the production opportunity set, resource constraints, and price parameters farmers face. The model is applied to two zones in Northwestern Mali, Sirakorola and Mourdiah, and solved using non-linear, dynamic programming. Data collected by the USAID IPM- CRSP/Mali project are used to specify the economic parameters of the model. A new technique for estimating the lower bound of a farmer’s production planning horizon is also developed and employed in the application of the model to Sirakorola and Mourdiah. The results of several model scenarios indicate that the availability of information regarding the efficacy of Striga control practices is a primary barrier to their adoption by subsistence farmers. The movement of Striga seed between fields, however, is of limited importance. The “optimal control practices” identified by the model depend on the size and demographic composition of the production unit (UP), the zone in which the UP is located, and the cash budget available to the UP. At low budget levels, the model suggests planting millet without fertilizer at a high density in Sirakorola and a low density in Mourdiah. At high budget levels, the model suggests planting millet at a high density in both zones while applying urea. The benefits of adopting the optimal set of practices are presented in both nutritional and financial terms, and can reach as much as a ten-fold increase in the nutritional content of and financial returns to a harvest. iii Acknowledgements I would like to thank the United States Agency for International Development (USAID) for its generous support of this project through the Integrated Pest Management Collaborative Research Support Program (IPM-CRSP). I would also like to thank Mali’s Institute d’Economie Rurale for its logistical support in collecting primary data, as well as the considerable intellectual contributions of its staff, including Amadou Diarra, Madame Gambi, Bourema Dembele, Makan Fofana, and countless others. In addition, my thanks go to the Office of International Research and Development at Virginia Tech and its Director, S. K. DeDada, for the valuable administrative support offered during the course of this study. I would also like to recognize the contributions of my advisory committee. Drs. George Norton, Jeffrey Alwang, and Bradford Mills kept me focused on the larger economic issues related to the project. Dr. John Caldwell provided invaluable insight into agricultural production systems in Mali. He also served as my primary technical advisor for the biological component of the model. Most importantly, however, I would like to thank my committee chair, Dr. Daniel Taylor. Dan provided both guidance and encouragement while allowing me the intellectual freedom to identify and pursue my own interests, for which I am grateful. In addition to his many professional contributions, Dan has served as a model dignity and courage for my personal life. His strength and commitment to family in the face of truly adverse conditions will be, without doubt, the most enduring lesson of my tenure at Virginia Tech. And, finally, my deepest thanks go to my wife, Susan, and my two children, Madeleine Jane and Joseph Justis, for their countless personal sacrifices. Without the emotional support they provided throughout, this study would not have been completed. Their names belong on my diploma as well. iv Table of Contents Abstract ................................................................................................................................. ii Acknowledgements ............................................................................................................... iv Table of Contents .................................................................................................................. v List of Tables......................................................................................................................... ix List of Figures ....................................................................................................................... xvi Chapter 1: Introduction Section 1.1: Introduction......................................................................................................1 Section 1.2: Basic Biological Characteristics of Striga ....................................................... 2 Section 1.3: Problem Statement ........................................................................................... 3 Section 1.4: Objectives......................................................................................................... 6 Section 1.5: Organization of the Dissertation ...................................................................... 7 Chapter 2: Literature Review Section 2.1: Introduction......................................................................................................9 Section 2.2: Striga Population Models................................................................................. 9 Section 2.3: Effects of Nitrogen Fertilizers on the Biological Parameters .......................... 18 Section 2.4: Effects of Hand-Pulling on the Biological Parameters .................................... 28 Section 2.5: Effects of Chemical Controls on the Biological Parameters............................ 29 Chapter 3: Methodology Section 3.1: Introduction......................................................................................................41 Section 3.2: Modeling Striga Population Dynamics............................................................ 41 v Section 3.3: The Economic Component............................................................................... 45 Section 3.4: Estimating the Value of Striga Eradication ..................................................... 46 Section 3.5: Identifying Barriers to Adoption...................................................................... 48 Section 3.6: Concluding Remarks on the Methodology ...................................................... 53 Chapter 4: The Economic Component Section 4.1: Introduction......................................................................................................55 Section 4.2: Defining the Representative Agricultural Production Unit.............................. 55 Section 4.3: Defining the Land Constraint........................................................................... 59 Section 4.4: Crop Associations Available in the Model ...................................................... 61 Section 4.5: Defining the Labor Constraint ......................................................................... 61 Section 4.6: Crop Production Functions .............................................................................. 66 Section 4.7: Defining the Cash Constraint........................................................................... 73 Section 4.8: Defining the Nutritional Constraint ................................................................. 74 Section 4.9: Input and Output Prices and the Discount Rate ............................................... 74 Section 4.10: Estimating the UP’s “Planning Horizon”....................................................... 76 Section 4.11: Concluding Remarks on the Economic Component ...................................... 78 vi Chapter 5: The Biological Component Section 5.1: Introduction......................................................................................................82 Section 5.2: Probability of Stimulation................................................................................82 Section 5.3: Probability of Germination..............................................................................86 Section 5.4: Probability of Attachment................................................................................86 Section 5.5: Probability of Emergence.................................................................................88 Section 5.6: Probability of Reproduction.............................................................................91