1. Department of Land Resource Management and Agricultural Technology, College of Agriculture and Veterinary Sciences, University of Nairobi , Nairobi, Kenya
2. German Institute for Tropical and Subtropical Agriculture (DITSL), Witzenhausen, and Institute of Agricultural Sciences in the Tropics (Hans-ruthenberg-institute 490g), University of Hohenheim , Stuttgart, Germany
3. Department of Agricultural Economics, College of Agriculture and Veterinary Sciences, University of Nairobi , Nairobi, Kenya