Trends and prospects for the productivity and sustainability of home garden systems in southern Ethiopia

Increasing population pressure and market developments are major drivers of change in land use and rural livelihoods. Home gardens are characterized by multipurpose trees and shrubs in intimate association with annual and perennial crops and livestock around the homestead which support the livelihoods of more than 15 million people in southern Ethiopia. The aim of this study was to gain a better understanding of how changes in home garden systems impact on agricultural sustainability. First, we studied dynamics in the cropping pattern and livestock population over the past two decades (1991-2013). To understand the diversity of home garden systems, farms were grouped into five types: Khat-based, Enset-cereal-vegetable, Enset-based, Enset-coffee and Enset-livestock based on the area share of crops and grazing land. Farm trajectories revealed a shift from the traditional Enset-based, Enset-coffee and Enset-livestock systems to 1) cash crop-oriented Khat-based systems in densely populated, market-proximate areas, and 2) combined food and cash crop-oriented Enset-cereal-vegetable systems in less populated and less accessible areas. Over the last two decades the area under khat (Catha edulis Forsk) expanded from 6% to 35% of the farm area, whereas the share of farm area devoted to both enset (Enset ventricosum (Welw.) Cheesman) and coffee (Coffea arabica L.) decreased from 45% to 25% in densely populated market-proximate areas. Meanwhile, in medium-populated, less accessible areas, enset and coffee together maintained a share of over 45% per farm, but cereals and vegetables gained in importance. Given the difficulty in quantifying the productivity of perennial crops, we used allometric relations to estimate the edible and commercial yield of both enset and khat plants. As home gardens produce several crops with a distinct purpose (e.g. food, feed, cash generation), the farm level productivity was expressed in three ways, in terms of annual crop yield, human edible energy yield and revenue. The lowest farm level crop productivity was attained in the traditional Enset-coffee systems (1820 kg DM ha-1), whereas the highest in the newly evolved Enset-cereal-vegetable systems (3020 DM kg ha-1). Energy productivity from food crops was higher in Enset-based systems (43 GJ ha-1) than in other systems whereas annual revenue was lowest in Enset-based systems (719 US$ ha-1) and highest in newly evolved Khat-based systems (6817 US$ ha-1). Introducing the high value cash crop khat and annual cereals in traditional home garden systems enabled smallholders to maintain food security and dietary diversity without jeopardizing plant species richness. Contrary to common claims, there was no positive effect of plant species richness on total crop and energy productivity, except for the positive effect on revenue in enset-oriented systems. The farm level gross margin attained in Khat-based systems was 4, 4, 2, and 2 times larger than in Enset-cereal-vegetable, Enset-based, Enset-coffee and Enset-livestock systems respectively. However, the shift away from enset and coffee, which received only organic fertilizer, to khat, which received solely inorganic fertilizer, led to a decline in N, P and K stocks by 20%, 70% and 30% respectively. As the profitability of khat cannot be matched by any other crop at current market prices, abolishment of khat cultivation is not an option for the small farms, as that would jeopardize their livelihood. One option for improvement is intercropping khat with food crops, such as maize and beans.  With population density expected to continually increase in the region, more improvement options tailored to the specific systems are required for sustainable development of home garden systems.