Agronomic and environmental studies of potato (Solanum tuberosum L.) and analysis of its value chain in Zimbabwe

Keywords: Irish potato, food security, stakeholder analysis, sustainability indicators, Cool Farm Tool-Potato, yield gap, resource use efficiency, LINTUL-POTATO model, Zimbabwe. Oniward Svubure (2015). Agronomic and environmental studies of potato (Solanum tuberosum L.) and analysis of its value chain in Zimbabwe. PhD thesis, Wageningen University, The Netherlands, with English summary, 220 pp. Irish potato is food for more than a billion people worldwide. In Zimbabwe, Irish potato is becoming an important food crop. The government declared it a national strategic food security crop on 18 May 2012. This major policy pronouncement, qualified Irish potato for government initiated farmer support initiatives such as mechanisation and irrigation capacity building. The growing importance of potato as a food crop is prefaced on rising food insecurity in the country coupled with the impact of the radical land reform of 2000 on agricultural production. The land reform completely restructured commercial agriculture when about 96 % of the original 12.5 million ha of large-scale commercial farmland in 1980 was taken up for resettlement by 2010. Two resettlement models were used, the A1 and A2 resettlement models. The former resembles the communal area land allocation system while the later are self-contained small to medium scale farm units ranging about 35 to 300 ha. The newly resettled farmers have started growing potato adding to the already existing communal area and the few remaining large-scale commercial farmers. It is in this context that the potential of the new agrarian structure to sustainably increase Irish potato production was investigated. Increasing potato production on a sustainable basis will enable the crop to assert itself as a national strategic food security crop and help ease the food security challenges the country is grappling with. A grower survey was conducted on the cultural practices, input use, average yield, and infrastructure for potato production. The survey data was used to categorise the growers. Only growers with a minimum 5 years continuous potato growing experience were targeted making the data collected dependable. Grower resource footprints of land, water, biocides and nutrients were calculated based on the actual yield, Ya. Further, the Ya data collected were used to calculate the yield gap, YG, based on the yield of the best performing growers, Yh, simulated yield potential, Yp, and water-limited potential yield, Yw, of the respective agro-ecological areas. The LINTUL-POTATO model was used to estimate Yp, Yw and water need. This model simulates potential dry matter production based on radiation use efficiency of intercepted light by the potato crop. Another model, the Cool Farm Tool-Potato was used to further distinguish and appraise the production systems in terms of yields, inputs and efficient use of energy as reflected in their CO2 balances. The model calculates the contributions of various production operations to the total greenhouse gas (GHG) emission. Consequently, grower practices which contribute the most to the GHG emission were identified and generic mitigation measures for each production system were suggested. Realising the growing importance of sustainability issues in agricultural production and the scarcity of evaluation protocols in cropping systems, the study developed a framework that can be used to evaluate cropping systems. The framework was constructed using the potato-based cropping systems in the Eastern Highlands of Zimbabwe. Finally, instead of just focusing on the production related aspects only, the study also took into cognisance the need to understand the performance of the entire Irish potato sector in Zimbabwe. A value-chain analysis was therefore conducted to evaluate the performance the Irish potato sector in the country. Irish potato production in Zimbabwe is still low. Experts estimate annual production at nearly 120,000 t from around 6,000 ha. The large-scale commercial and the A2 resettlement are large-scale, high input and mechanised systems with an average potato area of 9 ha per planting. The communal area and A1 resettlement are smallholder low input systems with average potato area per planting of 0.8 ha and animal-drawn equipment is used. On resource use efficiencies, the actual tuber yield ranged from 8 – 35 t/ha across all systems representing a yield gap of over 77 %. Comparing with the simulated average potential yield, the mean actual yield observed ranged from 8 to 35 % of the simulated potential yield, translating to a yield gap of 65 to 92 %. Hence there is a large potential to increase potato production in these environments. The nutrient use efficiencies range were: 97 to 162 g potato g-1 N, 93–105 g potato g-1 P2O5 and 97–123 g potato g-1 K2O. This was anticipated because of the high synthetic fertiliser use and the low actual yields reported. The biocide use efficiencies ranged from 0.5 to 0.9 kg potato g-1 active ingredient (a.i.) fungicide, and 8 to 15 kg potato g-1 a.i. insecticide. Regarding water use, the average water use efficiency based on irrigation water and rainfall, ranged from 2 to 6 g potato l-1, while the simulated potential water use efficiency from irrigation and precipitation ranged from 9 to 17 g potato l-1. The large gap observed between actual and potential water use efficiency shows the scope to improve crop management practices to increase actual yield while lowering irrigation water. On the CO2 balance of the systems, a high carbon footprint was reported with an average of 251 kg CO2 eq./t potato. The least average carbon footprint was 216 kg CO2 eq./t potato for the communal area, while the A2 resettlement system had the highest of 286 kg CO2 eq./t potato. The high carbon footprint was anticipated as a reflection of the systems’ inefficiencies in terms of low yields and high input use. Focussing on the performance of the entire Irish potato sector, value chain analysis showed considerable levels of value-addition and gross profit of at least 13 % at each linkage. While the sector enjoys government policy support, major factors impacting on the value-chain performance relate to high potato production costs, low yields, and lack of farmer training. On the proposed framework on cropping sustainability, the indicator thresholds serve to monitor farmer progress as they improve their practices towards the desired direction of sustainability. This study demonstrated that there is tremendous potential to increase potato output and help ease the food insecurity challenges the country currently faces.