PROGRESSIVE DECLINE IN SUGARCANE (Saccharum Spp. HYBRID) YIELD AND OPTIONS FOR ADDRESSING THE PROBLEM AT WONJI SHOA SUGAR ESTATE, ETHIOPIA

Abstract

Continuously declining yields of sugarcane have been the major challenge faced by the Ethiopian Sugar Industry. Over the last 70, 58 and 30 years, sugarcane yields have declined in Wonji-Shoa, Metehara, and Finchaa Sugar Estates by 48, 32 and 47%, respectively. However, to date, no comprehensive information has been generated on the root causes and management options of the decline in sugarcane yields. The objectives of this study were to identify the root causes of the progressive decline in sugarcane yields and suggest effective management strategies at Wonji Shoa Sugar Estate (WSSE). Therefore, the declining trends in yield were analyzed with respect soil type, variety and location by using long-term historical yield data (1954–2022) obtained from WSSE. To further comprehend the underlying causes of this decline, the changes in soil properties during the long-term intensive production of sugarcane were assessed through laboratory analysis of biosequentially sampled soils. Then, the minimum data set (MDS) and the key soil properties that exert a significant influence on cane yield were identified by using principal component analysis and stepwise multiple regression, respectively. Prevalence and changes in plant parasitic nematodes (PPNs) were also studied through soil and root sampling from various fields in the WSSE, and by conducting a pot experiment in a greenhouse. To manage the problem, the effectiveness of seven green manure crops were evaluated at WSSE under three methods of stool destruction in a split-plot design. Additionally, a simplified satellite-based approach for monitoring sugarcane plantations was evaluated by testing the accuracy of Landviewer Calculated Vegetation Indexes (LCVIs). Results revealed that 69% of the plantation fields were classified as "yield declining”, and the overall rate of the decline was 0.84 tons ha–1year–1. The rate of yield decline was significant in the older varieties, while non-significant in the newer varieties. Both Vertisol and Cambisol showed a significantly declining yield trends, despite the rate of decline was higher in the former than in the latter. The biosequential soil sampling result showed that organic carbon (OC), total nitrogen, available phosphorus (AP), exchangeable potassium (EcK), available sulfur (AS), zinc, and EC have significantly declined by 16%, 13%, 50%, 39%, 82%, 15%, and 58%, respectively, over the last seven decades. Additionally, the soil OC, AP, AS, and boron concentrations were below the critical values. The MDS of soil properties xxiv xxv at WSSE were identified to be silt, AP, EcK, OC, CEC, AS, sodium adsorption ratio, manganese and soil structural stability index, where the first three were found to play a dominant role in determining sugarcane yield. Furthermore, about ten genera of PPNs were detected in WSSE, of which four genera were newly identified. Over the long-term sugarcane production period, the population and diversity of PPNs increased by 36- and 4.5-fold, respectively. It was also observed that fumigation reduced the population of PPNs by 61% and produced 63% higher cane biomass than the unfumigated soil, suggesting the potential impact of PPNs on the observed yield decline. In the field experiment, cowpea, followed by lablab, dhaincha, and sunn hemp, were found to be the most effective green manures in terms of improving soil fertility and cane performance, with up to 17–20% yield advantage and 21–40% net economic benefits. The spraying method for cane stool destruction was nearly as effective as the cultivating out method. The satellite-based crop monitoring study revealed that the goodness-of-fit of the LCVIs to the sigmoid curve was significant, with 84–95% accuracy for all the indexes evaluated. The majority of the LCVIs also exhibited significant relationships with the yield and yield components of sugarcane. In conclusion, the long-term intensive production system has led to a significant reduction in the quality of soil, and most likely play a predominant role in the continuous decline in cane yields at WSSE. To mitigate this problem, green manuring and satellite-based crop monitoring offers a significant potential. Therefore, particular attention should be given to green manuring with legume plants such as cowpea, lablab, dhaincha, and sunn hemp after herbicide-induced destruction of sugarcane stools. It is also crucial to prioritize continuous replacement of outdated varieties, sustain the fertility of depleted soil, and optimize cultural practices particularly on fields with Vertisol soil types. The Sugar Estate should also adopt satellite-based crop monitoring which can enable plantation managers to take proactive measures before any significant damage occurs.