Effect of tillage system, residue management and nitrogen fertilization on maize production in western Ethiopia
Dilallessa, Tolessa Debele
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English: The sustainability of maize production in western Ethiopia is in question despite of favorable environmental conditions. A major reason for this phenomenon is severe soil degradation in maize fields. This soil degradation manifested often in low soil N fertility which inhibited maize yields. The situation is worsened by the financial inability of most farmers to purchase N fertilizer for supplementation. In these conditions two basic approaches can be followed to improve maize productivity in a sustainable way. Firstly, integrated cropping practices can be developed for maize to make better use of N from organic and inorganic sources. Secondly, maize genotypes can be selected that are superior in the utilization of available N, either due to enhanced uptake efficiency or because of more efficient use of the absorbed N. In this context, experiments were conducted to determine the integrated effects of tillage system, residue management and N fertilization on the productivity of maize, and to evaluate different maize genotypes for N uptake and use efficiency. The experiments on integrated cropping practices were done from 2000 to 2004 at five sites viz. Bako, Shoboka, Tibe, Ijaji and Gudar in western Ethiopia. They were laid out in a randomized complete block design with three replications. Three tillage systems (MTRR = minimum tillage with residue retention, MTRV = minimum tillage with residue removal and CT = conventional tillage) and three N levels (the recommended rate and 25% less and 25% more than this rate) were combined in factorial arrangement. Every year yield response, usage of applied N and changes of some soil properties were measured. In 2004 the same experiments were used to monitor the fate of applied N in the soil-crop system. Labeled urea was applied at the recommended rate to micro plots within the MTRR and CT plots for this purpose. During the initial two years of the experiments, there was no significant difference in grain yield between MTRR and MTRV and both were significantly superior to CT. However, during the final two years of the experiments, there was no significant difference between MTRV and CT and both were significantly inferior to MTRR. On average, the grain yield of MTRR was 400 and 705 kg ha-1 higher than that of MTRV and CT, resulting in consequent increases of 6.6 and 12.2%, respectively. The application of N increased the grain yield regardless of tillage system. An application of 92 kg N ha-1 was significantly superior to 69 kg N ha-1, but on par with the 115 kg N ha-1 application. Hence, the recommended fertilization rate of 92 kg N ha-1 for conventional tilled maize was also found adequate for minimum tilled maize in western Ethiopia. This rate remained economically optimum with a 20% decrease in the maize price and a 20% increase in fertilizer cost. The grain differences resulted from the tillage systems and concomitant residue management were attributed to significant changes in some soil fertility parameters, especially in the 0-7.5 cm layer. After five years both indices of organic matter, viz. the organic C and total N contents were significantly higher in the MTRR soils when the CT soils serve as reference. Similarly, the extractable P and exchangeable K contents of the MTRR soils were also higher than that of the CT soils. The only negative aspect of MTRR in comparison with CT was a decline in soil pH. A significantly higher grain N content was recorded with MTRR than with MTRV and CT. The stover N content was not significantly affected by the three tillage systems. However, grain, stover and total N uptake were consistently superior with MTRR compared to MTRV and CT. The NAE, NRE and NPE of maize for the same tillage system were consistently higher at the lower N level range of 69-92 kg ha-1 than at the higher N level range of 92-115 kg ha-1. At the lower N level range NAE and NRE were larger with CT than with the other two tillage systems. Both indices were higher with MTRR than with the other two tillage systems at the higher N level range. The NPE was not significantly affected by the tillage systems. However, the trend at both N level ranges was higher with MTRR than with MTRV and CT. The labeled urea study showed that the grain, stover and total biomass N derived from fertilizer was consistently higher for CT than MTRR. Conversely, grain, stover and total biomass N derived from soil was consistently higher with MTRR than CT. Therefore, the fertilizer N recorded in the MTRR soils was higher with MTRR than CT and mainly confined to the upper 45 cm. The fate of fertilizer N was in MTRR: 47% recovered by maize, 17% remained in the soil and 36% unaccounted for and in CT: 54% recovered by maize, 12% remained in the soil and 34% unaccounted for. The experiments on genotype comparison for N uptake and use efficiency were also done at Bako, Shoboka, Tibe, Ijaji and Gudar. In 2004 the response of five open-pollinated and five hybrid genotypes were evaluated at six N levels from 0 to 230 kg ha-1 with 46 kg ha-1 intervals. Only two out of the ten genotypes evaluated qualify as N use efficient. They were the openpollinated Ecaval 1 and the hybrid CML373/CML202/CML384. These two CIMMYT genotypes showed consistently higher NAE, NRE and NPE at low and high N applications as required. This was not the case with the two local genotypes that were included, viz. the open-pollinated Kulani and the hybrid BH 540. Based on the results that evolved from this study it is clear that: 1. Farmers should be encouraged to practice MTRR instead of CT since this change in tillage system could improve the productivity of maize on Nitisols in western Ethiopia. 2. On these Nitisols the conversion from CT to MTRR need not coincide with an adaptation in the recommended fertilization rate of 92 kg N ha-1. 3. The planting of N use efficient maize genotypes on Nitisols must be advocated to farmers, especially those who can not afford proper fertilization. Aspects that need to be investigated in future are: 1. Quantification of N mineralization and immobilization in the Nitisols when subject to MTRR and CT for maize production. 2. Losses of fertilizer N through volatilization, leaching and denitrification from the Nitisols when subject to MTRR and CT for maize production. 3. Suitability of other soil types which are used for maize production in western Ethiopia for MTRR instead of CT. 4. Performance of the N use efficient genotypes on other soil types which are used for maize production in western Ethiopia. 5. Crop rotation with N fixing crops.Afrikaans: Die afname in grondvrugbaarheid, in besonder N is een van die grootste beperkinge vir mielieproduksie in Wes-Ethiopië. Hierdie situasie word vererger deur die finansiële onvermoë van meeste boere om stikstofkunsmis te koop vir aanvulling. In hierdie toestande kan twee basiese benaderings gevolg word om mielieproduktiwiteit op ‘n volhoubare wyse te verbeter. Eerstens kan geïntegreerde gewasverbouingspraktyke vir mielies ontwikkel word sodat N vanaf organiese en anorganiese bronne beter benut word. Tweedens kan mieliegenotipes geselekteer word wat doeltreffend in die opname en gebruik van N is. In hierdie konteks is proewe uitgevoer om die effek van geïntegreerde bewerkingstelsel, gewasrestebestuur en stikstofbemesting op mielieproduktiwiteit te bepaal en ook verskillende mieliegenotipes vir doeltreffende stikstofopname en –gebruik te evalueer. Die proewe oor geïntegreerde gewasverbouingspraktyke is vanaf 2000 tot 2004 by vyf lokaliteite in Wes-Ethiopië uitgevoer. Drie bewerkingstelsels (MTRR = minimum bewerking met behoud van gewasreste, MTRV = minimum bewerking met verwydering van gewasreste en CT = konvensionele bewerking) en drie stikstofpeile (die aanbevole hoeveelheid en 25% minder en 25% meer as die hoeveelheid) is gekombineer in ‘n faktoriale rangskikking. Elke jaar is die opbrengsreaksie, gebruik van toegediende N en verandering in sommige grondeienskappe bepaal. In 2004 is dieselfde proewe gebruik om die lot van toegediende N in die grond-gewas sisteem te monitor. Gemerkte ureum is teen die aanbevole hoeveelheid op mikro persele binne die MTRR en CT persele vir die doel toegedien. In vergelyking met MTRR en CT het MTRR die graanopbrengs betekenisvol met onderskeidelik 6.6 en 12.2% verhoog. Net so het die toediening van N graanopbrengs verhoog en die agronomiese optimum, wat ook ekonomies winsgewend is, was 92 kg N ha-1. Die groter graanopbrengste wat met MTRR gerealiseer het, is toegeskryf aan die hoër inhoude van organiese materiaal, ekstraheerbare P en uitruilbare K wat met hierdie bewerkingstelsel na vyf jaar aangeteken is, veral in die 0 – 7.5 cm grond laag. Al drie indekse vir doeltreffende gebruik van toegediende N deur mielies, te wete die agronomiese (NAE), herwinnings (NRE) en fisiologiese (NPE) was deurgaans hoër by die laer stikstofpeilreeks van 69-92 kg ha-1 as by die hoër stikstofpeilreeks van 92-115 kg ha-1. Beide NAE en NRE was die grootste met CT by die laer stikstofpeilreeks en met MTRR by die hoër stikstofpeilreeks. Die NPE het ‘n geneigdheid getoon om by beide stikstofpeilreekse hoër met MTRR as met MTRV en CT te wees. Tydens oes het mielies gemiddeld 47 en 54% van die gemerkte ureumstikstof herwin vanaf onderskeidelik die MTRR en CT gronde. Hierteenoor was daar met oes nog 12 en 18% van die gemerkte ureumstikstof in die CT en MTRR gronde. Die gemerkte ureumstikstof waarvoor daar nie in die twee sisteme voor rekenskap gegee kon word nie was 36% vir MTRR en 34% vir CT. Die proewe oor die vergelyking van genotipes vir stikstofopname en –gebruikstreffendheid is op dieselfde lokaliteite gedoen wat voorheen na verwys is. In 2004 is die reaksie van vyf oopbestuifde genotipes en van vyf baster genotipes geëvalueer by ‘n stikstofpeilreeks vanaf 0 tot 230 kg ha-1 met 46 kg ha-1 intervalle. Slegs twee van die genotipes kwalifiseer as stikstofgebruiksdoeltreffend, naamlik die oopbestuifde Ecaval 1 en die hibried CML373/CML202/CML384. Hierdie twee CIMMYT genotipes se opbrengs was gemiddeld 5.9% by ‘n lae stikstoftoediening en 17.5% by ‘n hoë stikstoftoediening beter as die van hulle lokale genotipes. Die volhoubaarheid van mielieproduksie op Nitisols in Wes-Ethiopië kan bevorder word deur MTRR in stede van CT toe te pas met die huidige stikstofaanbeveling. Groot waarde kan tot die verandering in bewerkingstelsels toegevoeg word deur die plant van stikstofgebruiksdoeltreffende genotipes.