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Type: Theses
Title: Mechanisms contributing to wheat and maize yield under no-till, stubble retention and nitrogen fertilization in contrasting environments
Author: Kitonyo, Onesmus Musembi
Issue Date: 2018
School/Discipline: School of Agriculture, Food and Wine
Abstract: Tillage, stubble retention and nitrogen fertilization are management practices that influence the productivity and sustainability of rainfed cropping systems. However, the application of these practices is limited by our understanding of the mechanisms that contribute to crop growth and yield, including water and nitrogen use efficiency. Canopy development and patterns of leaf senescence alter the partitioning of water and nitrogen (N) use, both before and after flowering, which impacts grain yield. The two central questions for this research were: (1) what are mechanisms of canopy development that contribute to yield in no-till and stubble retention systems?; and (2) under what circumstances do they increase yield, water and N use efficiency? The aims for this thesis were to evaluate crop response to no-till, stubble retention and N fertilization, in the contrasting systems of wheat (Triticum aestivum L.) and maize (Zea mays L.), to better understand mechanisms that regulate crop growth, patterns of senescence and yield, in addition to water and nitrogen use efficiency (NUE). A physiological approach that linked the traits regulating crop growth and yield was used to interpret crop responses to treatments. It is hypothesised that there are similarities in the mechanisms operating in no-till and stubble retention systems that could be improved to increase yield. Field experiments were conducted in the dryland wheat growing environments of southern Australia and in maize systems in a sub-humid tropical environment in Kenya. In Australia, experiments were conducted at Roseworthy and Karoonda using two tillage treatments (conventional tillage, CT and no-till, NT), four rates of stubble (zero, low, moderate and high) and three N timings, splitting the application of 100 kg N ha⁻¹ between sowing, tillering (GS22) and awn emergence (GS49) in the ratios of 100-0-0, 25-50-25, 0-50-50. At Roseworthy, historic Australian wheat varieties were evaluated under NT with the retention of moderate amounts of stubble and under CT without stubble. In Kenya, field experiments were conducted at Embu research station to evaluate the responses of maize to CT and NT, three amounts of stubble (0, 3 and 5 t ha⁻¹) and N rates of 0, 80 and 120 kg N ha⁻¹, as well as timing the of supply of 80 kg N ha⁻¹ at sowing, six leaf stage (V6) and 12-leaf stage (V12) in the fractions of 0-0-0, 80-0-0, 27-53-0, 27-27-27 and 0-40-40. Wheat grain yield ranged from 1.5-3.2 t ha⁻¹, and the effects of tillage were marginal. Grain yield increased from bare ground up to the application of moderate amounts of stubble but reduced at high amounts of stubble. Benefits of water capture and storage did not improve with the application of high amounts of stubble. Crop growth rate (CGR) between stem elongation and flowering was inversely correlated with tiller numbers, and explained most of the treatment differences. Sowing application of N produced large vegetative biomass which led to a decrease in CGR and radiation use efficiency between stem elongation and flowering, resulting in a decrease in grain yield compared with delayed N supply. Five decades of selection has not provided greater adaptation to NT and stubble retention in Australian wheat, despite grain yield increases of 21 kg ha⁻¹ year⁻¹ between 1958 and 2011. Substantial changes in canopy architecture were detected from older taller varieties with closed canopies to modern short-stature varieties with more open canopies. Modern varieties had greener leaves but showed faster rates of leaf senescence compared with the older counterparts. Maize grain yield ranged from 2.3-5.3 t ha⁻¹, with small effects from tillage and stubble supply. Rate and timing of N supply produced large effects and modified crop response to tillage and stubble. When stubble was removed, grain yield reduced by 10% while water storage at sowing decreased by 8% under NT compared with CT. Crop growth rate between six-leaf stage (V6) and flowering, and nitrogen nutrition index (NNI) partially explained treatment differences. Retention of stubble reduced CGR and NNI compared with bare ground. The value of stubble in water storage at sowing, and crop growth and yield was greater in a season that received < 300 mm rainfall compared with where rainfall was > 600 mm. Delaying N supply increased NNI, CGR and traits associated with NUE and grain yield compared with sowing applications of N. Patterns of senescence in maize, at both whole-plant and canopy-layer scales were marginally impacted by tillage and stubble retention. Leaf senescence was primarily driven by N supply and sink size. Time to loss of 50% of maximum leaf greenness was earlier in fertilized crops but delayed in the unfertilized controls. Rate of senescence was faster in fertilized crops compared with unfertilized controls at both whole-plant and canopy-layer scales. Grain yield, kernel number and nitrogen remobilization efficiency were associated with a faster rate of senescence in the top and mid layer leaves but with slower rates of senescence in the bottom layer. There were similarities in treatment effects and the mechanisms that regulated crop growth and yield between the two systems: (1) Grain yield was a function grain number, which in turn was proportional to CGR during the critical period of determination. Strategic supply of N at sowing and later stages increased CGR during the critical period for grain set, improved NNI and increased RUE, hence higher grain yield; (2) Grain yield was maximized at 2-3 t ha⁻¹ of stubble as demonstrated by the analysis of yield gaps, potentially due to water capture and storage and the regulation of soil temperature which impacted emergence and early growth; (3) N supply and sink size modified the patterns of senescence in both crops, whereby faster rates of senescence were associated with higher grain yield; and (4) N supply modified crop response to tillage and stubble. Treatment interactions were few, and varied with N supply and season. Effects of tillage system were marginal and independent of season. A mechanistic approach is discussed, which links treatment effects and the mechanisms regulating grain yield. In conclusion, the mechanisms of canopy development and yield limitation operating in NT and stubble retention were similar in both cropping systems. Higher fertilizer N rates and better timing of N supply are required for yield improvement in NT and stubble retention systems. While NT alone reduced yield, moderate amounts of stubble can improve water storage and grain yield, but this is subject to seasonal rainfall. Critical thresholds of 2-3 t ha⁻¹ of stubble indicate amounts over this limit could be allocated to alternative uses. Results show the importance of interpreting crop responses to NT and stubble retention on the basis of physiological principles.
Advisor: Denton, Matthew
Zhou, Yi
Dissertation Note: Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Agriculture, Food and Wine, 2018
Keywords: Research by publication
stubble amount
nitrogen timing
crop growth rate
nitrogen nutrition index
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