Within-Plot and within-Plant Variability and Senescence Patterns of Maize Under Contrasting Tillage Systems, Stubble and Fertilizer Nitrogen.

This study quantified the within-plot and within-plant variability and the dynamics of senescence of maize in response to contrasting tillage systems; conventional (CT) and no-tillage (NT), different rates of stubble (0, 3 and 5 t ha^-1) and fertilizer N (0, 80 and 120 kg N ha^-1). Within-plot variability was assessed weekly throughout the crop cycle by measuring canopy greenness while within-plant variability was assessed by measuring the vertical profile of leaf greenness from anthesis to maturity. Senescence was scored visually from anthesis to maturity. Large within-plot variations in canopy greenness were measured under CT compared with NT during the reproductive phase but the differences disappeared as the crops aged. Stubble retention increased within-plot variability during the vegetative and the reproductive phases but variations decreased as the crop matured compared with bare ground. Fertilizer N reduced variations at plot level. Within-plant vertical profiles of leaf greenness did not differ between tillage systems and among stubble rates but fertilized crops had greener and increased leaf area index. Treatment interactions at both plot and plant level were inconsistent. Canopy senescence did not differ between tillage and stubble treatments but a time-course estimation of senescence revealed a fast rate of senescence of crops grown under stubble and N fertilization. Increases in the rate of senescence positively correlated with grain yield (R = 0.58) and grain protein (R = 0.37). These results imply that NT and stubble retention could reduce variability at the plot level, due to improved water and N economies. While senescence plays a regulatory role, the process could not be manipulated at different leaf positions, irrespective of the crop nutrition status.