Establishing optimum plant populations and water use of an ultra fast maize hybrid (Zea Mays L.) under irrigation
Yada, Gobeze Loha
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English: For each grain production system, there is an optimum row spacing and plant density that optimises the use of available resources, allowing the expression of maximum attainable grain yield in that specific environment. Introduction of the ultra-fast maize hybrids raised the question whether existing guidelines for row spacing and plant density were still applicable. This necessitated the integration of optimum row spacing by plant density to maintain productivity and sustainability the yields with the intention to increase water use efficiency. Field experiments were conducted for two successive cropping seasons (2008/9 to 2009/10) at Kenilworth Experimental Station of the Department of Soil, Crop and Climate Sciences, University of the Free State to evaluate the growth, agronomic performance, phenological development and water use efficiency of an ultra-fast maize hybrid at varying row spacing and plant densities under irrigation. The treatments involved in this study were three row spacings (0.225, 0.45 and 0.90 m) and five plant densities (50 000, 75 000, 100 000, 125 000 and 150 000 plant ha-1). The treatments were arranged in a factorial combination and laid out in a randomized complete block design (RCBD) with four replications. The largest block was used for periodic destructive sampling for growth analysis where a completely randomized design was adopted and replications consisted of five (5) single plants randomly selected. Regarding soil water monitoring, twenty neutron probe access tubes were installed prior to planting in the center of each plot in one of the three blocks of the agronomic study. Soil water content was measured at 0.3 m intervals to a depth of 1.8 m using a calibrated neutron probe. Measurements were made at weekly intervals from planting to crop physiological maturity where the volumetric reading was converted into depth of water per 1.8 m. Seasonal ET (water use) was determined by solving the ET components of the water balance equation. From this water use efficiency was computed as the ratio of total biomass/grain yield to seasonal ET. In each season crop growth, agronomic, phenologic and water use efficiency parameters were measured and the collected data were combined over seasons after carrying the homogeneity test of variances. Growth parameters, agronomic traits, phenology and water use efficiency of maize reacted differently to row spacing and plant density and the combination thereof. In general a slow increase in growth parameters during establishment was followed by an exponential increase during the vegetative phase. At the reproductive phase growth ceased following the onset of flowering. Photosynthetic efficiency (NAR) and CGR, averaged over row spacing, were highest at a plant density of 100 000 plants ha-1 at all growth phases. Reducing row spacing from 0.45 to 0.225 m and a plant density below or above 100 000 plants ha-1 showed LAI outside the optimum with respect to NAR for optimum seed yield. Row spacing, plant density and its interaction affected yield and yield components of maize significantly. Narrowing rows from 0.45 to 0.225 m and plant densities above 100 000 plants ha-1 as main or interaction effects led to the formation of smaller ears, a shorter ear length and diameter, low seed mass, favored plant lodging and development of barren plants with an obvious negative impact on grain yield. On other hand, plant densities below 100 000 plants ha-1 were insufficient to utilise growth-influencing factors optimally. Thus, growth analysis provided an opportunity to monitor the main effects and interaction effects of row spacing and plant density on crop growth at different growth and development phases. Row spacing and plant density combinations affected the phenological development of maize. Increasing row spacing from 0.225 to 0.90 m relatively prolonged the number of days to anthesis and silking. Regarding anthesis-silking interval (ASI), the lowest plant density had the shortest ASI while the higher plant densities had relatively longer ASI. Wide row spacing coupled with low plant density increased the number of days to physiological maturity and vice versa. Row spacing and plant density and their interaction affected water use efficiency of maize. Highest water use was observed at a plant density of 125 000 plants ha-1. Biomass WUE was highest at a row spacing of 0.45 m with a plant density of 125 000 plants ha-1 while the highest grain yield WUE recorded was at a row spacing of 0.45 m with a plant density of 100 000 plants ha-1. The overall combined effect of row spacing and plant density revealed that a combination of 0.45 or 0.90 m with 100 000 plants ha-1 to be the optimum for the selected ultra-fast maize hybrid under irrigation.