Response of Synthetic Nitrogen Forms on Maize Performance, Nitrogen Use Efficiency and Grain Protein Content in Kiambu County, Kenya
Isaiah Ogolla, Ochieng’
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The main cause of food insecurity in the developing nations more so in sub-Saharan Africa is the reduced crop productivity among the smallholder farms, which have low soil fertility. Such farms are characterized by low soil nitrogen (N) and inadequate use of inputs such as mineral fertilizers. Nitrogen is the greatest yield and quality-restricting nutrient in crop production globally with its management being one of the most critical aspects required for improving N use efficiency. In Kenya, the indiscriminate use of inorganic N fertilizers by smallholder farmers contributes not only to low maize yields but also impacts its nutritional protein content. A field study was set up, at Kenyatta University farm in Kiambu County for two growing seasons to evaluate the efficacy of N forms using three inorganic N fertilizer treatments: di-ammonium phosphate, calcium nitrate, and urea applied at four N levels (0, 25, 50, and 100 kg N ha−1) on maize performance, N uptake and use efficiency and the grain protein content. This study was laid up using a split-plot design where the whole plot being the three fertilizer treatments and the sub-plot being the four N levels. Treatments had three replications. Data on parameters of growth (plant height, biomass yield, leaf area index, Area of leaf, and leaf number plant−1) was collected. Plant tissues were analyzed for total N at three phenological phases: Vegetative (V-10), tasselling (VT) and harvesting (R6) to monitor N uptake and accumulation as well as determination of N use efficiencies. At maturity, yields on grain and stover were established and the grains analyzed for protein content. Data obtained were subjected for analysis of variance (ANOVA) by use of Genstat software and treatment means were further separated using Fisher's least significant difference (LSD) at p ≤ 0.05. Regression analyses were done on the measured variables to determine relationships between them. Results indicated significantly high (62 cm) plants height (at V-10 stage) in nitrate treated plots with a decrease of 8 and 14% for plants receiving ammonium and urea N forms, respectively. The lowest plant height (46 and 146 cm for V-10 and VT stage, respectively) were recorded in control plots with a peak (65 and 240 cm) registered in plots receiving 50 kg N ha−1. Regarding the area of leaf (LA) and leaf area index (LAI), plants treated with ammonium and urea registered 21 and 25% lower LA respectively, at V-10 stage compared to those receiving nitrate with a plateau at 50 kg N ha−1 rate. A similar trend was observed for LAI. At R6 stage, the highest stover yield (12 t ha−1) was noted in plots with nitrate treatment and lowest (8 t ha−1) in those fertilized with either ammonium or urea. Grain yield was significantly least (2.5 t ha−1), intermediate (2.6 t ha−1), and highest (2.9 t ha−1) in plots that received urea, ammonium, and nitrate respectively, with optimal values obtained at the intermediate rate (50 kg N ha−1). Treatment with nitrate enhanced grain protein more (11.6%) than urea or ammonium treatments. N uptake at tasseling was highest (42 kg N ha−1) under ammonium nitrogen, intermediate (22 kg N ha−1) in those receiving nitrate, and lowest (21 kg N ha−1) in urea treated plants. The highest (9.1 kg of total N uptake/kg N supplied) and lowest (4.7) N uptake efficiency (NuPE) were registered in plots treated with ammonium and urea respectively, with an intermediate value (8.0) obtained in plants receiving nitrate. Application at the rate of 25 kg N ha−1 resulted in the least NuPE with peak values registered at 50 kg (N ha−1). Agronomic N-use-efficiency (ANUE) differed among the rates with values ranging from 44 to 183 t ha−1 per N kg ha−1 applied in the order of 25 < 50 < 100 kg N/ ha. Data in this experiment indicate that for optimal growth, productivity, and protein nutritional benefits, maize production for consumption should be grown using nitrate N fertilizer at a 50 kg N / ha rate.