Nitrogen is considered as one of the most important inputs needed for increasing productivity of field crops. Split application of nitrogen is one of the ways to improve nitrogen use efficiency and reduce nitrogen losses particularly in Pakistan where soil temperature is comparatively high. Application of nitrogen at different growth stages and its optimum level is an important aspect of research required for enhancing crop growth and yield in Pakistan. Balanced amounts of nitrogen application at proper time according to the need of the crop decreases losses of N, increases yield and improves the quality of both grain and fodder resulting in greater net profit. There is also a need to understand how levels of N application affect the physiology and partitioning of assimilates in maize, Zea mays L., is an attempt to explore further avenues for proper nutrient management for improving yield on sustainable basis. Thus an experiment entitled "Physiology, Partitioning of Assimilates and Yield of Maize as Affected by Plant Density, Rate and Timing of Nitrogen" was designed and planted at the Agriculture Research Farm of the NWFP Agricultural University, Peshawar in 2002 and 2003. The 2 x 3 x 6 factorial experiment was conducted in RCB design with split-plot arrangement using four replicates. Factorial experimental treatments were two plant densities (60,000 and 100,000 plants ha-1) and three nitrogen rates (60, 120 and 180 kg N ha-1) applied to main plots, while six split application for N in different proportions were applied to subplots at different growth stages of maize variety, Azam, in two equal, three equal, three unequal, four equal, five equal and five unequal splits at sowing and with 1st, 2nd, 3rd & 4th irrigation at two weeks intervals. The crop was irrigated at two weeks interval i.e. 14, 28, 42, and 56 days after emergence. A uniform basal dose of 60 kg ha-1 P205 and 50 kg ha-1 K2 0 was applied and mixed with the soil during seedbed preparation.
Maize took more time to tasseling, silking and physiological maturity in 2002 than in 2003. Increase in plant density, N rates and the number of split application for N delayed tasseling, silking and maturity in maize. Number of leaves plant, mean single leaf area,. leaf area plant, SLA, LAI, LAR, AG R, CGR, ear and plant height were enhanced while increasing N rate and number of splits for N application. Leaf area was increased at the rate of 5.77 cm2 with one kg increase in the rate of N applied to maize. The highest leaf area of 515.48 cm2 was recorded in those plots to which N was applied in five splits with greater amount at later stages, while the minimum leaf area of 452.85 cm2 was recorded in plots which received N in two equal splits at sowing and first irrigation as recommended for maize. LAI of maize in the year 2002 was higher than in the year 2003. Increase in plant density and the number of splits application for N at later stages of maize increased LAI. In thick population of maize CGR at silking increased at the rate of 0.0175 g m-2 day-1, while in thin population CGR increased at the rate of 0.0093 g m2 day-1 with one kg increase in the rate of N. Light interception proportion was higher in the year 2002 than in 2003. Light interception proportion increased with increase in plant density. At maturity, maximum CGR of 19.40 g m-2 day-2 was recorded in plots to which N was applied in five unequal splits, while the minimum CGR of 16.87 g m-2 day-1 was recorded for plots to which N was applied in two equal splits. Increase in N rates and split applications for N increased AGR during seed filling duration. Plant and ear height, increased with increase in plant density, N rates and split applications for N. The higher density plots produced 10 cm taller plants than the lower density plots. Leaf weight plant-1, stem weight plant-1, ear weight plant-1, tassel weight plant-1, sheath weight plant-1 and total weight plant-1 increased with increase in N rate and number of split applications for N, and decreased with increase in plant density. Increase in N rate and number of split applications for N enhanced yield and yield components. Maximum of 88 ears per 100 plants were obtained in the plots to which the highest rate N was applied, while the lowest of 84 ears per 100 plants was recorded in the plots which received the lowest rate of N. The lower density plots gave sib1Jlificantly higher number of ears per 100 plants (94.87) as compared to 77.81 ears per 100 plants in plots of higher density. Heaviest seeds weighing 235.35 g/1000 seeds were observed in plots to which the highest rate of N had been applied, while the lowest weight of 222.58 g/1000 seeds was obtained in the plots to which the lowest rate of N was applied. Thousand seed weight increased with increase in N rates and number of split applications for N.
Grain yield increased with increase in plant density, N rates and number of split application for N. The highest grain yield of 5170 kg ha-1 was produced by the plots to which the highest rate of N was applied. Increasing plant density from 60,000 to 100,000 plants ha-1 increased grain yield from 4079 to 4603 kg ha-1, respectively. The highest grain yield of 4769 kg ha-1 was obtained from plots to which N was applied at five stages in equal amounts, while the lowest yield of 3874 kg ha-I was recorded in the plots which received N in two equal splits. Both stover and biological yields were increased with increase in plant density, rate and split application for N. Maize produced highest stover and biological yields in 2002 than ill 2003. Stover yield increase from a minimum of 4141 kg ha-1 at the lowest rate of N to a maximum 4909 kg ha-I at the highest rate of N. Biological yield increased at the rate of 16.3 kg. ha-1 with one kg increase in the rate of N applied to maize. Highest biological yield of 13895 kg ha-1 was recorded for the plots to which N was applied in unequal splits at five stages with greater quantity applied at later stage, while the minimum biological yield of 10938 kg ha-1 was recorded for the plots to which N was applied in two equal splits. Maximum harvest index of 39.36 % was noted in plots to which the highest rate of N was applied, while the minimum harvest index of 29.59 % was recorded in plots which received the lowest rate of N. Harvest index showed negative relationship with plant density. Protein concentration in maize grains increased with increase in N rates and split application for N. Proteins concentration and NUE increased with increase in N rates and number of split application for N. Protein concentration increased from a minimum of 8.15% at the lowest rate of N to a maximum of 9.25% at the highest rate of N. Nitrogen use efficiency showed positive relationship with plant density and negative relationship with N rates. Maximum NUE of 45.5 kg grain per kg N was noted when N was applied in five splits with the higher proportion of N applied in the later stages of maize, while the lowest NUE of 35.45 kg grain per kg N was obtained when N was applied in two equal splits. Higher rate of 180 kg N ha-1, about 50 % more than recommended rate for maize and split application for N increase grain yield and protein concentration in seeds and are recommended for in the irrigated tracts and high rain fed area of NWFP and Pakistan.