Response surfaces of growth and yield of wheat

In the 1976-77 and 1977-78 seasons four field experiments using a three factor central composite second order design in incomplete blocks were conducted to quantify the responses of autumn-sown wheats established at five seeding rates (170-580 seeds/m²) and fertilized at five rates of nitrogen (0-180 kg/ha) according to design-determined values of five early: late nitrogen ratios. The experiments compared the semi-dwarf Karamu with the standard Kopara, and irrigated with non-irrigated Kopara. Reproductive development was earlier in Karamu than in Kopara, but the responses in growth parameters, grain yield and plant nitrogen content to the three factors were similar except that Karamu was more responsive in terms of grain yield to early nitrogen and Kopara to seeding rate. Karamu appeared to be inferior in grain set per spike, so that overall there was no cultivar difference in the yield of grain and grain nitrogen. It would appear that grain yield of Karamu could be source-limited while that of Kopara sink-limited, and that Karamu was unable to mobilize as much nitrogen in vegetative structures for synthesis into grain protein as Kopara. Irrigation of Kopara during grain filling did not influence grain and grain nitrogen yields despite an increase in mean grain weight. Although irrigation at this time appeared to promote greater mobilization of nitrogen from the vegetative organs to the developing grains, it could cause a dilution of the nitrogen level in the grains. Higher than normal seeding rate tended to increase tiller and spike populations at harvest and reduce the other yield components, so that there was little or no increase in grain yield. Optimum yields were achieved at 375 seeds/m². In spite of differences in some growth parameters, seeding rate had generally no influence on the nitrogen content of the various parts of the plant after anthesis. The effects of nitrogen fertilizer in increasing vegetative growth were reflected in the positive responses in grain yield and enhanced nitrogen content in various plant parts including the grains. Early nitrogen applications tended to promote growth and yield, and later applications raised nitrogen concentration in the grains. The results are discussed in relation to the status of water and nitrogen supply in the soils, to changes in general weather conditions between the two seasons, and to possible causes of the variation in baking quality of wheat.