Response surfaces of “Kopara” wheat for seeding rate, and levels and times of application of nitrogen fertiliser

A three factor central composite second order design in incomplete blocks was used to quantify the yield responses of “Kopara” wheat established at five seeding rates (170–580 seeds/m 2 ) and fertilised at five rates of N at the onset of tillering (0–124 kg/ha) and again during spikelet growth (0–38 kg/ha). Seeding rate increments had little effect on grain yield but optimum yields were achieved at 375 seeds/m 2 . Increased spike populations obtained at higher seeding rates were offset by diminished grain set and the production of fewer spikelets per spike. Substantial increases in grain yield from N fertilisations at tillering were attributed to higher spike populations at harvest, with smaller increases attributed to the number of spikelets per spike and a small improvement in grain set. Application of N fertiliser during spikelet growth phase did not increase grain yield, for it was too late to have much influence on spike populations. Applications of N at any time increased grain N content. Kopara appeared to have a greater level of intertiller competition than in the semidwarf “Karamu”, so that the semidwarf could support a greater spike population through to maturity. Exposure to low temperatures and/or short days by sowing in winter appeared to eliminate the yield advantages characteristic of spring‐sown semidwarf wheats — increased numbers of spikelets per spike and better grain set. Seed set of both cultivars was limited by assimilate supply except at low tiller populations when grain set in the semidwarf appeared to be limited more directly by N nutrition. N content of Karamu grain was more influenced by agronomic treatment.