Effects of sowing rate, irrigation, and nitrogen on the components of yield of spring-sown semidwarf and standard New Zealand wheats

'Karamu', a Mexican semidwarf wheat, outyielded 'Aotea' and 'Arawa' by an average of 20% when spring sown because of better grain set per spikelet and by the formation of more spikelets per ear. Irrigation raised grain yields in all cultivars by increasing tiller survival. Sowing at twice conventional rates depressed grain yields in all cultivars by reducing grain set. Nitrogen fertilisation lowered grain yields of all cultivars by 7% in unirrigated plots by decreasing ear populations, apparently by intensifying water stress. Nitrogen applied at the same rate to irrigated plots increased ear populations and grain yields by 10%. Responses support the hypothesis that poor grain set in wheat in the field is caused by insufficient assimilate supply to the developing ear