Abstract
Wheat is conventionally sown in November in Yamaguchi. Early sowing (October) is known to decrease the number of spikes per area and the number of spikelets per spike. We examined the effect of nitrogen topdressing at the spike-development period on grain yield of the early sowing wheat. Three wheat cultivars were used: Iwainodaichi and Airakomugi showing medium degree of winter habit, and Akitakko showing strong degree of winter habit. These cultivars were sown early and conventionally in three growing seasons, 2003/2004, 2004/2005 and 2007/2008. Nitrogen was topdressed to each cultivar at their double ridge stage, terminal spikelet stage or flag leaf stage. The early sowing decreased the harvest index and decreased the grain yield by reducing spike number or grain number per spike in Airakomugi and Iwainodaichi. However, nitrogen topdressing increased the grain yields by increasing the higher harvest index or spike number. In Akitakko, the early sowing increased grain yield in some cases and never decreased it. In Iwainodaichi and Airakomugi, the early sowing advanced the spike development period about two months, to December or January, and increased the number of spikes having lighter grains or smaller number of grains. In Akitakko, the early sowing advanced spikelet development only ten days to March, and decreased the number of spikes with smaller number of grains.
