The effect of time of sowing on the grain yield of irrigated wheat in the Namoi Valley, New South Wales

The effect of time of sowing on the yield of 15 wheat cultivars grown under irrigation was examined at Narrabri, N.S.W. Sowing dates, which ranged from mid-April to mid-August, encompassed the period of sowing normally found with commercial crops. The length of the pre-anthesis period was affected by both temperature and photoperiod. There was evidence of a vernalization requirement for some of the winter and midseason cultivars, but, overall, photoperiod was the more important environmental factor determining pre-anthesis development. Each day's delay in sowing caused a delay of between 0.48 and 0.75 days in anthesis; the delays observed for spring wheats were generally greater than those reported for dryland wheat in eastern Australia. Winter cultivars generally did not show an optimum sowing or anthesis date. For spring cultivars, the optimum time of sowing was early June (range of about 3 weeks), while the optimum anthesis date was the last week of September (range of 1 week). Grain yields of spring cultivars were reduced by 6 and 16% per week's delay in sowing and anthesis respectively. Despite a non-limiting water supply, yields at late plantings were low, which was largely the result of hastened crop development and high temperatures during grain-filling. The environmental factors which determined the optimum sowing and anthesis dates were the incidence of early spring frosts and high spring temperatures. If frosts were not a factor at Narrabri, the optimum time of flowering would be 6-8 weeks earlier than the present optimum.