RESPONSES OF WHEAT CULTIVARS TO TIME OF SOWING IN THE SOUTHERN WHEAT-BELT OF WESTERN-AUSTRALIA

Experiments were conducted at 4 locations on an east-west transect in the 300-500 mm average annual rainfall zone in the southern wheatbelt of Western Australia, to determine whether promising crossbreds differ from existing wheat cultivars in their response to time of sowing. Nine cultivars and 2 crossbreds were examined at 3 sowing dates each year, ranging from late April to early July 1989, 1990, and 1991. Grain yield, grain quality (protein, hectolitre weight, grain weight, small grain sievings), crop development, and soil and weather variables were measured. The average decrease in grain yield with delay in sowing after early May was 20 kg/ha.day. All existing wheat cultivars and new crossbreds examined produced their highest yields when sown in early May. Yield decline for the crossbreds and cultivars was almost linear after early May; however, the rate of decline could not be entirely predicted from a knowledge of the crossbreds' maturities. Spear, the cultivar with longest maturity, was one of the highest yielding cultivars when sown in early May or June. The yields of the 2 shortest season cultivars, Kulin and Gutha, were only comparable to the yield of Spear when sown in June. Therefore, in the southern wheatbelt of Western Australia, we do not advise retaining a number of cultivars to suit a range of sowing times. The optimum flowering periods over the 3 seasons were 17 September-7 October in the medium rainfall zone of the southern wheatbelt and 3 September-23 September in the low rainfall area at the most easterly location, reflecting the importance of terminal drought. There is still a risk of frost damage to wheat crops in about 1 year in 3 for the periods estimated. Therefore some risk of yield loss from frost damage must be accepted if yields are to be maximised.