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Slow adoption by farmers in Western Australia of new wheat cultivars led us to propose that their grain yield advantage over the old cultivars may not be evident under the traditional agronomic practices used in the State. The experiments, including both tall (rht) and semi-dwarf (Rht) commercial cultivars, were sown from early to mid-May, up to early July at five locations each year in the central wheatbelt of southwestern Australia in 1986, 1987 and 1988. Semi-dwarf outyielded tall cultivars more when sown in May (0.65 t ha-l) than in early June (0.35 t ha-1) and did not outyield them at later sowings. It is postulated that the yield advantage of the semidwarfs is dependent on early sowing. The yield advantage of the semi-dwarfs was related to greater ear and kernel numbers. The relative reduction in kernel size with later sowing was greater in the semi-dwarfs than in the tall cultivars. The semi-dwarfs had larger harvest indices at the May sowings but the talk equalled or exceeded them at the sowings in June. We concluded that the semi-dwarfs used in our experiments were more sensitive to the stresses associated with later sowing in this environment than the tall cultivars. The optimum flowering period for the study area over the three seasons was 7 to 29 September. However, there was considerable variation from season to season, and we concluded that an adequate definition of the optimum flowering period should be based on time-of-sowing experiments over a range of seasons. |
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