Assessing specific agronomic responses of wheat cultivars in a winter rainfall environment

When new wheat cultivars are released for commercial production it is desirable to assist farmers to maximise the yield advantage by providing information about their responses to agronomic practices such as seeding rate and nitrogen (N) fertiliser. Over 3 years in 22 field experiments in the Mediterranean-type environment of Western Australia the response to seed rate and applied N fertiliser of current and recently released wheat cultivars was measured in factorial experiments under rain-fed conditions. A cross-site analysis showed that the environment × cultivar (location and year) or management (seed rate and N rate) interactions were relatively minor, explaining only 5% or less of the yield variation, in contrast to 89% accounted for by the environment. The analyses of individual experiments revealed that cultivars interacted more often with seeding rate (12 sites) than with applied N fertiliser rates (4 sites). Further, despite a frequent occurrence, the cultivar × seed rate interaction had only a marginal practical significance because the cultivar rankings at a site varied with season and the differences in optimum plant population were greater between sites and seasons than between cultivars at a site. The number of sites with positive and significant cultivar × N rate interaction was insufficient to generalise about the validity of the responses. The lack of any large differences between cultivars for their response to either seed rate or N rate implies the presence of a high inherent ability for compensation among yield components, thereby enabling the cultivars to exhibit an apparently high similarity for response to input levels. As such, it was not possible to generalise across environments in making clear suggestions for farmers to follow in respect of managing new cultivars differently from each other. It was concluded that despite the apparent desirability of providing information about differences between new and existing cultivars in their responses to seed and N rates at the time of release, they are more likely to be found where the differences between the cultivars are large, the testing sites are chosen so as to reduce the yield variance, and the yield level achieved in the experiments is above 2 t/ha.