Crop production in the high-rainfall zone of Western Australia (>450 mm average annual rainfall) is an increasing proportion of the state’s total farming system since the 1990s, when the profitability of animal production based on improved pastures was threatened. However, the yields of the dominant crops barley, canola and oats have often been insufficient to maintain whole-farm productivity in the changed system.

The aim of this study was to test the diagnostic approach to agronomic research as a means of increasing crop yields. Experiments were conducted at two farm sites over 5 years with treatments applied according to an initial diagnosis of the factors that may have been limiting production. The diagnosis of limiting factors was based on soil physical and chemical tests, plant tissue analyses and the observations of the farmers. The diagnostic tests were assessed against agreed standards. The highest yields in each year were compared with an estimate of the rainfall-limited potential yields. In both experiments, more than one factor was considered likely to be limiting crop and pasture production; therefore, factorial combinations of treatments were used, including deep-placed lime, deep-placed potassium and claying at one site, and deep ripping, raised beds and gypsum at the other. Split doses of nitrogen were applied to half of the plots after waterlogging events in some years.

The yield responses to the treatments changed each year but the highest yields were close to the calculated potential yield after taking account of estimated losses of water. Interactions between the factors were not often significant. That is, the responses were additive and independent, so they can be applied sequentially. No single factor could be identified as the most limiting at either site over the 5 years. However, the results suggested a hierarchy of measures that could be taken according to the least cost or the most profit principle, or according to farmer preference and convenience.