Crop sequences in Australia’s northern grain zone are less agronomically efficient than implied by the sum of their parts

• Income from 36% of individual crops exceeded 80% of their production frontier.
• A lower proportion of crop sequences exceeded 80% of their production frontier.
• Individual crops averaged 65% of their water and nitrogen-limited potential yields.
• In-crop weeds, pests and diseases reduced agronomic efficiency of individual crops.
• Crop intensity, crop configuration and fallow weeds may reduce sequence efficiency.

The question addressed in this paper is whether the agronomic efficiency of cropping sequences can be improved in the northern grain zone of Australia. Analysis of whole systems over multiple seasons is particularly important in cropping regions such as Australia’s northern grain zone, where farmers have the option of growing a variety of winter and summer crops and where fallowing is required to store water to safeguard yields of following crops. The study is based on a detailed longitudinal survey of the inputs and yields of 94 farmers’ fields over 7 seasons. The Agricultural Production Systems Simulator (APSIM) was used to benchmark the 94 cropping sequences and to determine if the productivity of these cropping sequences can be improved through changed management, particularly of nitrogen nutrition and timely sowing of individual crops. Comparison of surveyed and simulated yields were then used to determine the yield gap that can be explained by factors unaccounted for in APSIM, i.e. biotic stresses such as weeds, pests and diseases or inadequate operational management. Using a generalised N response curve, data from the simulation analysis were plotted relative to a normalised input–output relationship to determine their position relative to an efficiency frontier representing the simulated production function. This analysis was conducted separately for both individual crops and for whole sequences in individual fields. The analysis of 193 individual crops showed that the average yield of 1.90 t/ha was 65% of the simulated water and nitrogen-limited yield potential of 2.92 t/ha. Investigation of nitrogen unlimited and early sowing management options revealed that there was relatively little yield advantage (0.15 t/ha) to be gained from these management improvements. The remaining gap between observed yields and what the simulations suggest may be attributed to factors such as weeds, pests and diseases, sub-optimal operations or extreme weather events. Three production criteria were used to compare cropping sequences: (1) dollar income, (2) metabolisable energy yield, and (3) crude protein yields. While the income from 36% of the individual crops in the study was found to be more than 80% of their production frontier values, only 29% of whole cropping sequences achieved this benchmark. Similar results were achieved when crops and crop sequences were evaluated in terms of their metabolisable energy and crude protein yields. It is concluded that in order to increase the agronomic efficiency of cropping in Australia’s northern grain zone, attention should be focussed on the intensity and configuration of cropping sequences and on the management of fallows in addition to the management of individual crops.