Within-field variability of wheat yield and economic implications for spatially variable nutrient management

Economic justification for varying fertiliser inputs to match crop yield potential of different areas or zones in fields is limited by lack of understanding of the relationship between the extent of within-field yield variation and economic gains from zone versus uniform management. We conducted a survey of yield monitor data from 199 fields on the northern sandplain of the wheatbelt in Western Australia in order to document the extent of sub-field yield variation and test if variation is related to attributes such as yield and field area. The economic significance to zone management of the yield variation found in the survey along with variation in size of management zones, costs and prices, and soil fertility status was then assessed using a simple nutrient response model.Considerable variation occurred in yield within fields. Standard deviation varied from 0.2 to 1.2 t/ha and the difference in yield between the highest and lowest yielding thirds of each field varied from 0.5 t/ha in the least to 3.3 t/ha in the most variable field. Both small and large (10–172 ha), and low and high (0.6–4.9 t/ha) yielding fields exhibited variation that was potentially worth managing from an economic standpoint. Model results showed that the larger the difference in potential yield between zones, the greater the economic benefit from zone management. While yield contrast within fields can be increased with more zones, the economic advantage of more zones was small for the cases studied here. The potential economic benefits (from <$5 to $44/ha) increased with higher grain and fertiliser prices and depended on levels of soil nutrients in the different zones. Capturing the full value of the economic benefits in practice requires an accurate indication of yield potential in the different zones at the time when the fertiliser decision is being made. Yield maps can be utilised by growers to give estimates of within-field variation in yield potential and hence potential economic gains from variable rate application of fertiliser.