Water use, growth and yield of wheat in a subtropical environment

Wheat crops in southern Queensland grown on two different clay soils were studied in terms of growth, development, water economy, and uptake of nitrogen and phosphorus in a season when growing season rainfall was only 50 mm. No significant grain yield response to either nitrogen or phosphorus was detected, although growth response in the vegetative phase was apparent. Mean grain yields on the two soils were 254 and 277 g m-2. These are underestimates of yield potential because of losses due to a mouse plague. Mean yield inside metal mouse exclosures was 303 g m-2. On the higher-yielding site the water use determined from augered soil samples was 192 mm, comprising 50 mm of rain during the 139 days of crop growth and 142 mm of soil water conserved during the preceding summer. The pattern of water extraction was sequential removal of the stored water, starting from the soil surface and extending to a depth below 90 cm. Soil water potential after flowering reached well below - 15 bars. The water use efficiency for grain production was 1.58 g m-2 mm-1 of evapotranspiration, which is higher than most other reports for wheat crops found in the literature. A partial explanation for this high value is that most of the soil water (est. 86%) was transpired by the crop with only a minor proportion (14 %) lost by bare soil evaporation. In addition, the crop appeared to become progressively more adapted to water stress from early in the life cycle, and this stress resulted in slow extraction of subsoil water held at low potentials and therefore in conservation of soil water until the grain-filling phase. The results are discussed by comparing them with those of a crop grown mostly on current rainfall.