Prediction of grain protein in wheat and barley in a subtropical environment from available water and nitrogen in Vertisols at sowing

Summary. In many subtropical environments, cereal crops develop and mature largely on residual water in the soil. This research involves evaluation of the impact of plant available nitrogen (N) and water in soil at sowing on grain protein in wheat and barley in such a subtropical environment. Estimates of grain protein concentration of wheat (cv. Hartog) were made using plant available water and available N (soil nitrate-N and fertiliser N, where applied) at sowing using data obtained from an experiment conducted at Warra, Queensland, from 1987 to 1995. Treatments included: grass + legume leys of 4-year duration followed by continuous wheat with 0 or 50 kg N/ha.year applied as urea at sowing; 2-year rotation of lucerne and wheat; 2-year rotation of annual medics and wheat; 2-year rotation of chickpea and wheat, no-tillage wheat; and conventional tillage wheat. Fertiliser N as urea was applied to both no-tillage wheat and conventional tillage wheat at 0, 25 and 75 kg N/ha.year. The conventional tillage wheat also received N at 12.5 and 50 kg N/ha.year. Estimates of wheat grain yield required both rainfall during the fallow period or plant available water in the soil profile at sowing and rainfall from sowing to anthesis and, therefore, it could not be predicted precisely at sowing. Increasing plant available water (mm) in soil at sowing linearly reduced grain protein. In comparison, available N at sowing increased grain protein curvilinearly from 10.0% at 50 kg N/ha to 14.5% at 200 kg N/ha (0–120 cm depth). Variation in grain protein concentration was best accounted for by the available water : available N ratio at 0–90, 0–120 or 0–150 cm depths. The protein concentrations of wheat (cv. Hartog) grown in 1996 at Warra and Nindigully, and wheat (cv. Cunningham) grown from 1991 to 1995 at Billa Billa, and barley (cv. Tallon) grown in 1996 at Nindigully and Formartin, Queensland, were successfully predicted using the relationship between the available water : available N ratio and wheat grain protein concentration developed using data from Warra during 1987–95. Thus, available water should be matched by N supply at sowing to ensure the production of Prime Hard grade wheat and malting grade barley in the subtropical environment. As a ‘rule of thumb’, for 0–120 cm depth of soil sampling, each millilitre of available water matched with each kilogram of N per hectare of available N, at sowing, would produce about 13% protein wheat in this semi-arid region. It requires only 0.5 kg of N/ha for each millilitre of available water in 0–120 cm depth of soil to produce malting grade barley of about 10.5% protein concentration. Available water in soil at sowing can be approximated with rainfall during the fallow period, with rainfall (mm) : available N (kg/ha for 0–120 cm depth) ratios of 3.7 and 7.4 for respective 13 and 10.5% grain protein concentrations for both wheat and barley.