Simulation analysis of variation among seasons in winter wheat yields in Northern Italy.

Winter wheat growth and yield was observed in two sets of field experiments performed at the Experimental Farm of Padova University in Legnaro, over seven years beginning in 1981. A spring wheat model in which leaf area development was calculated as a function of temperature and biomass accumulation was utilized. The final number of leaves in each season was calculated based on a previous winter wheat model and the biomass accumulation was decreased when average daily temperature was below 10 \ub0C. In addition to the two modifications described above, the soil/crop nitrogen submodels were modified so that denitrification was a function of temperature and plant nitrogen uptake rate was sensitive to variations in daily temperature. Nitrogen leached below 60 cm was assumed to be no longer available to the winter wheat crop. The winter wheat model with the soil/crop nitrogen budged resulted in good agreement between the simulated and observed yields (less than 14 % difference existed in 12 of 14 environments). A strong negative correlation (r = -0.74) existed between the simulated yields and the simulated amount of nitrogen leached out of the root zone. Under the conditions of nothern Italy, retention of root zone nitrogen appears to be particularly important in achieving high winter wheat yields