Evaluation of the Accuracy of a Wheat Stem Nitrate Test in Predicting Nitrogen Requirements of Irrigated Durum eat

The procedure currently recommended by the University of Arizona for predicting the nitrogen (N) requirements of durum wheat has proven to be quite accurate at sites where grain yields exceeded 5,400 lbs/acre. However, the method slightly overestimated N needs when the yield possibility was below that level. Additional infonnation on the relationships between N rates, stem NO rN levels and grain yields are needed for the wide range of agronomic conditions found in Arizona. Three N fertility trials were conducted at the Maricopa Agricultural Center to: 1) document the accuracy of the currently recommended soil + stem NO3- Ntesting procedure in predicting the Nneeds ofdurum wheat on soils of varying residual N content and grain yield potentials; and 2) to evaluate the use of the current stem testing procedure on two dumm varieties 'Aldura ' and 'Westbred-881: The University of Arizona procedure was found to accurately predict the minimum amount of N required for optimum production of dumm wheat on two sites where yield potentials were 5,400 and 4300 lbs. grain /A, but it slightly overpredicted N rates on two sites with maximum yield levels of 5,400 lbs /a. 'Aldura ' consistently out yielded 'Westbred-881 ' by about 12 %but 'Aldura ' also averaged 0.78 %lower in grain protein content. Little statistical or practical differences were observed in the quantities of NOS- Ncontained in the stem tissue of these two varieties, which should simplify the interpretation of stem NO 3-N values for various wheat cultivars. The currently recommended procedure for predicting optimum N rates in durum wheat production has proven to be accurate when yield levels exceed 5,400 lbs. grain /A. A slight modification of the procedure may be needed to more closely predict N requirements on lower yielding sites