Late Season Tissue Tests for Critical Grain Protein Content in Durum, Maricopa, 1998

Proper nutrient management is necessary for successful production of durum wheat in the desert. If grain protein content is less than 13 %, significant economic losses to growers can result. Late season nitrogen (N) fertilization can resolve this problem, but tissue test guidelines have not yet been established. The objectives of this study were to: (i) correlate NO₃-N in dried stem tissue with sap NO₃-N, (ii) determine the minimum NO₃-N concentration in lower stem tissue at heading associated with the critical grain protein content, and (iii) determine whether flag leaf head, or whole plant total N at heading can be used as indicators of N status. In November 1997 two varieties of durum wheat, Mohawk and Kronos, were planted at the Maricopa Agricultural Center. Five N rates (0, 100, 200, 300, and 400 lbs/A) were applied in four split applications. Each treatment was replicated three times in a randomized complete block design. Samples were collected from the lower stem, flag leaf head, and whole plant from each plot at heading and analyzed for total N. Grain yields ranged from 1663 to 6916 lbs/A for Mohawk and 1529 to 7060 lbs/A for Kronos. Maximum yields were achieved at 200 lbs N/A for both varieties. Grain protein content averaged 8.6% to 13.4% (Mohawk) and 9.1% to 13.8% (Kronos). Correlation coefficients between stem NO₃-N and sap NO₃-N were 0.96 for Mohawk and 0.97 for Kronos. Lower stem sap critical NO₃-N concentration in Kronos is 1100 ppm NO3 N and 1700 ppm NO₃-N for Mohawk at heading for a grain protein content of 13 %. Lower dried stem tissue critical NO₃-N concentration in Kronos is 5500 ppm NO₃-N and 7500 ppm NO₃-N for Mohawk for a grain protein content of 13 %. Nitrogen concentration in flag leaves, heads, and whole plants were highly correlated with N rate. Therefore, N concentration in these tissues could potentially be used as indicators of late-season N status