FACTORS INFLUENCING GRAIN-N CONCENTRATION OF HARD RED SPRING WHEAT INTHE SEMIARID PRAIRIE

Prairie producers are now being rewarded with significant premiums for producing wheat (Triticum aestivum L.) of high protein concentration. We analyzed data from two 12-yr experiments conducted on a medium-tex tured Orthic Brown Chernozem at Swift Current, Saskatchewan, to determ ine and quantify factors influencing grain N concentration of hard red spring wheat grown on stubble land. Results of one of the 12-yr studi es, a snow management x fertilizer N, zero-tillage experiment, showed that under hot, dry conditions, grain N concentration was very high an d increased with moderate rates of fertilizer N (FN), then levelled of f at higher rates of N. Under cool, wet conditions, grain N first decr eased (due to N dilution by yield) then increased with further additio n of FN. Under warm intermediate moisture conditions, grain N concentr ation increased at moderate rates in response to FN. Data for the two 12-yr experiments were pooled and multiple regression, with backward e limination, and stepwise selection used to develop the relationship: G rain N (g kg(-1)) = -7.63 + 0.05 WU - 0.000094 WU2 + 0.30 SN -0.0022 S N2 - (0.0010 SN x WU) + (0.0017 FN x SN) + 0.0189 DD (R(2) = 0.64, P = 0.001, n = 262) where WU = water use (mm), SN = soil test N (kg ha(-1 )), FN = (kg ha(-1)), and DD = degree-days > 5 degrees C (degrees C-da ys) from 1 May to 31 August. WU was available spring soil water in 0- to 1.2-m depth plus 1 May to 31 July precipitation, and SN was NO3-N i n the 0- to 0.6-m depth, measured in the fall. We attempted to validat e this model using data from a long-term crop rotation and a fertilize r trial experiment in the Brown soil zone, a tillage x rotation experi ment in the Dark Brown soil zone in Saskatchewan, and an irrigation x N fertilizer experiment in the Brown soil zone of southern Alberta. Va lidation met with only modest success (R(2) up to 0.70, P = 0.001). Ge nerally, estimated grain N concentrations were lower than the measured values. Water use (negatively related) and temperature (DD) (positive ly related) were the most important factors influencing grain N, while FN and SN (positively related) were much less important. Because of t he complexity of response in grain N to the aforementioned factors, an d since farmers cannot predict weather conditions, fertilizer manageme nt to achieve high protein remains a challenge under dryland condition s.