Tailoring Nitrogen Recommendations: Varietal Response in Hard Red Winter Wheat to Nitrogen Fertilizer Application Timing.

Idaho has a unique climate that allows for the production of both spring and winter wheat as well as multiple market classes including hard red winter wheat.  With irrigation in southern Idaho and a mild climate with high rainfall (18-24 in) in northern Idaho, relatively high winter wheat yields are obtained with a state average of about 80 bu/A.  While achieving high yields, growers in Idaho often struggle to achieve adequate protein levels in hard red winter wheat.  There also have been genetic improvements in yield and varieties may respond differently to nitrogen fertilizer rates.  Trials were conducted in the 2013-2014 and the 2014-2015 growing seasons at Aberdeen (irrigated) and Moscow (dryland, rainfed).  At each location, split applications of nitrogen were applied with either 60% fall/40% spring or 40% fall/60% spring.  In addition, nitrogen was applied at anthesis in an attempt to boost grain protein.  At each location, six hard red winter varieties were selected with Norwest 553, LCS Colonia, LCS Azimut and WB-Arrowhead common among both locations.  In 2014 and 2015 the overall average yields were 56 and 86 bu/A for Moscow, and 177 and 118 bu/A for Aberdeen.  Yields in Moscow were below normal due to drought conditions, which were particularly severe during the spring of 2014.  Environmental stress also led to reduced yields at Aberdeen in 2015.  A combined analysis of data from Moscow and Aberdeen demonstrated that neither split application significantly impacted yield.  This suggests that lower rates of nitrogen applied in the fall could result in the same yield while reducing the risk for nitrogen loss due to leaching or volatilization.  Topdress application with 40, 60 or 80 lb nitrogen at anthesis failed to significantly increase grain protein.  Application of more than 40 lb of nitrogen under rainfed conditions resulted in phytotoxicity.