Nicole P. Anderson, Oregon State University, McMinnville, OR, Michael D. Flowers, Oregon State University, Albany, OR and Sarah K Del Moro, Oregon State University, Hermiston, OR
Winter wheat fields in western Oregon’s high rainfall region receive surface-applied nitrogen (N) fertilizers, mostly in the form of urea (46-0-0), in early spring when large amounts of precipitation are common. Nitrogen loss from ammonia (NH3) volatilization has been measured in surrounding regions where annual precipitation is low, but little work has been completed in high rainfall environments. Several studies have indicated that treating urea with the urease inhibitor (N-(n-butyl) thiophosphoric triamide (NBPT), trade name Agrotain®, minimizes N loss to volatilization. The objective of this two year study was to determine differences in grain yield, protein and test weight of soft white and hard red winter wheat when urea with and without Agrotain was broadcast on the soil surface at two study sites near Hillsboro (2012) and Corvallis (2012, 2013) in western Oregon. Ammonia volatilization was measured at each site with a modified passive flux method which consists of a rotating mast with glass tubes coated with oxalic acid at five heights (0.45, 0.75, 1.50, 2.25 m). At all site locations, one rotating mast was placed in wheat plots where either 157 or 190 kg ha-1 of urea was applied at Zadoks growth stage 30 and one mast was placed 91 m from the study locations where no fertilizer was applied. The average precipitation during the study period at both sites was 366 mm and 45 mm in 2012 and 2013, respectively. Average N loss 24 days after N application was 6.6 % and 30.5% of N applied in 2012 and 2013, respectively. In both years higher N rates consistently increased grain yield, protein and test weight for both wheat market classes; however, use of Agrotain treated urea only increased grain yield in 2013 for soft white wheat. The measured difference in grain yield when Agrotain® treatments were used is consistent with the amount of NH3 volatilization measured. Precipitation amounts immediately following N fertilizer application appear to heavily influence how much N loss from NH3 volatilization occurs in western Oregon.