Using Remote Sensing to Assess the In-Season Nitrogen Status of Perennial Ryegrass for Seed Production

Currently, perennial ryegrass (Lolium perenne L.) growers in western Oregon rely on yield goal estimates and experience to formulate N rates. This practice may result in insufficient or excessive N fertilizer rates in any given year. Plant tissue testing to determine the in-season plant N status may improve N management. However, tissue testing can be costly, time consuming, and difficult for growers to adopt. A possible solution to these problems is remote sensing. The objectives of this study were to determine (i) the critical whole-plant N concentration and N uptake values for perennial ryegrass, (ii) and if remote sensing in the form of aerial images could be used to assess the in-season plant N status of perennial ryegrass. Research was conducted at four sites in 2007, 2008, and 2009. Analysis found a critical whole-plant N concentration value of 28.0 g kg(-1) and a critical N uptake value of 157.9 kg N ha(-1). A significant and strong linear plateau relationship was found between relative near infrared (NIR) and both whole-plant N concentration (r(2) = 0.46) across site-years. Additionally, a significant and strong linear plateau relationship was found between the normalized difference vegetation index (NDVI) and N uptake (r(2) = 0.61) across site-years. Critical values obtained from these linear plateau relationships were similar to those found through tissue testing, indicating that relative NIR or NDVI may be used as an indirect measurement of whole-plant N concentration or N uptake in perennial ryegrass grown for seed.