Low-Altitude Aircraft Imagery for Assessment of Winter Cover-Crop Biomass and Nitrogen Content.

Cover crops are planted during the autumn to reduce nitrogen inputs to the surface water system.  The objective of this research is to determine a method to quickly assess the amount of nitrogen stored in the biomass before the cover crop is removed in the early spring.  Two digital cameras were mounted in the bottom of a Piper Cub aircraft: 1) true color with red, green and blue bands; and 2) color infrared with near-infrared, green and blue bands. Aircraft altitude and spatial resolution were determined based on user requirements of 50-mm pixels. Four on-farm field trials evaluating winter small grains were established on Maryland's Eastern Shore (wheat or barley, irrigated and non-irrigated) using a randomized block design with four fall fertilizer nitrogen levels (1, 15, 30, 45 kg N/ha) and six replicates per field. Crop above-ground biomass and nitrogen concentration were sampled at two locations within each 2.5-m wide by 50-m long plot, in April 2010, two weeks after a uniform spring fertility application (60 kg N/ha). There was little variation in nitrogen content among plots; most of the response to fall nitrogen observed in variation in aboveground biomass and leaf area index. Green normalized difference vegetation index [gNDVI = (NIR-G)/(NIR+G), where NIR is the near-infrared band and G is the green band] was significantly correlated with the variation of biomass. The color-infrared digital camera provided an effective means for monitoring crop growth; however, the ability of the true-color digital camera to detect variations in nitrogen content could not be determined.