NDVI 'Depression' In Pastures Following Grazing

Abstract

Pasture biomass estimation from normalized difference vegetation index (NDVI) using ground, air or space borne sensors is becoming more widely used in precision agriculture. Proximal active optical sensors (AOS) have the potential to eliminate the confounding effects of path radiance and target illumination conditions typically encountered using passive sensors. Any algorithm that infers the green fraction of pasture from NDVI must factor in plant morphology and live/dead plant ratio, irrespective of the senor used. Moreover, livestock grazing affects the morphology of pastures so the veracity of instrument calibration procedures applied under 'protected plot' conditions is questionable if the sensor is subsequently deploy as a 'calibrated sensor' into grazed fields. In this research we have simulated pasture grazing on establish plots of Tall fescue ('Festuca arundinacea') in a heavy clay (vertosol) soil and examined the effect of such grazing on the temporal NDVI values as derived using a Crop CircleTM sensor. Five plots with different soil moisture condition were maintained in the study period. Time domain reflectometer (TDR) was used to monitor volumetric soil moisture content (%) and NDVI measurements were taken on a daily basis. Following a grazing event (facilitated by uniformly mowing the grass to a height of 6 cm), biomass samples were collected on 3rd, 4th and 5th day along with coincident measures of the NDVI. For those plots with low soil moisture level (< ~37% of the full profile), the NDVI progressively decreased up to 2 or 3 days following the 'grazing' event, despite the plot biomass increasing due to regrowth. The NDVI values did not 'recover' until approximately 4 days after the 'grazing' event. However, for those plots of moderate to high soil moisture (>~37%) the NDVI-time curves monotonically increased with biomass re-growth immediately following 'grazing'. This has important ramifications for those intending to use NDVI as the basis for pasture assessment, particularly in situations involving short-term grazing rotation.