Evaluation of proximal sensing technologies for mapping bovine urine patches under grazing pastures

Abstract

Animal urine patches, particularly bovine, are the major source of gaseous and leaching losses of nitrogen (N) in livestock-grazed pastoral systems. One method of mitigating these losses is the application of N inhibitors to patches that slow down the N transformations, thus allowing more time for plant uptake. If inhibitors can be applied as spot-treatments, then the overall volume of chemical can be minimized without compromising inhibitor efficacy. To enable this, accurate and reliable methods of detecting patches within hours of deposition are required. In this study, we aimed to validate the output of the newly developed and modified instrument ‘Spikey-R’ for detecting and mapping urine patches via measurement of soil surface electrical conductivity with the aim of using these data for targeting in-situ spot treatment of detected patches. We compared measurements from Spikey-R against thermal imagery from a handheld camera taken during urine deposition, as well as imagery taken from a remotely piloted aircraft system (RPAS or ‘drone’) 2 weeks after deposition. Wetted areas, as a result of urine application, across all sites were 0.2–0.3 m2 (1 L urine), 0.3–0.5 m2 (2 L), and 0.3–0.8 m2 (3 L). Spikey-R data generally compared well with the reference map produced from the thermal imagery, with slightly larger mean patch areas that still fell within the bounds of uncertainty. Repeat Spikey-R runs over the same patches showed no significant difference in detected areas up to 48 h after application. The drone was successful in detecting most urine patches 14 days post-application and identified a much larger pasture response (average > 150% larger again) than the initial wetted area with sizes ranging from 0.4 to 0.7 m2 (1 L), to 0.5 to 1.1 m2 (2 L), to 0.5 to 1.5 m2 (3 L). Spikey-R proved to be an effective tool in a research setting; advantages and disadvantages of each technology are discussed in detail.