Measuring excreta patch distribution in grazed pasture through low-cost image analysis


Correspondence to: S.J. Dennis, AgResearch Limited, Lincoln Research Centre, Private Bag 4749, Christchurch 8140, New Zealand.



Nutrient losses from grazed pasture are an important non-point source of water pollution. The distribution of animal urine patches on pasture is an important factor in determining nitrate losses and influencing pasture growth, nutritive value and pasture acceptability to livestock, as a high amount of nitrogen (N) is deposited onto a small area of soil under a urine patch. Urine distribution may be recorded during or post-grazing. Measurements during grazing have been automated, but post-grazing measurement currently relies on manual observations that are time consuming and cannot be subsequently verified. To automate post-grazing measurements, aerial photographs were taken of grazed pasture approximately 14 d post-grazing using a standard digital camera. Pasture response areas were successfully identified by analysing the hue of the images using readily available software, yielding comparable results to manual counts. The majority of dung patches did not produce observable pasture responses, with only 14% of the visible response areas being associated with dung, so although this method cannot distinguish between urine and dung response areas, it is primarily influenced by urine. Provided photographs are taken in full sunlight with a high-quality camera, excreta patch numbers, areas and spatial distribution can be measured with a high degree of precision. Furthermore, the method is relatively inexpensive and applicable to a wide range of situations. A permanent photographic record of the pasture is also established, which allows verification of the analysis in future.