The ability to accurately estimate biomass of pastures is important to enable farmers preparing reliable feed budgets for their livestock. Available tools such as the rising plate meter (RPM) are time-consuming and labour intensive. Proximal sensing technology with a chance for analysing larger areas could provide regular estimates of pasture biomass with minimal labour requirements, eventually being automated. A method for contactless measuring of pasture height could provide an approach comparable to the plate meter concept. The objective of this study was to assess the suitability of stereophotogrammetry as a tool for non-destructive assessment of pasture biomass.

In two experiments, conducted 2016 (Exp. 2) and 2017 (Exp. 2) at the Lincoln University Dairy farm in Canterbury, New Zealand, the biomass of ryegrass based pastures was measured using stereophotogrammetry derived pasture height. Stereophotogrammetry was achieved as ‘structure from motion’ (here: camera on a moving platform), allowing to generate a three-dimensional canopy model after ‘stitching’ of the images. The pasture of predefined squares was then cut to ground level and oven dried to determine actual biomass yield (‘dry matter’). Three paddocks with different regrowth stages after grazing (low, medium and high biomass) were studied in each experiment. The accuracy of the proposed method was determined using linear regression analysis of ground measured biomass yield and plant height from stereophotogrammetry at the sampling squares. The stereophotogrammetric based results were also compared to biomass assessments with a rising plate meter and to the Normalised Difference Vegetation Index (NDVI) measured using an active optical reflectance sensor (Greenseeker, Trimble).

Overall, in both experiments there was a strong linear relationship between photogrammetry derived plant height and biomass (R²=0.92_{Exp. 1} and R²=0.78_{Exp 2}). The strong relationship between biomass and plant height was confirmed with the correlation of cut biomass to RPM (R²=0.91_{Exp. 1} and 0.78_{Exp 2}). NDVI had a less strong linear relationship with biomass (R²=0.65_{Exp. 1} and 0.66_{Exp 2}). In Experiment 2 grass volume of the sampling, squares were also derived using stereophotogrammetry. The pasture volume had a strong linear relationship with pasture biomass (R² = 0.83).

In conclusion, stereophotogrammetry could be used by farmers to measure biomass with accuracy comparable to that of the commonly used Rising Plate Meter. Measuring devices (e.g. simple digital outdoor cameras) could be mounted on any platform that regularly moves across the paddocks (e.g. an irrigator) and provides sward height profiles and thus yield data of the standing pasture biomass. Further research is required to validate the accuracy of the stereophotogrammetric approach throughout the year as well as under different site conditions and pasture management systems, including mixed species swards.