Unmanned aerial systems for monitoring seasonal dynamics of sward height under rotational grazing conditions

Efficient pasture management involves striking a balance between meeting the needs of the animals and maintaining the vegetation over time. For this purpose, monitoring sward condition, especially sward height, during the grazing season is critical. The use of unmanned aerial systems (UAS) could provide new insights into sward conditions for farmers to support adequate management. The effect of plant-animal interactions on the spatial heterogeneity of sward height and regrowth after grazing was evaluated in an experiment conducted in the spring and summer of 2022. The experiment involved rotational grazing, with animals changing paddocks every 3 days. UAS flights were coupled with sward height measurements using a sward stick, before and after grazing. As a first step, the possibility of monitoring the effects of grazing intensity and estimating sward height at different spatial resolutions using an easily calculated indicator from multispectral imagery, the Normalized Difference Vegetation Index (NDVI), was investigated. NDVI showed significant differences (P<0.05) between paddocks with high and low grazing intensity. Coefficients of determination (R2) ranging from 0.43 to 0.52 as the spatial resolution increased from 1.76 cm to 21.76 cm show that NDVI can potentially predict sward height when the spatial resolution is adequate. Further work based on a large database or other indicators or combinations of indicators is therefore needed to assess the seasonal dynamics of sward height under grazing conditions using UAS.