Normalized Difference Vegetation Index and Normalized Difference Red Edge could be useful tools for optimising grazing management in mixed pastures.

Pasture growth and quality are optimised when pasture is grazed at the point at which it achieves 95% light interception (LI), that is, when the canopy is capable of intercepting 95% of the incident solar radiation with its leaves. Thus, measurements of LI provide a good indicator of the ideal time to graze a pasture. However, measurements of LI are difficult since they require sensors to be placed in and above a pasture canopy throughout a pasture regrowth period and are affected by daily weather patterns. Therefore, an alternative tool for predicting pasture LI is desired. The Normalized Difference Vegetation Index (NDVI) and Normalized Difference Red Edge (NDRE) may provide useful alternatives to LI, since their measurement is less labour-intensive and is not affected by weather. In a mixed perennial ryegrass (Lolium perenne L.) + plantain (Plantago lanceolata L.) pasture at Massey University, New Zealand, the relationship between NDVI, NDRE and LI was investigated between early spring and late summer. NDVI and NDRE were measured by scanning eight plots (182m²) with a Rapid Scan(R) CS-45 canopy analyser (~300 readings), while LI was measured with a Spectrosense 2⁺(R) device, at three fixed locations within each plot. Positive logarithmic relationships were found between NDVI and LI (R² = 0.35) and NDRE and LI (R² = 0.30). These results suggest that NDVI and NDRE may be useful tools for predicting pasture LI, therefore indicating the most effective time to graze a pasture. However, future research is required to determine the limitations of their use for predicting optimal grazing times for different pasture species.