How to measure growth area index (GAI) in cereals

The crop’s green area index (GAI) is the ratio of green leaf and stem area to the area of ground on which the crop is growing. Use measurements of GAI during the growing season, alongside targets, to guide canopy management.

• By eye (simplest): Assess the green leaf area compared to the ground area by eye (see our reference photos)
• Use a machine (more accurate): Cut a known area of crop (e.g. 1m²), removing the leaves and feeding them through a leaf area machine
• Download an app: Some apps measure GAI from a photo taken vertically above the crop (see our reference photos). Multiple photos should be taken to get an average for a field. Apps are most accurate for growth stages (GS) up to early stem extension (around GS32)
• Exploit sensors: Whether handheld, tractor-mounted, on drones or satellites, such sensors measure reflectance of wavelengths of light. Various vegetation indices are then calculated, usually using near infrared or red wavelengths, which indicate crop cover

Images courtesy of ADAS.