Use of an Ultrasonic Distance Sensor for Estimating Cotton Plant Height

Accurate, rapid and continuous measurements of cotton canopy height can be useful in managing in-season crop height with growth regulators, harvest aid chemicals, in-season N management and for high-throughput phenotyping.  Surprisingly few studies have been published evaluating on-the-go crop height sensors in cotton.  The objective of this study was to assess the ability of a commercial, industry-grade ultrasonic distance sensor, Honeywell 943, to measure canopy height of irrigated cotton, by comparing sensor-based crop heights with manual height measurements, manual crop width measurements, and NDVI.  A second objective was to estimate the effect of N and water management on ultrasonic-based cotton crop canopy heights.  The study was conducted from 2016-2018 in Maricopa, AZ on a Casa Grande Sandy loam under subsurface drip irrigation with cotton cultivar 'DP 1549 B2XF'.  NDVI and ultrasonic height data showed very high correlation, although NDVI plateaued out at the end of the season.  Manual measurements of plant height and width correlated well with ultrasonic plant height measurements, although only 60 hand measurements were made each week.  Ultrasonic plant heights were about 10 cm lower than manual plant heights, as there are few good leaves to target at the top of the plant.