Comparison of weight estimate and rising-plate meter methods to measure herbage mass of a mountain meadow

Abstract

A rising plate meter (RPM) and ocular estimation of herbage fresh weight (OCES) were compared as double sampling methods to measure herbage dry weight (DWT) in a mountain meadow grazed by cattle (0, 2.5, 3.2, and 6.9 AUM/ha) and deer. On 8 dates, 5 to 10 plots were clipped and 50 to 100 plots were estimated in 2 or 4 pastures, each of which had 6 vegetation types, resulting in 120 groups of observations. Whereas 11 different calibration lines were necessary to calibrate the OCES (r2 =0.74$ to 0.91), 17 lines were needed for the RPM (r2=0.04 to 0.82). Average residual standard deviations (Sy.x) were 653 for OCES vs. 846 kg ha-1 for RPM. The different calibrations for OCES were caused by differences in the %DM of the herbage (dates and meadow type), whereas RPM calibrations were affected by grazing treatment, date, meadow type, and observer. When the same number of clipped and estimated plots were used for both methods, OCES was 24% more precise than RPM. To obtain a precision of +/- 200 kg ha-1 (P=0.05) OCES required 697 fewer clipped plots for the whole experiment than RPM, but OCES field costs were 3% higher. If calibrated on net readings (before-after clipping) RPM overestimated herbage mass, relative to clipped plots and OCES. The lower cost per RPM reading was counterbalanced by greater precision and generality of OCES calibrations.