A comparison of four methods of herbage mass estimation

The objective of this study was to compare the accuracy of four non-destructive techniques for estimation of herbage mass (HM) in swards rotationally grazed by dairy cows. Visual estimation (VE) of HM, a rising plate meter (RPM), the Hill Farm Research Organisation sward stick (SS) and the pasture probe capacitance meter (PPCM) were evaluated. Estimates were obtained on five occasions (July, September and November, 1997 and April and May, 1998). Mean dry matter (DM) yields (kg/ha) of available herbage cut to 40 mm above ground level (AH) and total herbage cut to ground level (TH) were 2161 (s.d. 850.2) and 2762 (s.d. 890.9) kg, respectively. Relationships were obtained for VE, RPM, SS and PPCM with AH, and for RPM, SS and PPCM with TH. For AH, the proportions of variance (R²) accounted for by the model were 0.95, 0.94, 0.92 and 0.72 for VE, RPM, SS and PPCM, respectively. The corresponding residual s.d. values were 193, 222, 249 and 458 kg DM/ha, and the coefficients of variation (CVs) were 9%, 10%, 12% and 21%, respectively. For TH, the R² values were 0.88, 0.87 and 0.76 for RPM, SS and PPCM, respectively. The corresponding residual s.d. values were 318, 331, and 442 kg DM/ha, and the CVs were 12%, 12%, and 16%, respectively. It is concluded that VE was the most accurate method of HM prediction and that the PPCM method was an inaccurate predictor of HM.