Visual Assessment versus Compressed Sward Heights as Predictors of Forage Biomass in Cool-Season Pastures

 

R. E. Vibart, S. L. White–Bennett, J. T. Green, Jr., and S. P. Washburn

 

Abstract

            Two methods to predict pasture biomass for fescue-ladino clover and orchardgrass-ladino clover swards were studied. Visual assessment by an experienced observer was compared to compressed heights using a 0.2-m² acrylic plate meter that exerted 6.0 kg/m² pressure on the sward. Regression analyses were conducted for each method using clipped and dried plant material to ground level from 0.25- m² quadrats used as the referral method. Sampling included 150 observations for fescue on 38 d and 119 observations for orchardgrass on 30 d from 1995 to 1998. Estimates of biomass were sequential from least to most invasive: visual assessment; compressed sward height; and clipped biomass. Higher coefficients of determination (R²) and lower errors of prediction (S_xy) for regressions indicated improved prediction of forage biomass. Regressed across sampling days and all observations, visual assessments (R² = 0.69 and 0.65, S_xy = 685 and 624 kg DM/ha for fescue, and orchardgrass, respectively) were more accurate in predicting forage biomass than were compressed sward heights (R² = 0.37 and 0.47, S_xy = 998 and 775 kg DM/ha for tall fescue and orchardgrass, respectively). Regression equations within sampling days were also more precise for visual assessment than for compressed sward heights for fescue (R² = 0.94 vs. 0.83 ± 0.03; P < 0.01; S_xy = 297 vs. 442 ± 48; P < 0.05; n = 38) and tended to differ for orchardgrass (R² = 0.94 vs. 0.89 ± 0.02; P = 0.07; S_xy = 247 vs. 321 ± 42; P = 0.16; n = 30).   Compared to compressed sward heights, visual assessment had a higher percentage of predictions with R² ≥ 0.8 (92 vs. 68% of 38, P < 0.05) for fescue and a numerically higher percentage (93 vs. 83% of 30) for orchardgrass. Reasons for lower accuracy using compressed sward heights were not clear. Indentations in soil surface, differences in composition, maturity, or ground cover could contribute to less precision using the plate meter. Visual assessment proved to be a simple and accurate method of predicting forage biomass in cool season pastures.

 

Key Words: visual assessment, compressed sward height, pasture biomass.