Estimating Herbage Standing Crop with Visual Obstruction in Tallgrass Prairie

We evaluated the visual obstruction method as a non-destructive means of estimating herbage standing crop in tallgrass prairie. Prediction models were developed for both plot-level and pasture-level estimates by regressing standing crop from clipped plots on visual obstruction measurements (VOM) from 48, 20sample trials. Trials were conducted year-round on burned and non-burned sites in different seral stages and with various levels of productivity and grazing pressure. Separate models were required for burned and non-burned pastures, but both applied across all other variables and were unaffected by community heterogeneity. Coefficients of determination were 0.95 and 0.90 for burned and non-burned pastures, respectively. Use of a more precise measurement scale for visual obstruction did not improve the prediction models. Models for standing crop based on individual quadrats explained less variation than models based on transect averages. The highest correlations with visual obstruction were obtained with 20 x 50 cm quadrats placed adjacent to the measurement pole and oriented toward the observer. The visual obstruction method required little training and mean deviations of student readings from those of the trainer were less than 1 cm. Sampling efficiency is improved with the visual obstruction method because it is reasonably accurate and 6 times faster than clipping. Standing crop estimates can be calculated immediately and less field equipment is needed.