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United States Department of Agriculture

Agricultural Research Service

Title: Estimating Herbage Standing Crop in Tallgrass Prairie with the Visual Obstruction Method

Authors
item Vermeire, Lance - TEXAS TECH UNIV.
item Gillen, Robert

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 20, 2000
Publication Date: January 1, 2001

Interpretive Summary: Reliable estimates of the amount of forage in a pasture are critical for making informed decisions about grazing management. Gathering such information is difficult and usually relies on hand clipping of several individual samples from across a pasture. This method is useful for research but is usually not practical on actual farms and ranches. A more rapid alternative method makes visual estimates of forage height and then converts the height measurements to weight. This method has been useful in some types of pastures but the conversion factor from height to weight is different for different types of forages. We tested the visual method in tallgrass prairie vegetation. The visual method was accurate and at least 6 times faster than hand clipping. The conversion factor from height to weight was not different between seasons of the year. The conversion factor was different between pastures burned within the last year compared with pastures not burned. Results were equally good whether height measurement were made to the nearest 2.5 cm (1 inch) or the nearest 5 cm (2 inches). The method requires little training or specialized equipment. We recommend the use of the visual method as a practical alternative for measurement of forage weight but care must be taken to use the proper conversion factor for burned or unburned pastures.

Technical Abstract: We evaluated the visual obstruction method as a non-destructive means of estimating herbage standing crop in tallgrass prairie. Prediction models were developed for quadrat and pasture-level estimates by regressing standing crop from clipped quadrats on visual obstruction measurements (VOM) from 48, 20-sample 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 the field estimates 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 VOM 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 six times faster than clipping. Standing crop estimates can be calculated immediately and less equipment is needed in the field.

Last Modified: 11/24/2014
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