Estimating Shrub Forage Yield and Utilization Using a Photographic Technique

Abstract

We assessed a photographic technique to estimate shrub yield and utilization of common snowberry, snowbrush, and heart-leafed willow found in mixed-conifer rangelands. We determined the correlation between green leaf area size (LA) and forage yield (Y) and compared plant utilization estimated by photographic technique (ULA) to actual utilization (UY) values. Shrub forage yield and utilization were determined by hand plucking in five to eight increments. Before the first plucking, and after each subsequent plucking, we took two photos that were perpendicular to each other at the intersection of the shrub. Each photo was evaluated using image-processing software to produce red (R), green (G), and blue (B) color band images. Green leaf area was determined as follows: pixel = ([G-R] + [G-B])/ (G+R+G+B); Green leaf area, cm2 = (Calibration [C], cm2∙pixel-1) × (LA, pixel). Green leaf area calculated from the photographic technique was strongly correlated (r2 = 0.83-0.94, P < 0.001) with forage yield for all three shrubs. A strong correlation (r2 = 0.95, n = 142) was detected between utilization values estimated through green leaf area size and actual values. Utilization estimated by LA did not differ (P = 0.60, n = 142) from actual utilization values. In summary, our results indicate the photographic technique could be used once calibration curves are developed, to measure available browse yield and utilization of shrubs in a nondestructive approach.