Measuring Legume Content in Pastures Using Digital Photographs

Quantifying botanical composition is important for evaluating the effects of management on legume content and of legume content on pasture yield and quality. The standard for measuring botanical composition is hand separation of clipped samples. An alternative is taking point counts of botanical components on photographs of the pasture. The latter was tested on a rotationally stocked pasture, with photos taken at 24 random sample areas, areas clipped at ground level, and samples hand separated into grass, legume, and forb fractions. Photos were evaluated with a grid in Microsoft PowerPoint. Point counts were calibrated to hand-separated values using linear regression. Grass and legume point-count components were not significantly different from hand-separated values (P = 0.05) but underestimated the forb fraction. Calibration regressions had R² values ranging from 0.45 to 0.98. The precision of this technique is dependent on the number of photos per pasture, the number of points counted per photo, and the number of paired samples taken for calibration. In cool-season grass–clover pastures, 12 or more photos per pasture and 100 or more points per photo are a good balance between photo number and points per photo. For calibration, 12 or more paired samples should be used. Photo point counts appear to be a practical method of measuring grass, legume, and forb components in rotationally grazed pastures.