Estimating conservation tillage residue using aerial photography

A new, inexpensive aerial photointerpretation technique for estimating the percentage of grain straw residue cover on field surfaces uses low-altitude, large-scale, small-format photography in conjunction with 35mm projectors and a modified light table. The 35mm reference and test frames represent subareas of fall-planted fields, which vary in previous year's residue cover, soil color, and soil texture. Ground data collected include line transects for estimating the cover percentage. Two film types, natural color and colorinfrared, were acquired in 70mm film at two scales (1:4,000 and 1:12,000), then reframed in 35mm mounts for independent testing by interpreters with varying experience. Based on four residue cover classes, the 1:12,000-scale, color-infrared photographs, with an average correct value of 70.7%, were significantly better (P = 0.05) than the other film types and scales. Based on two residue cover classes (<30 percent, ≥ 30 percent), the 1:12,000-scale, color-infrared photos again produced the highest overall correct value (91%), but the value was not significantly different from the 1:12,000 color (88.3%) or the 1:4,000 color-infrared (88.3%) photos. This method of estimating straw residue cover percentage shows promise in monitoring conservation tillage and soil exposed to rainfall erosion. The method could also substantially reduce the field time needed to ensure compliance of agricultural conservation practices in U.S. Department of Agriculture cost-sharing programs.