ABSTRACT:
Nine devices were used to visually measure percent residue cover on fields following a crop of winter wheat (Triticum aestivum L). The devices were four lines with various arrangements of bead markers, a measuring tape, and four wheels with markers located on or near the wheel perimeter. Emphasis was on wheat residues, including low residue cover as found following intense tillage of Pacific Northwest non-irrigated wheatlands. Sources of measurement variation were studied for each of the nine devices to determine if any of the methods/devices were preferable for documenting residue cover in conditions of extended weathering and multiple tillages. The fields were located in the Southern Great Plains and the Columbia Plateau area of the Pacific Northwest. A total of nine field sites were intensely measured, with three operators taking 1000 point readings with each device. Residue cover varied from 6 to 84%-cover (“%-cover” is the unit of measure). Field conditions ranged from fallow with as many as seven tillages to long-term no-till. Measurement precision and operation time were similar for the four line and wheel type devices. Precision, as evaluated by the mean range of data values, ranged between 7.7 to 9.7%-cover. Excluding another type of transect line, the measuring tape, field time to take 1000 points averaged only 22.3 min. The tape took 50% more field time and is not recommended. Any differences in measured cover caused by the line bead arrangements or wheel designs, were either small or were masked by large variations among operators and spatial differences across the fields. Other factors, including length, height, stubble row spacing, and initial amount of residue, as well as surface roughness were also either masked or not important. Measurement variations by operators within replications and across replications were of the same order of magnitude, at ± 2 to 7%-cover. When combined, these sources of variation generally exceeded the arbitrary criteria of allowable variations of ± 3%-cover (originally based on ± 10% of 30% mean cover, which defined the critical value of residue cover for a tillage system to be “Conservation Tillage” (CTIC, 1995)). Of course, as residue cover values become small, such as 6 to 10%-cover, then the ± 3%-cover is really an allowable tolerance of ± 33 to 50%. This points out the need either for more appropriate allowance criteria or for the development of measurement instruments which will achieve more precision than is currently possible with any of the visual-measurement devices/methods used in this study. Results showed that fields with low amounts of weathered wheat residues may be measured with the same devices and methods as fields with higher amounts of residues. The range of observed values and absolute variation became smaller as cover decreased. Because the field sites spanned a wide range of conditions, the results should apply to most dryland wheat production areas.
Footnotes
J. E. Morrison, Jr., is an agricultural engineer with the USDA-Agricultural Research Service, Temple, TX 76502; R. W. Rickman is a soil scientist with the USDA-Agricultural Research Service, Pendleton, OR 97801; D. K. McCool is an agricultural engineer with the USDA-Agricultural Research Service, Washington State University, Pullman, WA 99164-6420; and K. L. Pfeiffer is a conservation agronomist with the USDA-Natural Resources Conservation Service, WNTC, Turdaud, OR 97204-3225.
- Copyright 1997 by the Soil and Water Conservation Society
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