The Value of Those Stalks
Even the casual observer driving around Great Plains sunflower areas in winter can attest to the obvious difference in snow trap in a tilled or flattened harvested field versus one with thousands of upright stalks per acre. But just how much difference is there? And how much benefit do those standing stalks provide in terms of postharvest soil erosion control?
USDA’s Natural Resources Conserva-tion Service (NRCS) was seeking some quantifiable answers when it asked researchers at the USDA-ARS Central Great Plains Research Station to undertake studies documenting the impact of standing sunflower stalks. After three years and five studies at the Akron, Colo., station, lead investigator David Nielsen has reported some revealing data — data which should be helpful to NRCS as it updates its sunflower residue erosion control formulas.
It’s common for sunflower ground in the central Great Plains to be fallowed following the sunflower harvest, with winter wheat then planted on those fields the next August or September. For years, however, there has been widespread concern about wind erosion during that fallow period, Nielsen notes, “due to the assumption that low residue amounts left after harvest provide inadequate protection for the soil surface.” But based on his research, the USDA research agronomist believes those stick-like standing stalks have a significantly bigger impact than what they’re credited for in the soil-loss calculation models used to date.
Nielsen’s research — which stretched from the fall of 1992 through the spring of 1995 — was designed to measure the effects of differences in standing sunflower stalk height and population on (1) wind velocity within the sunflower residue, (2) snow depth and (3) changes in soil water content during the winter and spring months following the sunflower harvest. Nielsen looked at the relative effect of flattened stalks and those at two different heights (ranging from 17 to 29 inches, depending upon the respective study).
Stalk populations ran from about 10,600 to 26,100 per acre. Three of the studies were conducted in USDA plots at Akron; the other two in commercial sunflower fields near the northeastern Colorado community. Row spacings were 30 inches in all instances.
To obtain accurate readings of wind speed, “cup anemometers” (see above photo) were placed at various heights among the flattened and standing stalks; and an anemometer and wind vane were also set up in the fields at a height of 79 inches above surface level. Wind speeds were measured every minute, with 15-minute average values computed and saved by on-site dataloggers. Measurements from the 79-inch height were used as the reference wind speed against which lower-elevation speeds were compared.
Soil water content was measured at various depths and times throughout the studies. Snow depth measurements were taken at four locations in each plot of four of the five studies.
Stalk diameters of 40 plants surround-ing each anemometer mast were measured as part of the effort to gauge the “silhou-ette” factor (i.e., cumulative effect of height times diameter times population).
What were the conclusions of the three-year USDA research project?
First, cutting height of the stalks plays the key role in reducing wind velocity, and hence soil erosion, as well as in trapping snowfall. All three factors — cutting height, plant population and individual stalk diameter — are important, Nielsen states, but cutting height leads the group.
Based on his research, Nielsen has developed a formula to predict the effects of the various parameters. Utilizing this formula, he offers an example:
In a field with 16,000 stalks per acre at an average stalk diameter of two-thirds of an inch (measured at roughly two inches below the top of the stalk), changing the cutting height from 10 to 30 inches would triple the silhouette factor and end up reducing wind speeds by 26 percent (this at eight inches above the soil surface).
What does all this imply for NRCS soil-loss calculations? According to Nielsen, such a magnitude of change in the silhouette factor corresponds to a reduction from 45 percent down to 19 percent in the soil-loss ratio used by NRCS in its revised wind erosion equation.
Does wind direction as compared to sunflower row direction play a role in this equation? Yes, but a small one, Nielsen says. He finds only about a four-percent reduction in wind velocities at the soil surface when those winds are blowing perpendicular to the rows as opposed to being parallel with the rows.
Nielsen found no consistent relationship between plant population or cutting height and the quantity of residue mass. That inconsistency was, he says, due to substantial variability in water availability during sunflower growing seasons and the resulting amounts of residue mass in the respective studies.
Do higher plant populations necessarily increase a field’s total silhouette factor since there are that many more stalks? No, says Nielsen. The increased number of stalks is largely offset by the decrease in individual stalk diameter; so total silhouette area remains about the same.
The effect of sunflower stalk height on snow depth in the Great Plains studies is illustrated by Figure 1. While the four studies reflected in Figure 1 consistently showed an advantage from higher cutting heights, that advantage was more pronounced in some fields than others. That’s due to the varying amounts and timing of snowfall received by the respective fields — and to differences in measured wind speed.
For example, “the overwinter period for Study 1 was one of nearly continuous snow cover from November 21, 1992, through March 15, 1993,” Nielsen reports. “Conse-quently, with each new snow event, the effect of differences in snow catch due to stalk height increased.” As of January 11, 1993, the 27- to 29-inch standing stalks in Study 1 had trapped more than three times the snow trapped by the flattened stalks.
By comparison, during the winter of 1993/94, the Akron area did not receive any large snowfalls accompanied by high winds; nor were there extended periods of continuous snow cover. So the differences between standing stalks and flattened areas — while still evident — were not nearly as pronounced (Figure 1 / Studies 3 and 4).
Not surprisingly, the taller sunflower stalk heights and corresponding increases in snow catch translated into significantly better overwinter soil water storage (Figure 2). For the two studies in which snowfall was accompanied by strong winds, about three inches of additional soil water were stored under the tall standing stalks as compared to soil where the stalks had been flattened. “Water use-yield relationships for this area of the central Great Plains suggest that this amount of increased soil water would result in an increase in wheat yield of about 18 bushels per acre in a wheat/sunflower rotation,” Nielsen observes. (Strong winds did not accom-pany snowfall in the other two studies. Correspondingly, there were not any significant differences in snow trap and resulting soil water storage between standing and flattened stalks.)
One final related note: Most soil erosion in the central Great Plains is produced by winds of 35 miles per hour or greater, Nielsen states. Picking a wind speed of 35 mph, he analyzed the nine-year period of 1987 through 1995 to see how many times in each month that particular wind speed was achieved or exceeded at Akron. “March/April/May is the big erosive period here,” he comments. “If we can protect the soil during those months, we generally don’t have a lot of problems.”
Over that nine-year period, Nielsen found that each of those three months averaged just one day with potential for winds of 35 mph or greater. His message? If central Great Plains sunflower stalks can be left standing well into May, they will not only trap moisture during the winter months, but will also be of significant help in reducing soil erosion throughout the spring.
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