Sunflower Residue During Fallow Periods
Sunflower has, for decades, been commonly labeled as a “low-residue” crop whose postharvest ground is particularly prone to wind erosion. True? Untrue? Partially true?
All three answers could be correct, depending upon the farming area and management system. Lighter soils, low plant populations, mediocre yields and bare ground following harvest are ingredients for a potentially serious erosion problem. At the other end of the spectrum, however, winter and spring winds are of little threat when strong yields and standing sunflower stalks are mixed with a minimum- or no-till production system.
In many High Plains rotations, sunflower is followed by a summer fallow period to recharge soil moisture profiles, with weed control during fallow achieved through the use of wide sweeps (undercutters). How does that program impact residue quantities and, correspondingly, soil erosion control during the fallow period?
Recent studies at the USDA-ARS Central Great Plains Research Station near Akron, Colo., have examined the loss of sunflower residues during summer fallow as affected by two types of management: no-till and sweep-till (conventional).
The USDA project, spearheaded by soil scientist Merle Vigil, measured sunflower residue loss rates in both no-till and conventional-till sunflower and then sought to predict the relationship between at-harvest residue mass and percent residue cover over the course of the fallow period. Sunflower was planted in 1994 and 1995, with the various measurements taken during the ensuing fallow years of 1995 and 1996.
Vigil suggests it’s inaccurate to automatically paint sunflower as a “low-residue” crop. He points, for example, to studies which have measured as much as 7,000 pounds per acre of sunflower residue immediately after harvest. He also notes that research and grower experience have left little doubt about the superiority of surface crop residue under a no-till management system as opposed to conventional tillage. What’s been missing, Vigil believes, is reliable information about (1) the rate of loss of sunflower residue and (2) the mechanisms involved with that residue loss.
“Perhaps the true issue is residue durability,” the USDA scientist ventures. “If that’s the case, the question becomes: How should sunflower be managed during the summer fallow phase to maximize the retention and amounts of surface crop residue?”
One segment of Vigil’s research sought to correlate the height and mass of standing sunflower stalks with residue cover over time. The study’s at-harvest sunflower populations ranged between 10,000 to 14,000 per acre, with the height of the harvested stalks being either 18 inches or 25 inches. Weeds in the tilled plots were controlled with a sweep plow (32-inch V-blade undercutter), while glyphosate (Roundup) provided weed control for the no-till plots. All treatments were replicated six times in each of the study’s two years.
The line transect method* was used to determine percent residue cover, with measurements taken monthly throughout the summer fallow period. Surface residue mass was measured at the beginning of each fallow period and again at the end of the fallow period each year. All soil was cleaned off the collected residue prior to the residue weight being recorded on an oven-dry basis.
For the stalks from the 1994 no-till sunflower harvest, Vigil took measurements in mid-October 1995 — 286 days after harvest. For the 1995-harvested no-till sunflower, stalk evaluations were made in late September of 1996 — 265 days after the crop was harvested. He found that a greater percentage of the shorter stalks (18 inches) were still standing as of wheat planting time, as compared to the taller (25-inch) stalks. The stand count advantage of the shorter stalks as compared to the taller ones was significantly higher in 1995 than in 1996, but the trend was similar. The reasons could be multiple, he says, ranging from more damage to taller stalks from spray booms and tractor axles, to the taller stalks having more area exposed to winds.
The second segment of Vigil’s study was designed to develop a relationship between sunflower surface residue mass and the percent residue cover (as measured by the line transect method). Both weed residue and sunflower residue were collected, cleaned and weighed separately, with standing stalk mass also taken into account.
What did Vigil find?
He reports that in 1995 (’94-crop sunflower), the no-till ground averaged 2,700 pounds of residue per acre on the soil surface as of wheat planting time (mid-September). No-till also maintained greater than 40-percent residue cover. Meanwhile, the sweep-tilled sunflower ground had just 1,900 pounds of residue on the soil surface as of wheat planting time and just 20-percent residue cover.
The benefits from no-till were not as great the following year. “In 1996, the no-till-managed sunflower had significantly more surface residue up to about June 14,” Vigil indicates, “whereas after that date, both the no-till and the conventional-till-managed sunflower had less than 30-percent residue cover. On July 19, both had less than 20-percent residue cover, indicating no real advantage for no-till by this date.”
Poorer weed control and thus a higher amount of weed residue on the first year’s no-till sunflower ground contributed to overall residue cover. “Knowing the weed pressure [level] that can be economically tolerated by sunflower would be beneficial for producers in our region,” Vigil says.
Can use of the line transect method help sunflower producers accurately esti-mate the relationship between flat surface residue mass (pounds per acre) and percent residue cover? Yes, Vigil concludes. “We found that 97 percent of the variability in surface crop and weed residue could be described by a simple linear equation,” he says. That equation is as follows:
Residue (pounds per acre) = 60.5 x PRC-LT. PRC-LT is percent residue cover as measured by the line transect method. (See footnote on page 13 for explanation.)
“This relationship is important, because percent residue cover/line transect measurements are easy to make, whereas actual residue mass measurements are difficult,” Vigil notes. “We now have a relationship, so if someone wants to take a quick line transect measurement, he can use this equation to say, ‘That’s how many pounds of flat or nearly flat residue I have.’ ” The equation would not measure standing stalk effect. — Don Lilleboe
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