Tillage & N Effects
Since 1985, the USDA-ARS Northern Great Plains Research Laboratory at Mandan, N.D., has been gathering data on the effects of tillage and nitrogen on sunflower production under an annual cropping system. This research was initiated in response to growing interest in conserva-tion tillage systems — spawned in part by farm program compliance requirements — as means of preserving soil and water. The research also has been interwoven with the desire of many Great Plains producers for alternatives to a traditional wheat/fallow rotation which would better utilize their land resources and benefit them economically.
Carried out on a 65-acre site, the USDA-ARS research has compared fallow versus continuous cropping; three types of tillage systems (conventional, minimum-till and no-till); and various rates of nitrogen fertilizer. During the 10 study years summarized here (1985-1994), annual precipitation at the south central North Dakota location averaged 15.7 inches, ranging from a low of 5.8 inches (1988) to a high of 22.6 inches (1993). The long-term (119 years) annual precipitation average at the Northern Great Plains Research Laboratory is 16.4 inches, with the May-September portion being 11.3 inches.
Of the three tillage treatments in the sunflower experiments, “conventional till” was defined as having less than 30-percent residue cover at planting; “minimum till” plots had 30- to 60-percent residue cover at planting; and “no-till” was interpreted as more than 60-percent residue cover at the time of planting. Nitrogen fertilizer was applied at rates of 30, 60 and 90 pounds to each crop in the annual cropping system (spring wheat, winter wheat, sunflower). Broadcast ammonium nitrate (34-0-0) prior to planting was used as the N source. Two sunflower hybrids — one a early maturing variety and other being of medium maturity — were planted each year, though not the same two over the entire 10-year period.
Sunflower followed winter wheat in the continuous cropping rotation, with fall weed control on the wheat stubble usually accomplished with a Roundup treatment. Stubble was left standing over winter in all tillage treatments to trap snow and enhance soil water storage upon spring melting.
Spring tillage operations prior to sunflower seeding consisted of the following:
• In conventional till, a granular herbicide (Treflan or Sonalan at 0.9 to 1.0 pound active ingredient per acre) was applied with an air-assist granular applicator mounted on an undercutter. This took place during the initial tillage operation (shallow, about two inches deep) in mid-April. It was followed by a tandem disk pass in mid-May, just before sunflower planting.
• In minimum-till plots, the granular herbicide was applied in mid-April via the same method as in conventional till. The second incorporation was with the under-cutter in mid-May.
• The no-till plots received a fall-spray application of Surflan or a granular applica-tion of Sonalan at a rate of 1.25 pounds active ingredient per acre in late October. When needed, these plots also received a spring burndown treatment of Roundup.
The sunflower crop was seeded around the third week in May each year with a 800 Cyclo air planter. Approximately 23,000 seeds were dropped in 36-inch rows.
What has this decade-long investigation revealed regarding the production of sunflower in this type of environment and under the above-described system?
Table 1 summarizes precipitation, grain yield, sunflower residue yield, stalk-grain ratio, test weight and oil content across all tillage and nitrogen treatments. As shown in the table, the Mandan site experienced several years of below-normal precipitation during 1985-94, including three straight years (1988-90) of severe drought. The data show that sunflower was able to utilize deep soil water supplies during that drought period; but the 1991 yields were very poor since the deeper soil profile had not been recharged. Yields did rebound along with precipitation during 1992-94.
Table 2 is of particular interest, as it indicates the effect of tillage and nitrogen fertilization on sunflower yields and test weights across the 10-year spectrum.
In terms of grain yield, Table 2 shows there was not a large difference among the three tillage treatments when averaged over 10 years. The average yield ranged from 1,328 pounds of seed under a minimum-till system to 1,237 pounds in conventional till. The no-till sites were in the middle at 1,299 pounds per acre.
There was a significant response to nitrogen fertilization. As Table 2 indicates, the average 10-year yield ranged from 1,356 pounds per acre for sunflower receiving the 90-pound N rate, down to 1,215 pounds for the 30-pound rate. Not surprisingly, the 60-pound was in the middle at 1,292 pounds.
The amount of crop residue (stalks) is an important consideration in controlling soil erosion following a sunflower crop. Residue/stalk production ranged widely over the 10-year study period: from 1,224 pounds per acre in 1990 (a drought year) up to 4,530 pounds in 1987. The 10-year average was 2,768 pounds of residue per acre — an amount which, if left largely intact after harvest, should be able to significantly reduce soil and water erosion potential.
As might be expected, the stalk-to-grain ratio increased in dry years (Table 1). Stalk yield also increased in accordance with higher N rates and, when compared among tillage treatments, was highest under the no-till system.
Nitrogen rates and tillage treatments did have an effect on seed test weight (Table 2), though not a large one. Neither test weight or oil content (Table 2) were adversely affected by the annual cropping system.
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