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Narrow Rows & Weed Control

Saturday, February 1, 1997
filed under: Planting Systems

Can narrow rows and certain other cultural practices help control weeds while simultaneously benefitting sunflower yields? Two years of USDA-ARS research at Mandan, N.D., and Akron, Colo., have produced some enlightening — though not necessarily consistent — data pertaining to that question.

The 1995 research objectives of USDA researchers Donald Tanaka (Mandan) and Randy Anderson (Akron) were to determine the influences of row spacing on (1) light penetration within the sunflower canopy at Akron; (2) weed biomass production at Mandan; and (3) sunflower seed and residue production at both locations.

Previous studies at North Dakota State University had documented that seed yield under narrow rows was either similar to, or better than, that under wide rows at the same per-acre plant populations. Quicker canopy development and a resulting better suppression of weed populations was considered an important reason for that outcome — and provided the motivation for the more-recent USDA research.

At Akron, adverse weather resulted in the 1995 sunflower plots being planted on June 21 — later than planned. Per-acre plant populations were 16,000 and 20,000 on both 18- and 30-inch rows. Special equipment was used to measure solar radiation three times each week from the six-leaf plant stage until flowering. All measurements were taken within one hour of solar noon.

At Mandan, sunflower was planted in four different row spacings: 15-inch, 30-inch, 37.5-inch and in paired rows 7.5 inches apart on 37.5-inch centers. Weed biomass under each spacing configuration was measured when the sunflower reached the flowering stage of plant development.

The 1995 Akron measurements found that light interception was reduced by 15 percent with the 18-inch rows as compared to 30-inch rows. Plant population within a particular row spacing did not, however, affect light penetration. The Akron 18-inch rows arrived at 50 percent or less light penetration by the time the sunflower reached the 10-leaf stage — which was 12 to 14 days sooner than the 30-inch rows.

“Significant reduction in weed development and growth can occur when light penetration within the canopy is less than 50 percent,” note Tanaka and Anderson. That determination, along with the narrow-row data from Akron, would help explain why weed biomass production at Mandan was significantly less for 15-inch rows than with the wider rows. Weed biomass was 1,370 pounds per acre in the 15-inch rows, but rose to 3,200 pounds with the 37.5-inch rows. Biomass totals for the 30-inch and paired rows were similar: 2,480 and 2,640 pounds, respectively.

Despite less inter-row light penetration and lower weed biomass production in the 1995 trials, “narrow rows did not significantly increase total dry matter, seed yield, residue yield, head diameter or residue-to-seed ratio when compared to wide rows at either location,” the USDA researchers report.

With 1995’s results in hand, Tanaka and Anderson decided to examine whether sunflower producers can use narrower rows to reduce herbicide inputs while achieving weed control similar to conventional control in wide rows. The ’96 research focused on a production system — which they termed their “cultural” system (no residual herbicides) — that included narrow rows, an increased plant population and a delayed seeding date. Comparing this system to a “conventional” system, the objective was to see what kind of interactive impact there would be on weed management without residual herbicides.

Each system — conventional and cultural — was placed under both no-till and minimum-till conditions at both the Mandan and Akron locations. The no-till conventional herbicide was Prowl at 1.5 pounds ai/A, with the no-till cultural treatment consisting of Roundup (0.37 pound ai/A) applied about 10 days prior to planting). The minimum-till treatment for the conventional 30-inch rows was Sonalan (1.0 pound ai/A, incorporated twice), with the minimum-till cultural treatment consisting strictly of a tillage pass with an undercutter prior to planting.

A John Deere 750 no-till drill was used to plant the sunflower at both locations. At Akron, the conventional planting took place on June 5; the cultural planting on June 20. The Mandan dates were May 23 and June 10, respectively. Row spacing on the conventional was 30 inches; that of the cultural, 15 inches. Plant populations were 16,000 (Akron conventional), 20,000 (Akron cultural) and 29,000 (both conventional and cultural at Mandan).

Conventional system sunflower at Akron was planted just prior to peak weed emergence, while cultural system sunflower was planted after peak weed emergence. That circumstance “reduced weed biomass and density by eight-fold when compared to the conventional systems.”

However, the situation was markedly different at Mandan, where weed biomass was two to three times greater in the cultural systems. “Because of the wide range in temperature fluctuations at Mandan for late May and June, weeds did not exhibit an emergence curve similar to the one at Akron,” the USDA researchers explain. “Most of the weeds emerged after planting the cultural system.” Their conclusion? “It appears that because of the differences in the agro-ecosystem, planting sunflower later, as was done in the Akron cultural system, may not be a practical method of controlling weeds in sunflower at Mandan.”

Looking at the light penetration issue, Tanaka and Anderson report that despite having been planted two weeks later, the cultural system at Akron reached the key 50-percent light penetration level within one week of the conventional sunflower. That development could have reduced crop-weed competition and contributed to what ended up being very similar results in terms of sunflower seed yield and head diameter between the two systems.

At Mandan, weed competition in the cultural systems resulted in final seed yields sharply lower than the conventional. Head diameter and kernel weight also were significantly less. Both systems produced similar amounts of sunflower dry matter and residue at Mandan.

Tanaka and Anderson conclude that “the potential exists to control weeds in sunflower using cultural management techniques rather than herbicides.” The study shows that narrow rows can increase weed control; but combining narrow rows with other cultural practices magnifies that control. Practices such as seeding date appear to hold more potential as a weed control aid in the Central Great Plains rather than in the Northern Great Plains, they add. — Don Lilleboe
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