INTRODUCTION
A majority of South Carolina's soybeans are planted in the early summer after wheat harvest (doublecropped). Traditionally, farmers have burned or buried wheat residues prior to soybean planting, both of which are now considered environmentally-unfriendly practices. During the early 1990s, Southeastern farmers became interested in using both narrow row widths (less than 30 inches) and conservation tillage (at least 30% residue cover on soil surface) to produce doublecropped soybeans. The soil surface is shaded earlier in the growing season when narrow row widths are used, thereby reducing the risk of soil erosion and making the crop more competitive with weeds. At the beginning of this project, we felt the increased competitiveness of the crop with narrow row widths would be important for conservation-tillage systems where cultivation is generally not a weed-management option. In addition, we envisioned the weed-control benefits from narrow row widths would aid farmers who grow herbicide-resistant crops since the herbicides applied to these crops, such as glyphosate, have no residual weed-control activity in the soil. For any type of narrow-row system, we felt deep tillage would be necessary to promote root penetration through Coastal Plain soil hardpan layers.
OBJECTIVE
Develop economically and environmentally sustainable narrow row and conservation tillage soybean production systems.
APPROACH
The interactive effects of surface tillage (disked versus no-surface-tillage), deep tillage, row width (7.5 inch versus 30 inch), crop rotation, soil type, and genotype were examined in a series of experiments for doublecropped and full-season soybean intitiated between 1993 and 2001. In most cases, the field experiments were conducted using methods similar to those described by Frederick et al. (1998). Seeds were planted at a rate of 4 and 8 seeds per foot of crop row when the 7.5-inch and 30-inch row widths were used, respectively. In most cases, the variety Northrup King S75-55 was planted in these studies.
RESULTS
For doublecropped soybean, highest seed yields were found when the soybean was planted no till in narrow row widths and the soil was deep tilled with a ParaTill prior to planting (Table 1). This early study was conducted on a highly productive Goldsboro sandy loam soil and rainfall amounts were above normal in all three years. Soybean yield increases due to narrow row widths were greater with deep tillage and no-surface tillage. Subsequent research conducted using large (1/8 acre) research plots at another location supported these results, although seed yields were lower in this experiment than in the previous study because growing conditions were much drier (Table 2). In addition to higher seed yields, we found maximum shading by the soybean leaves occurred more than 4 weeks earlier with the 7.5-inch row width than with the 30-inch row width which was observed to result in better weed control (data not shown).
Growers on the Coastal Plain frequently ask whether planting with conservation tillage reduces the need for deep tillage. After 5 years of research, we have found no indication that the yield response to deep tillage is diminished over time with conservation tillage (Fig. 1), compared to disking the soil (Fig. 2).
Benefits from narrow row widths have been reported to be greater when soybean is planted late. Late planting reduces soybean branch growth and branch yield, which can be compensated for by planting at a higher seeding rate in a more narrow-row configuration (Board et al., 1990; Frederick et al., 1998). We found substantial seed-yield increases with narrow row widths for both full-season and later-planted doublecropped soybean, although the doublecropped soybean had less branch and total seed yield than the full-season soybean (Table 3). The higher plant population used with the narrow row width resulted in a proportional increase in seed yield originating from the main stems and a smaller increase in seed yield originating from the branches. The lower branch and total seed yield per plant with narrow row widths probably was due to the more severe early-season drought stress that occurred with the narrow row widths (Frederick et al., 1998). Drought stress primarily reduces seed yield on the branches, but has little effect on seed yield from the main stems (Table 4). Because branch growth and branch seed yield are very sensitive to environmental stresses, we hypothesize that soybean varieties with good mainstem yield potential may be more suitable for narrow row width systems, especially for later-planted doublecropped soybean. In 2000, we began testing lines and varieties for seed-yield differences under narrow row culture (Table 5).
CONCLUSIONS
Planting with narrow row widths (7.5 inches) consistently increased soybean seed yield, even in dry years. Based upon our research results, farmers should expect yield increases of 15 to 20% or more with narrow row widths under most growing conditions on the Coastal Plain of South Carolina. Yield increases with narrow row widths were usually greater when conservation tillage and deep tillage were used. Data indicate deep tillage is more important when planting with narrow row widths and/or conservation tillage than when traditional practices are used.
REFERENCES
Board, J.E., B.G. Harville, and A.M. Saxton. 1990. Branch dry weight in relation to yield increases in narrow-row soybean. Agron. J. 82:540-544.
Frederick, J.R., P.J. Bauer, W.J. Busscher, and G.S. McCutcheon. 1998. Tillage management for doublecropped soybean grown in narrow and wide row culture. Crop Sci. 38:755.
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| Fig. 1. Seed yield response to deep tillage over time for doublecropped soybean grown with (DT) and without (NO) deep tillage. Soybean was produced using conservation tillage and the same experimental plots each year. Source: Frederick, Bauer, Busscher, and McCutcheon, 2002, unpublished data. Source: Frederick, Bauer, Busscher, and McCutcheon, 2002, unpublished data. |
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| Fig. 2. Seed yield response to deep tillage over time for doublecropped soybean grown with (DT) and without (NO) deep tillage. The soil was disked before planting and the same experimental plots were used each year. Source: Frederick, Bauer, Busscher, and McCutcheon, 2002, unpublished data. |