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Soybean Production Systems for Claypan Soils¹

In the Midwestern USA, early planting, narrow-row spacing, and good weed control are recommended practices for soybean (Glycine max L. Merr.) production. Reduced tillage is being increasingly recommended as a means of reducing fuel use and soil erosion. Little multifactor research has been conducted to define the optimum combination of tillage, planting date, row spacing, and weed control. Two levels of each of these factors were tested in a 2⁴ factorial experiment. Treatment levels for each factor were: planting date (1 May vs. 1 June); row spacing (38 vs. 76 cm); weed control (preemergence herbicides vs preemergence + postemergence herbicides); and seedhed preparation (conventional vs. no-till). Additional treatment combinations evaluated the effects of mechanical cultivation in wide rows. The study was continued for 5 years on a Mexico silt loam, mainly Udollic and Mollic Ochraqualf soil near Columbia, Mo. Compared to their counterparts, average yield advantages for narrowrow spacing, conventional tillage, June planting, and preemergence + postemergenee weed control were, 5 (not significant), 31, 43, and 51 %, respectively. Lack of yield response of ‘Williams’ soybeans to narrowrow spacing and early planting was attributed to limited rainfall and incomplete weed control. Average weed control was 18, 12, and 31% better with conventional tillage, June planting, and herbicides applied preemergence and postemergence than with their counterparts. Other treatment comparisons showed no difference in soybean yields between bentazon (3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) applied postemergence and two timely cultivations. The availability of improved herbicides for selective weed control along with improved planting equipment may lead to an acceptance of continuous no-tillage for a corn (Zea mays L.)-soybean rotation. Planting date should be planned so that critical stages of soybean development coincide with periods of probable moisture availability. When mid-season rainfall is normally deficient, planting soybeans about 1 June may achieve better yields than earlier (1 May) plantings on soils with moisture-holding characteristics similar to those of the silt loam Udollic and Mollic Ochraqualf soils of central Missouri.

Key Words: Weed control • Tillage • Stand • Lodging • Soil moisture • Narrow rows • Glycine max Merr.

² Agricultural Engineer, USDA, ARS, Soybean Production Systems Research Unit, 102 Bldg. T-12, UMC, Columbia, MO 65211 and professor of agronomy, Univ. of Missouri, Columbia, MO 65211.

Received for publication May 14, 1982.

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