No abstract

The competitive effects of sicklepod(Cassia obtusifoliaL.) on soybeans(Glycine maxL. ‘Bragg’) were studied on Malbis sandy loam soil and Chesterfield sandy loam soil during 1968 to 1970. Weed density was a linear function of soybean yield for the densities studied (0 to 15 weeds/m2). The regression coefficient of weed density (weeds per square meter) on bean yield (kilograms per hectare) was −92 and −101 for the two soils, respectively. When the data were combined across both soils and years, the regression coefficient was −95. Densities of 7.7 weeds/m2reduced soybean yield 19 to 32% and 34 to 35% on the two soils, respectively. The weight of weeds at harvest was also inversely correlated to soybean yield. Sicklepod was more competitive when grown 15 to 30 cm from the drill than in the drill. Soybean yields were reduced in some instances when sicklepod was allowed to compete for as little as 4 weeks. If weeds were removed at 2 to 4 weeks after soybean emergence, no further weed removal was needed for maximum yield of soybeans.

Cropping systems for minimum or no tillage have been developed to produce corn (Zea mays L.) yields equal to or higher than yields obtained in conventional tillage systems, but limited research has been conducted with tillage systems involving corit and soybean [ Glycine max (L.) Merr.J rotations. The objective of this study was to compare different cropping sequences of corn, soybean, and wheat (Triticum aestivum L.) in conventional, strip, and no tillage. A field experiment was conducted on a Hartsell fine sandy loam (fine-loamy, siliceous, thermic Typic Hapludults). In 1981, corn yields with no tillage were 30o/o lower than those from conventional tillage systems. No corn yield differences were observed in 1982 and 1984 due to tillage or crop rotation; however, in 1983, strip and no tillage in conjunction with soybean in the rotation increased corn grain yields by 12%. Soybean yields in strip and no tillage decreased 16o/o compared to conventional tillage yields in 1981, but in subsequent years, soybean yields increased with those systems. A significant tillage x rotation interaction in 1981, 1982, and 1983 was caused primarily by a buildup of soybean cyst nematode (Heterodera glycines Ichinohe) (SCN) population with conventional tillage and continuous soybean. Rainfall affected soybean yields more with conventional tillage than with strip or no tillage. The conservation tillage systems (strip or no-tillage system) in combination with corn-soybean rotation for both full-season or double-cropped soybean gave the most consistent yield increase for the 4 yr.Published in Agron. J. 80:76-80(1 988).

Soybean (Glycine max (L.) Merrill) yields in the past have varied considerably from year to year in the southeastern United States. Many factors are important in soybean production, but water stress at critical growth periods appears to be one of the most frequently limiting factors. Soybeans were grown on a Lucedale fine sandy loam soil to determine the effects of three soil water regimes at two row widths and three plant population levels on water use, rate of plant growth, and bean yield of two varieties. Water use rates by soybeans were influenced more by soil water regime than by row width or variety. Average rates during the season ranged from 0.05 to 0.84 cm/day, depending on the amount of available water in the soil. Variety had the greatest influence on plant height. Final plant heights ranged from 10 to 15 cm greater for ‘Bragg’ than for ‘Hampton 266.’ Heights were increased by irrigation and plant population level, but were not affected by row width. Average bean yields were influenced more by irrigation or variety than by row width or plant population. Average bean yields by soil water regimes were 2,020, 2,420, and 2,490 kg/ha for no irrigation, intermediate, and high irrigation, respectively. Bragg averaged 2,420 kg/ha and Hampton 266 averaged 2,150 kg/ha. Row width and plant population showed little influence on average bean yields.